cavity-nesting ducks: why woodpeckers matter · trees nor woodpeckers are available (kear in...

Birdwatchers, at least in Britain, tend tothink of ducks as nesting in the open andon the ground, but cavity-nesting is actu-

ally normal, if not obligatory, in almost one-third of the 162 members of the wildfowl family(Anatidae). Geffen & Yom-Tov (2001) suggestedthat hole-nesting has evolved independently atleast three times within the group. I think that itis more likely to have arisen five or six times;indeed, hole-nesting could be the primitivecondition. In any event, the habit occurs amongwhistling-ducks (Dendrocygnini), shelducks(Tadornini), in two or three different groups of

surface-feeding ducks (Anatini), includingthose which we used to call ‘perching ducks’,and in the seaducks (Mergini).

Because ducks are incapable of making holesfor themselves, however, they must rely onnatural agents for the construction of nest cavi-ties, and this dependence has profound implica-tions for their lifestyles, breeding productivityand conservation. As Eberhard (2002) pointedout, because they are not excavators, cavity-adopting ducks have limited control over thelocation of their nests, and rarely nest colo-nially. The skills of a variety of other animals

217© British Birds 96 • May 2003 • 217-233

Cavity-nesting ducks:why woodpeckers matter

Janet Kear

ABSTRACT This paper poses a number of related questions and suggests someanswers.Why were there no resident tree-hole-nesting ducks in Britain until

recently? Why is the Black Woodpecker Dryocopus martius not found inBritain? Could the supply of invertebrate food items, especially in winter, be

responsible for the distribution of woodpeckers in western Europe? It isconcluded that, in Europe, only the Black Woodpecker can construct holes

large enough for ducks to nest in, and that this woodpecker does not occur inBritain & Ireland because of the absence of carpenter ants Campanotus. A pleais made for the global conservation of dead and dying timber, since it is vital

for the biodiversity of plants, insects, woodpeckers and ducks.

Goosander Mergus merganserRosemary Watts/Powell

are employed, but the bird family which con-tributes most to the process of hole-making, as‘primary cavity excavators’ (Aitken et al. 2002),is the woodpeckers (Picidae), of which there arejust over 200 species (Winkler et al. 1995).

To give a few examples: Audubon (1835)observed the North American Wood Duck Aixsponsa laying for three successive years in holesmade and abandoned by the large, and probablynow extinct, Ivory-billed WoodpeckerCampephilus principalis. Indeed, all early Amer-ican explorer-naturalists (who were mostlyEuropeans) noted with interest that the WoodDuck nested in woodpecker holes. Bellrose &Holm (1994) actually speculated that the duckevolved its particular size and slim shape inorder to take advantage of cavities created by awoodpecker which is slightly smaller thanIvory-billed but much commoner – the PileatedWoodpecker Dryocopus pileatus. They made thepoint that the range of the Wood Duck fitscompletely within that of the Pileated Wood-pecker.

The Wood Duck’s nearest relative, the Man-darin Duck Aix galericulata, breeds in tem-perate forests similar to those inhabited by theWood Duck, but on the opposite side of thePacific Ocean. The males of these two beautifulducks differ substantially in plumage and a littlein size – the Wood Duck being stouter – but the

females, which alone incubate the eggs, areextremely similar in appearance and body pro-portions (Madge & Burn 1988). Since it hasbeen suggested, quite convincingly, that the sizeand shape of the American bird evolved toenable it to sit in a woodpecker hole, it seemedsensible to look for an Asian woodpecker, aboutthe same size as the Pileated, which may havehad a similar influence on the Mandarin. Whileit is not as rare as used to be assumed, the Man-darin is still uncommon. The forests ofManchuria, where it formerly bred, have beenalmost totally destroyed, and its breedingbiology is little studied (Shurtleff & Savage1996). The only woodpecker which occurs inJapan in sufficient numbers, and can producecavities large enough for the Mandarin, is theBlack Woodpecker D. martius. As a breeder, thewoodpecker is more or less confined toHokkaido and northern Honshu – and so is thenesting Mandarin. Black Woodpeckers are plen-tiful among the steep mountains of easternHokkaido, where Mandarins and GoosandersMergus merganser also occur in the nestingseason. As the female Goosander uses BlackWoodpecker holes in Europe (and PileatedWoodpecker holes in America; Mallory & Metz1999), it is likely that the Goosander and theMandarin adopt such cavities in Japan as well.Perhaps at one time, the Mandarin nested in

218 British Birds 96 • May 2003 • 217-233

Cavity-nesting Ducks: why woodpeckers matter

154. Female Wood Duck Aix sponsa investigating nest sites, Canada.

219British Birds 96 • May 2003 • 217-233

Cavity-nesting ducks: why woodpeckers matter

155. Male and female Mandarin Ducks Aix galericulata,Virginia Water, Surrey. In Japan, the current breeding range of the Mandarin closely matches that of the Black Woodpecker Dryocopus martius (plate 156),

which is found chiefly in Hokkaido and northern Honshu.

Black Woodpecker holes in other parts of thelatter’s extensive range, so that the WoodDuck/Pileated Woodpecker association in NorthAmerica was mirrored by one involving Man-darins and Black Woodpeckers in Asia.

A number of holarctic seaducks are cavity-nesters, and many likewise depend on wood-peckers. Nearly 100% of Buffleheads Bucephalaalbeola use holes made by a smaller woodpecker,the Northern Flicker Colaptes auratus. SmewMergellus albellus, Hooded Merganser Lophodytescucullatus and Common Goldeneye B. clangulaalso use cavities made by large woodpeckers, asdoes Barrow’s Goldeneye B. islandica in NorthAmerica (Evans et al. 2002). The population ofabout 2,000 Barrow’s Goldeneyes in Iceland usescrevices in the lava rock, however, since neithertrees nor woodpeckers are available (Kear inpress).

The requirement for a degree of adaptabilityand accommodation by cavity-nesting ducks isobvious. The Goosander will occasionally nestamong boulders, in a hole in the ground or acavity in the bank, so treeless areas are some-

times used (Sharrock 1976; Mallory & Metz1999). Wood Ducks and Mandarin Ducks, on theother hand, are obligate tree hole-nesters, notlaying at all unless an elevated cavity can befound. By no means all will use woodpeckerholes, but many prefer them if they are available.In North America, Pileated Woodpecker nestsconstituted only 5% of cavities deemed ‘suitable’by researchers in one study of over 300 suchholes; nonetheless, 20% of cavities actually usedby Wood Ducks were old woodpecker nests(Soulliere 1990). Why should they seek out awoodpecker hole rather than one which hasmerely rotted? By choosing a site in which toexcavate their nest, woodpeckers are maximisingtheir chances of being free of interference fromtheir own kind, free from last year’s nest parasites(20-50% of Black Woodpeckers may reuse lastyear’s hole if it was successful, but PileatedWoodpeckers seldom do; Bull & Meslow 1977;Winkler et al. 1995), and, most of all, free fromegg-eaters such as climbing snakes andmammals. In Europe and Asia, mammalianpredators include Pine Martens Martes martes,

Japanese Martens M. melampus, Stone MartensM. foina and Raccoon Dogs Nyctereutes procy-onoides, while in North America, Common Rac-coons Procyon lotor and the large Fox SquirrelSciurus niger are potential predators. Wood-peckers intend their nests to be, so far as pos-sible, out of the reach of predators, with fewperches or ledges to give a foothold, and with asmall entrance, so that an enemy has difficultygaining access. Since woodpeckers are usuallyterritorial, their holes are dispersed through theforest, and often concealed under dense canopycover (Evans et al. 2002). This may not be thecase with natural cavities; so, from the duck’spoint of view, old woodpecker holes are morelikely to be cryptic and impregnable, and to

produce live duck-lings at the end of 30days of incubation.

Cavity capacitymay seem a tight fitfor a duck and hereggs, as most femaleducks are twice aslarge as a wood-pecker. The size ofthe cavity excavatedby the woodpecker,however, as opposedto that of the entryhole, is determinedby the size of four orfive full-grownfledglings because,unlike ducks, youngwoodpeckers growup within the cavity.Nest-hole capacitywill, therefore, begreater than thatneeded for an adultwoodpecker pluseggs. The PileatedWoodpecker nesthas a rounderentrance than thatof the Black Wood-pecker: 10.8 cm ×8.7 cm was theaverage of 13entrances (Haramis1990), and theWood Duck seemsquite capable of

getting through a hole of that size. Black Wood-pecker entrance holes, at 13 cm × 8.5 cm, aremore oval (Winkler et al. 1995), and a femaleSmew or Mandarin must turn through 90° toenter. Cavity depth and width are less easy tomeasure, but the average of 13 Pileated Wood-pecker nests was 56 cm deep by 23 cm wide(Bull & Meslow 1977) – long enough to make ithard for a Raccoon to reach the bottom with itsarm, and quite wide.

Even those few birds, including woodpeckers,which make burrows in the ground, can beuseful to ducks. In South America, for instance,ground-burrowing woodpeckers (probablyCampo Flicker Colaptes campestris) sometimesprovide a home for the Speckled Teal Anas flavi-

156. Black Woodpecker Dryocopus martius, Finland, February 1987.

rostris (Nores & Yzurieta 1980). In Africa, theAfrican Pygmy-goose Nettapus auritus, at 250 gthe smallest of all ducks, nests in the disusedholes of barbets Megalaima spp. as well as thoseof woodpeckers. The female Ruddy-headedGoose Chloephaga rubidiceps sometimes nestsin penguin burrows, as does the FalklandSteamerduck Tachyeres brachypterus, and theTorrent Duck Merganetta armata may use theabandoned burrow of the Ringed KingfisherMegaceryle torquata (Kear in press).

In Argentina, Speckled Teals choose aban-doned chambers in the crowded compoundnests of Monk Parakeets Myiopsitta monachus,and may nest colonially – a rare phenomenonamong ducks; the large, enclosed stick nests are5-20 m high in the tree canopy. At the begin-ning of the twentieth century, parakeet nestswere built only in Tala Celtis tala trees but, sincethen, exotic and much taller Eucalyptus gumshave been introduced. These are selected almostexclusively by parakeets and ducks as providinghigher and better security from terrestrialpredators (Gibson 1920; Hudson 1920; Weller1967; Port 1998a,b). Absolute height is not aproblem, since young wildfowl of hole-nestingspecies hatch without a fear of heights (Kear1967) and, being small and light in weight,jump without harm to join their parent on theground beneath.

In Australia and Madagascar, there are nowoodpeckers; nevertheless, many ducks arecavity-nesters, and so storms, fire, moisture,fungus, ants (Formicidae) and termites(Isoptera) must be relied upon to provide holes,mostly in trees but in other structures as well(Simpson & Wilson 2001). The Radjah Shel-duck Tadorna radjah in Australia adopts cavitiesmade by termites and fungal infections (Frith1982). The grey teal group, including ChestnutTeal Anas castanea, Sunda Teal A. gibberifrons,Andaman Teal A. (gibberifrons) albogularis andBernier’s Teal A. bernieri, are essentially birds ofthe Grey Mangrove Avicennia marina oceanfringe of a tropical and semi-tropical rangewhich stretches from Madagascar to Australia,and all are cavity-nesters (Young et al. 2001;Young 2002; plate 157). Cavities in mangrovetrees are caused by the fall of side branches, andthen by rot and termite action. Once rot setsinto the crown, the tree will not live for long, soa cycle of maturing and dying swamps is essen-tial for successful duck reproduction.

The endangered White-winged Duck Cairina

scutulata of the tropical wet forests of southeastAsia is yet another obligate cavity-nester; itscontinued presence in largely cleared areas ofsoutheast Sumatra has been attributed to itshabit of nesting in rot-created holes in rengastrees Gluta spp. These trees are frequently leftuncut in former swamp forest on account oftheir irritant sap, which is poisonous to humans(although apparently not to monkeys and squir-rels which eat the fruit), and so are rejected bytree-cutters and the timber trade (Green 1992).Thus, the duck can sometimes survive in placeswhich are otherwise cleared of trees. While theEgyptian Goose Alopochen aegyptiacus in SouthAfrica is not an obligate hole-nester – abouttwo-thirds of the population nests on theground – it will nest in elevated positions up to60 m high and many of these are in holes (Mil-stein 1993); Brown et al. (1982), Maclean (1993)and Milstein (1993) noted sites which includedold Hamerkop Scopus umbretta, crow Corvusspp. and African Fish-Eagle Haliaeetus vocifernests, cliff ledges, tree holes, burrows, churchsteeples, caves and buildings.

Most shelducks use mammal burrows, ratherthan tree holes, except for the Paradise ShelduckTadorna variegata of New Zealand, which, untilrecently, lived where there were no mammals

157. Hole in mangrove tree used by Bernier’s TealAnas bernieri, Madagascar.

(the females of this species, like Barrow’s Gold-eneyes in Iceland, use rock crevices instead). TheCommon Shelduck T. tadorna must have beenfar less numerous in Britain before the Normansintroduced the burrowing Rabbit Oryctolaguscuniculus. Rabbits were initially kept by monksas a source of food, and housed in specialwarrens which were bordered by high banks andgorse hedges. The Dissolution of the Monas-teries in the first half of the sixteenth centurymeant that the warrens were sold into private

hands and, eventually, the rabbits escaped topopulate the countryside, and their burrowsprovided underground nesting sites for shel-ducks. The Australian Shelduck T. tadornoideshas likewise adopted the burrows of introducedrabbits; formerly, they must have used holesprovided by ground-burrowing marsupials,some of which are now extinct. The SouthAfrican Shelduck T. cana almost always selectsburrows made by Aardvarks Orycteropus afer.

To summarise, many wildfowl use cavities for

158. Bob Tilt examining the nest of a Pacific Black Duck Anas superciliosa,Barrenbox swamp, near Griffith, New South Wales, Australia, early 1950s.

160. Typical nesting site of Pacific Black Duck Anas superciliosa in a Red Gum Eucalyptus camaldulensis tree, AngoraSwamp, Booligal, New South Wales, Australia, October 1980.This site is within a spout caused by a limb breaking

off and revealing the hollow interior.

159. Nest, with eggs, of anincubating Pacific Black Duck

Anas superciliosa in a site similar tothat shown in plate 158, NSW,Australia, October 1980. Note that there is a complete lack ofnesting material, simply copious

down and a few breast/bellyfeathers from the female.

tralia

nesting, although not all are obligate hole-nesters. A variety of agents create the holes, andsome of the most important of these are wood-peckers. Both wildfowl and woodpeckers arethought to have evolved at much the same timein the Cretaceous period (approximately 144 to66.4 million years ago), quite early in the evolu-tion of the birds. So, wildfowl have been aroundfor a long time – long enough to get used to,and make use of, the woodpeckers.

The missing ducksUntil a pair of Goosandersproduced young in Perth-shire in 1871, no tree-hole-nesting duck had bred inBritain. There are now twomore: the human-intro-duced Mandarin Duck,which has been breedingsuccessfully in the region ofVirginia Water, Berkshire,since 1930 (Shurtleff &Savage 1996), and the self-introduced Common Gold-eneye which first nested inthe Scottish Highlands ofStrathspey in 1970 (Dennis& Dow 1984). All three havespread since their initialhatch (fig. 1). The

Goosander, despite being culled by fishermenconcerned for salmon Salmo salar stocks, is nowbreeding throughout Scotland, Wales andEngland as far south as Devon. The Mandarin isstill commonest in the Thames Valley, where, 15years ago, it was thought to have reached a pop-ulation size of at least 7,000 individuals (Davis1988). More than 100 pairs of Common Gold-eneyes now produce young every year in Scot-land. In 1999, a pair bred in the Borders region,

161. Female Common Goldeneye Bucephala clangula, prospecting for nest cavities, Highland, spring 1996.

162. Goosanders Mergus merganser, Germany, November 1998. Female Goosanders use Black WoodpeckerDryocopus martius holes in Europe, and Pileated Woodpecker Dryocopus pileatus holes in America.

which may suggest a slow spread southwards,and individuals are seen increasingly during thesummer in England (Ogilvie et al. 2001).

Why did tree-hole-nesting ducks take so longto reach Britain? Providing the answer to thatquestion is one of the purposes of this paper –and while I cannot be certain, I am going tosuggest some explanations.

Fig. 1. Distribution of Mandarin Duck Aixgalericulata, Common Goldeneye Bucephala clangula

and Goosander Mergus merganser in Britain.Reproduced from The New Atlas of Breeding Birds inBritain and Ireland: 1988-1991 (Gibbons et al. 1993),

with kind permission of the BTO. Large dotsrepresent evidence of breeding within the 10-km

square concerned, while small dots indicate that thespecies was present during the breeding season.

Distribution of woodpeckersLeaving aside ducks for a moment, what dictatesthe distribution of woodpeckers, especially thelarger ones, to whose ranges our northern tree-nesting ducks seem so tied? In particular, why arethere no Black Woodpeckers in Britain despite athriving population just across the channel inFrance and Belgium, at the edge of a huge rangestretching across Europe and Asia to Japan?

Let me also pose a related query: why arethere no woodpeckers of any size in Ireland?They occur on the Isle of Wight, Anglesey, andJersey, but not on the Isle of Man nor in Ireland.Are woodpeckers such poor flyers that theycannot negotiate the Irish Sea? Hutchinson(1989) thought that the absence of wood-peckers, along with other forest birds such asTawny Owl Strix aluco, European NuthatchSitta europaea, and Marsh Parus palustris andWillow Tits P. montanus, was due to theirinability to reach Ireland at all, and theirabsence does support the hypothesis that immi-

Mandarin Duck Common Goldeneye

Goosander

gration rates are dependent on distance fromthe source pool (MacArthur & Wilson 1967).Fossils of the Great Spotted Woodpecker Den-drocopos major have been found in Co. Clarefrom the time of the last glaciation, whenIreland was joined to Britain, but only theGreen Picus viridis and Great Spotted Wood-peckers have been recorded as recent vagrants,and according to Hutchinson (1989) most ofthe Great Spotted Woodpecker records are ofirruptive Continental birds. Lack (1969), on theother hand, thought that the difficulty of dis-persal across the Irish Sea could not be thefactor responsible for the lack of woodpeckers,nuthatches etc. in Ireland. The fact that theContinental Great Spotted Woodpecker is occa-sionally irruptive suggests that it is quitecapable of reaching Ireland and the Isle of Man– it just does not thrive once it gets there. Someexplanation other than their inability to flythere is necessary for the missing Irish wood-peckers.

Woodpeckers do have a curious worldwidedistribution (Winkler et al. 1995). Althoughoccurring in the Bahamas, the Canaries and thePhilippines, they are not found on some otherconspicuous islands and continents, such asNew Zealand, Australia, New Guinea, Mada-gascar (where the Aye-Aye Daubentonia mada-gascariensis is said to fill their niche by eatinginsects from rotting wood with a hugely elon-gated third finger – Young 2002) or on theGalapagos, where the Woodpecker FinchCamarhynchus pallidus classically performs thatrole (Lack 1947). There seems to be no evidencethat Black Woodpeckers have been wiped outfrom Britain – the species has not been presentsince the last Ice Age (10,000-12,000 years ago)at least. Similarly, Ireland seems to have had norecent resident woodpeckers. Fungal decay oftrees large enough for cavity excavation is a pri-ority requirement. In Belgium, 40% of BlackWoodpeckers make their nests in Beech Fagussylvatica wood, often in the avenues of Beecheswhich line busy roads, and only 20% in pinesPinus. The Beech is a hardwood tree which issubject to heart rot and so becomes easy for thebirds to excavate. Historically, it is a native ofonly the southeast corner of Britain, but BlackWoodpeckers also excavate the decaying woodof pine and spruce Picea, so that the compara-tive rarity of Beech until recently does not seemsufficient reason for the absence of BlackWoodpeckers.

Woodpecker food supplyI believe that the clue to woodpecker distribu-tion in Britain & Ireland lies in their foodsupply, and in the availability of two kinds ofwood-living insects – beetles (Coleoptera) andants. The birds collect these insects with the aidof a long tongue and sticky saliva. The varietyand size of insects becomes fewer and smallerrespectively as one goes west from the Conti-nent to England and then across the Irish Sea.Sixty-two species of longhorn or woodboringbeetle (Cerambycidae) occur in Britain, partic-ularly on oaks Quercus spp., willows Salix sp.,poplars Populus spp. and pines, but this is only aquarter of the 250 species occurring in centralEurope, and the Continental ones are typicallylarger in body size (McLean & Speight 1993). ABlack Woodpecker may consume 900 barkbeetle larvae or 1,000 ants in a single meal, andthe largest beetle larva recorded as being takenwas over 6 cm long (Cramp & Simmons 1977)– there is nothing of this size in Britain. Whyhave the larger longhorn beetles not spreadmore widely? Presumably because they do notfly powerfully, as there is no strong selection in

Fig. 2. The number of ant species per British vice-county and the zones where daily sunlight in Mayaverages 5-6 hours. Black: over 30 species; dense

dots: over 20; light dots: over 10; blank: 1-10.Reproduced from the New Naturalist Ants (Brian

1977) with kind permission of HarperCollins.

favour of long-distance dispersal – the nextdead tree in an unmanaged forest is usually notfar away (McLean & Speight 1993). As adults,they are dependent on the pollen of flowers,and are often to be seen feeding in the sunshine,or on warm evenings. Possibly our summers arenot warm enough nor, especially, dry enoughfor them, or perhaps our winters are not suffi-ciently cold.

All European woodpeckers take ants, notably,of course, our own Green Woodpecker, whichtakes about 2,000 daily, mostly collected fromlawns and meadows. Pileated and Black Wood-peckers are especially fond of carpenter ants ofthe genus Campanotus, which, althoughextremely widespread, do not occur in Britain &Ireland. Unlike the longhorns, ants live in,rather than feed on, dead wood. Their distribu-tion again seems to be determined mainly bytemperature, and a map of ant abundance (fig.2) closely mirrors one showing abundance of

British breeding woodpeckers (fig. 3). A bias tothe south is obvious: of the 42 species of antfound in Britain & Ireland, 33 occur in Dorset,31 in Hampshire, 29 in Surrey, 18 in NorthWales, 14 in the northern Highlands of Scot-land, 18 in Leinster and nine in Ulster (Brian1977). I believe that it is the availability of long-horn beetles and ants, especially in winter, thatdetermines woodpecker success in Britain.

Nevertheless, British woodpeckers obviouslydo eat other things besides beetle larvae andants. The Great Spotted Woodpecker seems par-ticularly adaptable; since the 1960s, it has comeonto bird tables, where it consumes fat, andseeds such as peanuts. Indeed, the fact that iteats spruce and pine seeds on the Continent,and that these fail in some years, is probably thecause of its occasional irruptive behaviour. Italso takes nestling birds and moths. The twolarger British woodpeckers have spread andincreased in the last 100 years as the climate has

Fig. 3. Distribution of Green Woodpecker Picus viridis and Great Spotted Woodpecker Dendrocopos major inBritain. Reproduced from The New Atlas of Breeding Birds in Britain and Ireland: 1988-1991 (Gibbons et al. 1993),with kind permission of the BTO.These maps represent regional variation in relative abundance, from high (dark

red) to low (light blue).The data were analysed so that each of the ten colours (excluding white) coversapproximately one-tenth of the coloured area on the final maps.The numbers in the key represent frequency ofoccurrence (the proportion of tetrads visited in each 10-km square in which the species was recorded).These

maps show that Green Woodpecker strongholds are in southeast England and the Home Counties, extending westto the New Forest; in wooded areas flanking the River Severn; the Welsh Marches and South Wales; and scattered

concentrations in the Brecklands of East Anglia, Devon and the Lake District. Great Spotted Woodpeckers aremost abundant in southern England, with other hotspots in Gloucestershire, Gwent and the Welsh Marches.

Compare these patterns with the ant distribution shown in fig. 2 (page 225).

Green Woodpecker Great Spotted Woodpecker

ameliorated, and they are presumably helped byincreasing forestation, which has assisted thespread of the wood ants Formica spp. TheGreen Woodpecker has invaded Scotland, theGreat Spotted Woodpecker has moved north,and both are thought to have benefited, tem-porarily, from Dutch Elm disease (Sharrock1976). The Lesser Spotted Woodpecker Dendro-copos minor eats moths (Lepidoptera) andgreenfly (Aphididae) in Britain (but seems tofeed its nestlings on tree-climbing ants), and itsnumbers are currently declining for reasonswhich are not obvious.Ants and beetles mayhave multiplied andspread in Britain duringthe last half-century, butthey are not counted soregularly nor completelyas birds, and few distrib-ution maps have beencompiled.

So I may not beentirely correct in sug-gesting that the compar-ative lack of ant andbeetle diversity in Irelandaccounts for the lack ofwoodpeckers there. CliveHutchinson wrote in1989 that ‘the only cer-tainty at this stage is thatmuch more remains tobe said on the subject’. Ido, however, feel thatfood supply, especially inwinter, is likely to be thecritical factor. Moreover,I suspect that the reasonwhy there is no thrivingpopulation of BlackWoodpeckers in Britainis the absence of car-penter ants, which aresuch an importantelement of their winterfood across a range thatextends from France toJapan. The closest car-penter ant to theseislands in Europe is thetree-dwelling Campan-otus herculeanus, which,as its name suggests, is

large. It lives in rotten wood and in big colonies;it therefore needs large old trees (Holldobler &Wilson 1990). Maybe our trees are generally toosmall for it. It also likes warm dry summers andcold winters. It has ‘anti-freeze’ arrangements inits physiology so that it is able to hibernate, sta-tionary, within the wood, unless it is dug out by aprobing woodpecker. Apparently, C. herculeanusoccurred in Britain during the last interglacialperiod, but died out; today, it would probablyfind our climate too warm and our winters toounpredictable.

163. Male Great Spotted Woodpecker Dendrocopos major, Kent.Food supply, in particular the comparative lack of ant and beetle diversity,

may be a critical factor in explaining why this species is absent from Ireland.

To return to the missing tree-hole-nestingducks, I suggest that Britain historically had nosuch birds because of the absence of BlackWoodpeckers – the only creature large enoughto excavate safe, sizeable cavities in wood. Why,then, do we now have three? Three changes mayhave influenced this situation, to varyingdegrees. One is that predator-proof cavitieshave become available from another source –from humans, in the form of nestboxes.Another factor has been a drastic decrease inour largest tree-climbing and egg-eatingpredator, the Pine Marten, brought almost toextinction during the last two centuries bygamekeepers who treated it as vermin (Corbet& Southern 1977). A third factor might beclimate change – we entered another warmingperiod in the 1880s, and our springs havebecome milder. Warmer springs are likely tohave been relevant to both Common Goldeneyeand Goosander as they are early nesters and layon shorter daylengths than other species in theseaduck group (Murton & Kear 1978). I aminclined to believe that the Mandarin Duck isalso affected by early spring temperatures (Kear1990), and would not have survived its releaseinto Britain if the climate had been that of thepre-1850s when the River Thames regularlyfroze in winter.

NestboxesSecure nesting sites are a priority requirementof cavity-nesting ducks, and boxes which can berendered predator-proof can be an excellenttool for increasing the local productivity (Zicus1990; Ludwichowski et al. 2002). In the case ofBlack-bellied Whistling-duck Dendrocygnaautumnalis in Texas, hatching success was 44%in cavity nests but 77% in nesting boxes pro-tected with predator guards (Bolen 1967).

Almost all Scottish Common Goldeneyes usenestboxes, but it took 14 years for the first birdto lay in one of the boxes supplied (Dennis &Dow 1984) – perhaps the individual was afemale that had herself hatched in a box on theContinent. The small British breeding popula-tion is still largely dependent on boxes ratherthan on natural holes, and I doubt that theCommon Goldeneye would be nesting so suc-cessfully were it not for those nestboxes. Anabsence of large tree-climbing predators prob-ably contributed to that initial success – thougha gradual increase in Pine Martens since the1920s (Corbet & Southern 1977), owing to areduction in keepers and the growth of newconifer plantations, has meant that martensnow take Common Goldeneye eggs in theHighlands occasionally (Dennis & Dow 1984).

Obviously, Mandarin Ducks would not be

164 & 165. Nestbox (left) sited for CommonGoldeneyes Bucephala clangula, and a similar boxcontaining a completed clutch (right), Sweden.

breeding in Britain if someone had not intro-duced them. The initial breeding of a sizeablepopulation of flying Mandarins was in boxesplaced around a waterfowl collection atFoxwarren Park in Surrey (Kear 1990; Shurtleff& Savage 1996). Their successful escape andspread, into a relatively predator-free environ-ment (domestic cats Felis silvestris catus andGrey Squirrels Sciurus carolinensis do not, onthe whole, take eggs as large as those of ducks),was aided by the boxes which were put uparound Windsor Great Park, in Berkshire. Forinstance, in the six seasons before 1985, anaverage of 56 ducklings annually left theWindsor boxes (Davis & Baggott 1989a,b).

The Goosander arrived by its own means,and its colonisation seems to be part of thespecies’ natural increase and spread throughwestern Europe. Its initial success was not dueto boxes, although its spread, in Northumbriafor instance, may have been, and some birds inStrathspey certainly make use of boxes put upfor Common Goldeneyes. Perhaps the mostimportant factor here was the comparativescarcity of a tree-climbing egg-eater; if the PineMarten, which also nests in hollow trees, hadbeen present when Goosanders started tocolonise and breed in the 1870s, I believe thatthey would have had greater difficulty inbecoming established.

Competition for holesSizeable cavities, suitable for ducks, are desir-able if not essential for a wide range of othervertebrates and invertebrates, and competitionfor their possession can be considerable(Conner et al. 2001; Semel & Sherman 2001;Aitken et al. 2002). In Madagascar, for example,many parrots (Psittacidae), Comb DucksSarkidiornis melanotos, African Pygmy-geeseand nocturnal lemurs (Lepilemur and

Cheirogaleus) spend time in holes (Young2002), and compete with the Bernier’s Teal fornest sites. Wood Ducks rival Black-belliedWhistling-ducks for cavities in the south oftheir range in Texas (Bolen & Cain 1968), andHooded Mergansers, Buffleheads, goldeneyes,owls (Strigiformes), and mammals such asmartens, squirrels and bats (Chiroptera) farthernorth (Bellrose & Holm 1994). Wood Duckshave been seen trying to usurp Pileated Wood-pecker nests while they were still in use by theowner and excavator (Conner et al. 2001).Introduced European Honeybees Apis melliferaand Common Starlings Sturnus vulgaris(Kerpez & Smith 1990) provide further prob-lems for the ducks.

Tree-hole-nesting ducks in continentalEurope clash with Eurasian Jackdaws Corvusmonedula, European Rollers Coracias garrulus,Stock Doves Columba oenas, owls and starlings,hornets (Vespidae) and bats (Winkler et al.1995), among others. In Brazil, the criticallyendangered Brazilian Merganser Mergus octose-taceus has to share a limited tree-hole resourcewith Muscovy Ducks Cairina moschata, toucansRamphastos spp., parrots and mammals such asWhite-eared Opossums Didelphis albiventris(Silveira & Bartmann 2001). Parrots, cockatoos,owls, tree kingfishers (Halcyonidae), opossums(Didelphidae), bats, reptiles, bees(Hymenoptera) and introduced Common Star-lings and Common Mynas Acridotheres tristiscompete with ducks for cavity nests in Australia(Simpson & Wilson 2001). It is unusual forthere to be enough large cavities present tosatisfy local demand.

Conservation implicationsThe biodiversity of whole habitats needs pre-serving in order to maintain successful breedingpopulations of cavity-living animals, including

166. Brood of recently hatched Common Goldeneyes Bucephala clangula, reared in nestbox, Highland, 1973.

ducks. Many primary cavity excavators areessential to the lifestyle of others. Agents besideswoodpeckers can play a positive role; anincrease in the number of North AmericanBeavers Castor canadensis has been suggested asa factor in the North American Wood Duck’scomeback since the 1920s and 1930s, as naturalcavities in ‘snags’ standing in water tend tohatch more ducklings (Beard 1953; Haramis1990). The ant-eating Aardvark is listed asEndangered by IUCN; its conservation is vital ifthe South African Shelduck is to find sufficientnesting opportunities.

In Poland, Mikusinski et al. (2001) found apositive relationship between woodpeckerspecies richness and the numbers of other forestbirds. They suggested that the woodpeckergroup is a good indicator for assessing aviandiversity, and that in regions of Europe wheredata on forest bird numbers are not readilyavailable, woodpecker surveys provide an excel-lent assessment tool. Sadly, many species ofwoodpecker are in decline because of habitatloss, and nestboxes do not work well for them(Winkler et al. 1995). It was the destruction andfragmentation of the old pine and hardwoodforests of southeastern USA, as well as over-hunting, that eliminated the Ivory-billed Wood-pecker (Jackson 1996); the same was probablytrue of the extinct Imperial WoodpeckerCampephilus imperialis of Mexico, which, at 563 g, was the largest of the woodpeckers. TheBrazilian gallery forests contain few old, large

holed trees because most have been removed byhumans and, perhaps as a consequence, thehole-nesting Brazilian Merganser is reduced tojust 250 individuals (Silveira & Bartmann2001). Mangroves, the home of many of thegrey teal group, and of great value as fish nurs-eries, are being destroyed at an alarming rate toprovide farmed prawns for western markets. InAustralia, governments are encouraging theplanting of vast numbers of new trees whilecontinuing to allow the clearing and logging ofold-growth forest and woodland (Simpson &Wilson 2001), and this goes on elsewhere.

Where a shortage of safe nesting sites forducks is limiting productivity, artificial onesneed to imitate a range of features. The NorthAmericans, in an environment still full ofpredators, make great efforts to create boxes forWood Ducks which resemble tree trunks andwoodpecker holes – because those are the oneswhich the ducks prefer. Maybe the Japanese, intrying to increase the numbers of MandarinDucks, should do the same. If no woodpeckerholes are available for the ducks to make a com-parison with the site in which they werehatched (as in Britain), then all sorts of boxesseem to be acceptable. In the tropics, artificialnest sites will need to be examined frequently asthey themselves may decay, or be swept away inhurricanes.

A different benefit of nestbox schemes is thatthe construction and erection of boxes caninvolve the public and hunters’ organisations,

167 & 168. Female Common Goldeneyes Bucephalaclangula, investigating chimney pots as potential

nesting sites, Strathspey, Highland, spring.This speciessometimes attempts to nest in chimneys.

such as ‘Ducks Unlimited’ in North Americaand in New Zealand (www.ducks.org). As wellas providing local employment, the construc-tion of nestboxes can help to demonstrate prac-tical conservation techniques (Fredrickson et al.1990). Their use was pioneered long ago in cap-tivity by aviculturists who wanted their tameducks to breed; the development of structureswhich are predator-proof has been in the handsof biologists working with wild populations.The Orinoco Goose Neochen jubata of theAmazon and the rare Scaly-sided MerganserMergus squamatus in far-eastern Russia areamong species which have benefited recently,and taken successfully to artificial sites (Kear inpress). The installation of boxes is suggested forthe even rarer Brazilian Merganser in order totest whether the availability of good-qualitynest sites is limiting breeding success (Silveira &Bartmann 2001).

Boxes may seem to be the answer to manyproblems of nesting-site shortage, but shouldnot be employed to the detriment of an interestin conserving natural sites (Aitken et al. 2002;Evans et al. 2002). Bellrose (1990) thought thatnestboxes made only a small contribution (4-5%) to the juvenile component of the WoodDuck’s autumn population, despite some100,000 boxes being available – more than forany other cavity-nesting duck. Extra boxes maynot ensure extra young. As Poysa & Poysa(2002) pointed out, following a 12-year study ofbox-nesting Common Goldeneyes, density-dependent features such as competition, preda-tion and food supply also limit reproductivesuccess. The tendency for females to dump eggscan increase in boxes which appear identical(Bolen 1967; Semel et al. 1988; Evans et al.2002). Semel & Sherman (2001) investigatedthe situation in a colour-marked, box-nestingpopulation of Wood Ducks over seven breedingseasons. They concluded that a scarcity of pre-ferred nesting sites was probably the key factorbehind four characteristics of cavity-nestingducks, including Wood Duck: natal philopatry,nest-site fidelity, aggressive competition for nestsites and high levels of intraspecific parasitism.Evans et al. (2002) found larger clutch sizes,lower nesting success and different majorpredators of Barrow’s Goldeneyes nesting inboxes compared with those laying in naturalcavities. They concluded that studies ofBarrow’s Goldeneye which use nestboxes maynot be representative of a population that uses

natural holes. On the other hand, Buffleheadsnesting in boxes were found to behave more likebirds nesting in natural sites. Ideally, a thoroughinvestigation of population ecology should beundertaken before any management measure,such as the provision of nestboxes, is put inplace. In the real world, such perfection isseldom achieved – time always seems far tooshort.

Woodland, especially moist and floodedwoodland, needs protection worldwide. Inorder to ensure that woods and forests containhealthy populations of primary excavators, suchas woodpeckers, dead and dying timber isneeded. Ironically, even the predators maysuffer if no old wood is available: Brainerd et al.(1995) thought that woodland managementreduced the number of natural den sites formartens in Scandinavian forests, and madethem dependent upon the Black Woodpeckerfor the provision of breeding cavities. InBritain, veteran trees and dead timber arerequired for the conservation of numerous rarelichens, fungi and spiders (McLean & Speight1993), and 20% of British insects are dependenton dead wood. Woods must be mature (over-mature from the forester’s point of view) andnot ‘tidy’ and disease-free. Old, rot-prone treestend to be removed by foresters during the thin-ning process, so that the cavity-bearing, dead-wood component is now largely missing frommanaged forest. ‘Limbs with decay should notautomatically be cut back to sound wood, cavi-ties should not be drained, filled and sealed, anddamage to bark should not automatically beregarded as in need of remedial treatment. Out-right felling of an ancient tree should beregarded as unacceptable’ (Key & Ball 1993).Generally, fungi and insects which rot deadwood do not attack healthy trees (Winter 1993),although there are exceptions.

Dead wood matters – it turns to gold. Almostall agents responsible for cavity constructionneed moribund timber in order to start theprocess. In a natural, unmanaged forest, 50% ofthe wood will be dead or dying, and the slowprocess of decomposition is necessary to returnnutrients to the soil. Attitudes must be changed;heart rot and rot holes in deteriorating wood-land should be welcomed and not deplored, notjust for the sake of hole-nesting ducks, but for ahuge range of other organisms whose livesdepend on wood that is rotten.

Acknowledgments

I am extremely grateful for the help of Tim Davis, and forthe advice of Drs Johnny Birks, Clem Fisher, Peter Fullagar,Mary Lanyon, Pat Morris, Jeff Port, David Snow and GlynYoung.

References

Aitken, K. E. H.,Weibe, K. L., & Martin, K. 2002. Nest-sitereuse patterns for a cavity-nesting bird community ininterior British Columbia. Auk 119: 391-402.

Audubon, J. J. 1835. Ornithological biography.Vol. 3. Adamsand Charles Black, Edinburgh.

Beard, E. B. 1953.The importance of beaver in waterfowlmanagement at the Seney National Wildlife Refuge.J.Wildl. Manage. 17: 398-436.

Bellrose, F. C. 1990. History of wood duck management inNorth America. In: Fredrickson, L. H., Burger, G.V.,Havera, S. P., Gruber, D. A., Kirby, R. E., & Taylor,T. S.(eds.), Proc. 1988 North Am.Wood Duck Symp. St Louis,Missouri.

Bellrose, F. C., & Holm, D. J. 1994. Ecology and managementof the Wood Duck. Stackpole, Mechanicsburg,Pennsylvania.

Bolen, E. G. 1967. Nesting boxes for Black-bellied TreeDucks. J.Wildl. Manage. 31: 794-797.

Bolen, E. G., & Cain, B.W. 1968. Mixed Wood Duck-TreeDuck clutch in Texas. Condor 70: 389-390.

Brainerd, S. M., Helldin, J-O., Lindstrom, E. R., Rolstad, E.,Rolstad, J., & Storch, I. 1995. Pine Marten (Martesmartes) selection of resting and denning sites inScandinavian managed forests. Ann. Zool. Fennici 32:151-157.

Brian, M.V. 1977. Ants. Collins, London.Brown, L. H., Urban, E. K., & Newman, K. 1982. The Birds of

Africa.Vol. 1. Academic Press, London.Bull, E. L., & Meslow, E. C. 1977. Habitat requirements of

the Pileated Woodpecker in northeastern Oregon.J. Forestry 75: 335-337.

Conner, R. N., Shackelford, C. E., Saenz, D., & Schaefer, R. R.2001. Interactions between nesting PileatedWoodpeckers and Wood Ducks. Wilson Bull. 113: 250-253.

Corbet, G. B., & Southern, H. N. 1977. The Handbook ofBritish Mammals. Blackwell, Oxford.

Cramp, S., & Simmons, K. E. L. (eds.) 1977. The Birds of theWestern Palearctic.Vol. 1. OUP, Oxford.

Davis, A. 1988.The distribution and status of the Mandarinin Britain. Bird Study 35: 203-208.

— & Baggott, G. K. 1989a. Clutch size and nesting sites ofthe Mandarin Duck Aix galericulata. Bird Study 36: 32-36.

— & — 1989b. Egg-laying, incubation and intraspecificnest-parasitism by the Mandarin Duck Aix galericulata.Bird Study 36: 115-122.

Dennis, R. H., & Dow, H. 1984.The establishment of apopulation of Goldeneyes Bucephala clangula breedingin Scotland. Bird Study 31: 217-222.

Eberhard, J. R. 2002. Cavity adoption and the evolution ofcoloniality in cavity-nesting birds. Condor 104: 240-247.

Evans, M. R., Lank, D. B., Boyd,W. S., & Cooke, F. 2002. Acomparison of the characteristics and fate of Barrow’sGoldeneye and Bufflehead nests in nest boxes andnatural cavities. Condor 104: 610-619.

Fredrickson, L. H., Burger, G.V., Havera, S. P., Gruber, D. A.,Kirby, R. E., & Taylor,T. S. (eds.). 1990. Proc. 1988 NorthAm.Wood Duck Symp. St Louis, Missouri.

Frith, H. J. 1982. Waterfowl in Australia. Angus & Robertson,Sydney.

Geffen, E., & Yom-Tov,Y. 2001. Factors affecting the rates of

intraspecific nest parasitism among Anseriformes andGalliformes. Anim. Behav. 62: 1027-1038.

Gibbons, D.W., Reid, J. B., & Chapman, R. A. 1993. The NewAtlas of Breeding Birds in Britain and Ireland: 1988-1991.Poyser, London.

Gibson, E. 1920. Further ornithological notes from theneighbourhood of Cape San Antonio, Buenos Aires. Ibis11: 1-97.

Gorman, G. 1998.The spread of Black Woodpecker inEurope – will it reach Britain next? Birding World 10:390-395.

Green, A. J. 1992. The status and conservation of the White-winged Wood Duck Cairina scutulata. IWRB SpecialPublication No. 17, Slimbridge.

Haramis, G. M. 1990. Breeding ecology of the WoodDuck: a review. In: Fredrickson, L. H., Burger, G.V.,Havera, S. P., Gruber, D. A., Kirby, R. E., & Taylor,T. S.(eds.). Proc. 1988 North Am.Wood Duck Symp. St Louis,Missouri.

Hepp, G. R., & Bellrose, F. C. 1995.Wood Duck (Aixsponsa). In: Poole, A., & Gill, F. (eds.), The birds of NorthAmerica. No. 169. AOU,Washington, and Academy ofNatural Sciences, Philadelphia.

Holldobler, B., & Wilson, E. O. 1990. The Ants. Springer-Verlag, Munich.

Hudson,W. H. 1920. Birds of La Plata. Vol. 2. Dutton, NewYork.

Hutchinson, C. D. 1989. Birds in Ireland. Poyser, Calton.Jackson, J. A. 1996. Ivory-billed Woodpecker. In: Rodgers, J.

A., Kale, H.W., & Smith, H.T. (eds.), Rare and endangeredbiota of Florida. University Press of Florida, Gainsville.

Kear, J. 1967. Experiments with young nidifugous birds ona visual cliff. Wildfowl 18: 122-124.

Kear, J. 1990. Man and Wildfowl. Poyser, London.— (ed.). In press. Ducks, geese and swans (Anseriformes).

Bird Families of the World. OUP, Oxford.Kerpez,T. A., & Smith, N. S. 1990. Competition between

European Starlings and native woodpeckers for nestcavities in Saguatos. Auk 107: 367-375.

Key, R. S., & Ball, S. J. 1993. Positive management forsaproxylic invertebrates. In: Dead Wood Matters. EnglishNature Science 7, Peterborough.

Lack, D. 1947. Darwin’s Finches. CUP, Cambridge.— 1969.The numbers of bird species on islands. Bird

Study 16: 193-209.Ludwichowski, I., Barker, R., & Bräger, S. 2002. Nesting area

fidelity and survival of female Common GoldeneyesBucephala clangula: are they density-dependent? Ibis144: 452-460.

MacArthur, R. H., & Wilson, E. O. 1967. The theory of islandbiography. Princeton University Press, Princeton.

Maclean, G. L. 1993. Roberts’ Birds of Southern Africa. (6thedn). John Voelcker Book Fund, Cape Town.

Madge, S., & Burn, H. 1988. Wildfowl: an identification guideto ducks, geese and swans of the world. ChristopherHelm, London.

Mallory, M., & Metz, K. 1999. Common Merganser (Mergusmerganser). In: Poole, A., & Gill, F. (eds.), The Birds ofNorth America. No. 442.The Birds of North AmericaInc., Philadelphia.

McLean, I. F. G., & Speight M. C. D. 1993. Saproxylicinvertebrates – the European context. In: Dead WoodMatters. English Nature Science 7, Peterborough.

Mikusinski, G., Gromadzki, M., & Chylarecki, P. 2001.Woodpeckers as indicators of forest bird diversity.Conservation Biology 15: 208-217.

Milstein, P. le S. 1993. A study of the Egyptian GooseAlopochen aegyptiacus. Unpubl. PhD thesis, Univ.Pretoria.

Murton, R. K., & Kear, J. 1978. Photoperiodism in

waterfowl: phasing of breeding cycles andzoogeography. J. Zool., Lond. 186: 243-283.

Nores, M., & Yzurieta, D. 1980. Aves de ambientes acuáticosde Córdoba y Centro de Argentina. Secr. de Estado deAgricult. y Ganad, Córdoba.

Ogilvie, M. A., & the Rare Breeding Birds Panel. 2001.Non-native birds breeding in the United Kingdom in1999. Brit. Birds 94: 344-381.

Port, J. 1998a. Reproductive strategies of an arborealdabbling duck: the Speckled Teal Anas flavirostris ineastern Argentina. Unpubl. PhD thesis, Univ. Minnesota,Minneapolis.

Port, J. 1998b. Long-term pair bonds and male parentalcare in Speckled Teal Anas flavirostris in easternArgentina. Wildfowl 49: 139-149.

Poysa, H., & Poysa, S. 2002. Nest-site limitation and densitydependence of reproductive output in the CommonGoldeneye Bucephala clangula: implications for themanagement of cavity-nesting birds. J. Appl. Ecol. 39:502-510.

— & — 2001. Intraspecific parasitism and nest-sitecompetition in Wood Ducks. Anim. Behav. 61: 787-803.

Semel, B., Sherman, P.W., & Byers, S. M. 1988. Effects ofbrood parasitism and nest-box placement on WoodDuck breeding ecology. Condor 90: 920-930.

Sharrock, J.T. R. 1976. The Atlas of Breeding Birds in Britainand Ireland. Poyser, Berkhamsted.

Shurtleff, L. L., & Savage, C. 1996. The Wood Duck and theMandarin:The northern wood ducks. Univ. California

Press, Berkeley.Silveira, L. F., & Bartmann,W. 2001. Natural history and

conservation of Brazilian Merganser Mergusoctosetaceus at Serra da Canastra National Park, MinasGerais, Brazil. Bird Conserv. Int. 11: 287-300.

Simpson, K., & Wilson, Z. 2001. Birds that nest in treehollows. Bird Observer 811: 15-18.

Soulliere, G. J. 1990. Review of Wood Duck nest-cavitycharacteristics. In: Fredrickson, L. H., Burger, G.V.,Havera, S. P., Gruber, D. A., Kirby, R. E., & Taylor,T. S.(eds.). Proc. 1988 North Am.Wood Duck Symp. St Louis,Missouri.

Weller, M.W. 1967. Notes on some marsh birds of CapeSan Antonio, Argentina. Ibis 109: 391-441.

Winkler, H., Christie, D. A., & Nurney, D. 1995.Woodpeckers. Pica Press, Robertsbridge.

Winter,T. 1993. Deadwood – is it a threat to commercialforestry? In: Dead Wood Matters. English Nature Science7, Peterborough.

Young, H. G. 2002. Predicting the ecology of Madagascar’sendemic dabbling ducks using captive populations andrelated taxa: implications for conservation. Unpubl. PhDthesis, Univ. Canterbury, Kent.

Young, H. G., Lewis, R. E., & Razafindrajao, F. 2001. Adescription of the nest and eggs of the Madagascar TealAnas bernieri. Bull. BOC 121: 64-67.

Zicus, M. C. 1990. Nesting biology of Hooded Mergansersusing nest boxes. J.Wildl. Manage. 54: 637-643.

Prof. Janet KearJewells Lodge, Umberleigh, Devon EX37 9EY

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