Study of Cuckoos and Reed Warblers Ian Wyllie Plates 48-51 INTRODUCTION

In the spring of 1973 wildlife photographer Maurice Tibbies began to make a film for television on the Cuckoo Cuculus canorus under a two-year contract for the BBC and Time-Life Films. At a site close to my home in Cambridgeshire a young Cuckoo, about two weeks old, was found in the nest of a Reed Warbler Acrocephalus scirpaceus; this was filmed by Tibbies and we subsequently found a further twelve young of different ages and three Cuckoo eggs, all in Reed Warbler nests. We had discovered a seemingly excellent site but it was by then late June and the end of the Cuckoo's laying period. After visiting various Cuckoo habitats all over Great Britain, Tibbies decided that this site in Cambridgeshire offered the best opportuni­ties for filming one of the most cryptic aspects of the Cuckoo's behaviour—the egg-laying. This paper recounts some of the observa­tions made on Cuckoos when Tibbies returned to the site in 1974 to complete the film.

Until 1922, when E. P. Chance published his book The Cuckoo's Secret, there were several theories concerning the act of egg-laying. The most generally accepted of these, based on sightings of Cuckoos carrying eggs in their beaks, was that the egg was laid on the ground, picked up in the beak and thus placed in the host species' nest. Chance (1922, 1940) disproved this and showed that the Cuckoo layed directly into the nest. As well as collecting scientific data, which have since been analysed by Lack (1963) and Seel (I973)> Chance made numerous assertions that we considered sufficiently reliable to help us in our attempts to film the egg-laying. In essence, these were as follows: only the male Cuckoo calls 'cuckoo' and only the female 'bubbles'; the female maintains a territory and deters other females wishing to use that area; each female parasitises one main host species; she locates the nests and lays her eggs unaided by the male; she may visit the nest to deter­mine its precise location prior to egg-laying; when a nest is selected for egg-laying the female becomes determined to use that nest; if a selected nest is destroyed or a suitable nest cannot be found she may lay almost anywhere, even on the ground or in the nest of an unsuitable host species; the egg can be retained in the oviduct for several hours (but not indefinitely) until conditions for laying are suitable; each female lays eggs which differ in size, colour and markings from those of other Cuckoos, i.e. each female can be identified by her eggs; the eggs are laid at intervals of not less than

IBril, Birds,6&: 369-378, September 1975] 3%

37° Cuckoos and Reed Warblers

two days, i.e. eggs are not laid on consecutive days; eggs tend to be laid in 'clutches' of about five, with a longer interval (than two days) between each clutch; the egg is usually laid after midday following a varying length of time spent by the female watching the host's nest from a suitable vantage point; each egg is usually laid when the host species is forming a clutch, but preferably when the host has two eggs; the female 'glides' to the nest in hawk-like fashion when about to lay; she lays directly into the host species' nest, usually removing one egg; the time taken to lay an egg is extremely brief— about nine seconds; cuckoos take no further interest in the host's nest after egg-laying; where host species' nests are plentiful a female may lay up to 25 eggs in one season.

Further investigations are required to verify many of these claims but, for the purpose of our studies, they were accepted as true. It should be noted that Chance's data were collected on Cuckoos parasitising Meadow Pipits Anthuspratensis and that we were attempt­ing to apply them to Reed Warbler-Cuckoos.

To film a Cuckoo laying an egg it was necessary to predict where and when a particular female was going to lay. We knew from our experience in 1973 that Reed Warblers were the main host species, so a constant watch had to be kept on all Reed Warbler nests that could be found in the area. When the first Cuckoo egg appeared we could, according to Chance, expect that subsequent eggs would be laid in the afternoons on alternate days in nests with one, two or three eggs. Also, we could expect to see the bird watching or visiting a chosen nest before an egg was actually laid.

The study site in Cambridgeshire consists of a number of small Phragmites reed-beds, some of which are surrounded by bushes and mature trees. A plan of the site (fig. 1) shows the area used by Cuckoos with the positions and distances between the relevant habitat features. The site was visited on 85 consecutive days from 28th April until 21st July, during which over 300 hours' field-work were logged. Visits were usually made in the morning, in the evening and all day at week-ends. Notes were kept on the Cuckoos' where­abouts and activities, on the contents and stages of development of Reed Warbler nests, and on the eggs and young of Cuckoos.

FIELD OBSERVATIONS A minimum of eight Cuckoos were in the area at some time during the 1974 season. At least two were males which were frequently heard calling against one another. Evidence for six females was established from the variation in egg-colour by which we also labelled them. These included Cuckoo P laying pink eggs similar to those Of a Robin Erithacus rubecula; Cuckoo B laying brown eggs like those of a Skylark Alauda arvensis; Cuckoo s laying a spotted egg like

Cuckoos and Reed Warblers 37i

Fig. 1. Sketch plan of Cambridgeshire site in 1974 showing distribution and order of laying of eggs from six Cuckoos Cuculus canorus

that of a House Sparrow Passer domesticus; Cuckoo BL laying eggs with a bluish tinge; Cuckoo G laying grey eggs similar to those of a Pied Wagtail Motacilla alba; and Cuckoo L laying a very large grey egg.

Arrival The site was first visited on 28th April when a single male Cuckoo (identified by call) was in evidence, together with at least one singing male Reed Warbler. The first female (identified by the bubbling call) arrived one week later and was heard on 4th May at 07.00 hours (the times given throughout are BST). The second male arrived on 6th May and immediately began to sing against the first, which resulted in much aggression and displaying between the two. On 8th May both males were seen displaying to a female and thereafter Cuckoos were frequendy seen in twos and threes and, on one occasion, four were seen together. A calendar of Cuckoo activities in 1974 is shown in fig. 2.

The first pair of Reed Warblers was seen on 12th May and the first signs of nest-building were noticed on 18th May. The two male Cuckoos were heard and seen daily until and including 23rd May, but there was no further sign of an obvious female until 26th May, by which time two Cuckoo eggs had been laid (see table 1). No Cuckoos were heard or seen on 24th and 25th May although Cuckoo p had almost certainly laid her second egg of the season in the afternoon of 24th May. All the Cuckoos, therefore, had become very secretive at the beginning of the laying period.

372 Cuckoos and Reed Warblers

Egg-laying A female Cuckoo 'bubbled' in the morning of 26th May, and at 13.15 hours she took up position in a hedge about 20 metres from a Reed Warbler nest containing three eggs in Southtrack reed-bed (see fig. 1). At 14.15 hours the Cuckoo dropped with a crash into the dead reeds about two metres from the nest. She emerged a few seconds later to return to her previous position and within five minutes, accompanied by a male Cuckoo, flew to South reed-bed. To our surprise Cuckoo P had not laid and there was no further sign of Cuckoos that day. The following morning Cuckoo p's diird egg was found in a nest with two of the host's eggs in South reed-bed.

The fourth and fifth eggs of this particular bird were laid in nests containing no eggs and they, too, were not discovered until the morning after they were laid. By the end of May, Cuckoo P had succeeded in laying five eggs, probably in the afternoons on alternate days, without our having witnessed one of the layings.

From our brief notes made in 1973 it was apparent that three of these five eggs had been laid in nests located almost exactly where young Cuckoos had been found the year before. We decided, there­fore, to pay rather more attention to sites where Reed Warblers had been cuckolded in 1973.

One such site (West reed-bed in fig. 1) consisted of a small patch of reeds surrounded by bushes, where both male and female Cuckoos were often seen. By 2nd June the only pair of Reed Warblers using this site had completed their nest, so a completely camouflaged hide was constructed under the bushes about two metres away. On 3rd June the warbler laid her first egg and at 15.00 hours, having given no warning, Cuckoo p scrambled through the reeds, removed the Reed Warbler's single egg and laid her own direcdy into the host's nest. The whole operation took only 11 seconds and can be seen in the film The Private Life of the Cuckoo.

Five other Reed Warbler nests at a suitable stage for egg-laying by Cuckoos were also available on 3rd June, including two more

At feast one male present At least two males present At least one female present Egg-laying by Cuckoo P Egg-iaying by Cuckoo B Egg-laying by Cuckoo BL Egg-laying by Cuckoo S Egg-laying by Cuckoo G Cuckoo eggs in nests Cuckoo young in nests Fledged young Cuckoos present

3 0 ^ 31 30 31 31 Apr May June July Aug

Fig. 2. Calendar of Cuckoo Cuculus canorus activities from observations at Cam­bridgeshire site, 1974

■ i : . ^ ^

| » , ■ - "T". . . : ,n

j,4::;~t; , .«1| 1

E D 1 .. " •> 11

|.*'.i,S«8KK«l

a 1 . .1

1 -„ „ - 1

Cuckoos and Reed Warblers 373 Table I. Dates -when eggs of Cuckoos Cuculus canorus were laid, Cambridge­

shire, 1974 Asterisks refer to Reed Warbler Acrocephalus scirpaceus pairs which were cuckolded

twice. The data in parentheses are assumed or estimated Reed Warbler No. host's

Date Time (BST) Cuckoo pair number eggs Site

(22-5) 24-5 26.5 28.5 30.5 (1.6) 3-6 3-6

(6.6) 7.6 7.6

(11.6) 12.6 16.6 17.6 17.6 19.6 23.6

(27.6) 29.6

i-7 (2-7) (3-7) (6.7) (6.7) 11.7

— after 11.30 after 14.20

— — —

15.00 after 12.45 before 19.00 after 14.30

I7-I5 —

before 19.00 11.30-20.30

19.20 before 20.00 after 18.00

16.15 —

18.15 after 18.00

— before 14.20

— —

before 20.40

P P P P P B P B BL B S B G G B BL B B B B B BL B BL L B

35 4

25 14 12*

Sedge Warbler 7*

36* 33

2 11 12* 42 H 5

24 10 53 36*

7* 8

38 9

45 54 6

(2) 2 2 0 0

(2) 1 1

(2) 3 4

(2) 2 3 2 1 2 2

— 2 1

— (0 — 2

East Southtrack South Withy Bridge River West Central Typha Southtrack Central Bridge Northeast Withy Southtrack South Central North track Central West Northtrack North Southtrack North River Southtrack

nests containing single eggs. One of these was used by Cuckoo B for her second egg of the season, her first having been laid in the nest of a Sedge Warbler Acrocephalus schoenobaenus about 500 metres from the main reed-beds around the track (see fig. 1). This was the only Cuckoo egg found in anything but a Reed Warbler nest and suggests that Cuckoo p, having established a laying pattern in the main reed-beds, had prevented Cuckoo B from using 'her' nests. Cuckoo B had moved into the main reed-beds at the same time that Cuckoo P laid her last known egg. (On the morning of 26th May three Cuckoos were sighted at Central reed-bed, two apparently chasing the other to River reed-bed. Although only one bird 'cuckoo'd', it was thought that this had been Cuckoo P and her mate chasing the other male known to be in the area. However, it could just as easily have been Cuckoo p driving Cuckoo B from the main reed-beds with a male Cuckoo attempting to court both females.)

374 Cuckoos and Reed Warblers

A regular pattern of egg-layings on alternate days was not recorded after Cuckoo p's fifth egg. Nevertheless, the known dates of subse­quent layings tended to be at multiples of two-day intervals. For example, Cuckoo B laid on 3rd, 7th, 17th, 19th, 23rd, and 29th June, and 1st and n t h July. As with Chance's observations, undetected eggs could have been laid. (At least one egg was un-located, the fledged youngster being seen on 18th August.) Cuckoo B was filmed in the act of egg-laying on 23rd and 29th June (see plate 50b).

Nest-robbing An alarming number of Reed Warbler nests were predated through­out the season from 28th May onwards. Although the nests were easier to find than usual owing to the poor growth of reeds caused by the lack of spring rain, the only natural predator thought to steal eggs was a pair of Jays Garrulns glandarius. Human disturbance by bird-watchers, ramblers, fishermen and even egg-collectors was responsible for the loss of some nests and eggs.

Predation of Reed Warbler nests by Cuckoos (apart from warbler eggs taken during egg-laying) was recorded on nine occasions and circumstantial evidence suggested that Cuckoos frequendy took and ate eggs from nests on other occasions (see table 2).

On 7th June, at 17.05 hours, a Cuckoo was photographed taking the only egg from a Reed Warbler's nest in South reed-bed (plates 49, 50). Ten minutes later Cuckoo s laid in a nest with four eggs that had been incubated for four days in Central reed-bed. Although Cuckoo s was possibly a young, inexperienced bird, the observations suggest that the dominant female Cuckoo (at that time Cuckoo B) attempted to prevent her from laying by robbing the egg from the only suitable nest in the territory. No further eggs of Cuckoo s were found, so she may have been driven out of the area or forced to use some other host species.

Table 2. Host-nest predation by Cuckoos Cuculus canons, Cambridgeshire, 1974

Contents of Date

7.6 9.6

22.6 22.6

29.6 29.6 30.6

5-7 8.7

Time (BST)

17-05 12.30 07.00 11.30

13.00 18.00 11.30 15-15 16.30

Nest number

25a 47 80 32

9° 75 75 86

103

nest

1 egg 2 eggs 3 eggs 1 egg (infertile) 2 young 2 eggs 4 eggs 2 eggs 4 eggs 3 eggs

Predatioj

" egg 2 eggs 3 eggs 2 young

2 eggs 2 eggs 2 eggs 4 eggs 3 eggs

Cuckoos and Reed Warblers 375 At 12.30 hours on 9th June (an assumed laying-day) the contents

of a nest with two eggs in Bridge reed-bed were robbed, accom­panied by a series of excited bubbling calls. Several observers were standing only about 50 metres from the nest and a hide had been placed about four metres away in anticipation of laying. The repeat-nest of this pair of warblers was also robbed on 17th June when three eggs were taken. Furthermore, a female Cuckoo was witnessed robbing a two-egg nest at this spot at 17.16 hours on 27th June 1973. It seemed that a female Cuckoo was repeatedly victimising the Reed Warblers using this particular site.

Another nest-robbing incident is of special note as it involved the predation of nestlings, an activity which has not been fully recorded previously (see Chance 1940, p 167). Cuckoo p's sixth egg, laid on 3rd June in West reed-bed, mysteriously disappeared by n t h June (it was possibly taken by egg-collectors). Two of the remaining three Reed Warbler eggs hatched on 18th June, the third being infertile. At n . i o hours on 22nd June two Cuckoos, presumably females, were heard bubbling excitedly from the vicinity of the nest. At 11.40 hours, after eight bubbling calls, a Cuckoo was seen eating a young Reed Warbler. The four-day old nestling was held by the abdomen, shaken in the same manner as a caterpillar, and finally swallowed whole. An inspection of the nest and its surroundings revealed no sign of the other young warbler but the infertile egg was still present and intact. Cuckoo B was filmed laying in the repeat-nest of this pair of warblers one week later.

Summary counts By the end of the 1974 season, 26 eggs from six female Cuckoos had been found and 27 were known to have been laid. One bird had laid a minimum of 12 eggs, another six, a third four, and three others one or two eggs each (see table 1). The laying period ex­tended from 22nd May until the uncommonly late date of n t h July. Male Cuckoos were heard from 28th April until 10th July and females from 4th May to 14th July. (In 1973 the last female was seen on 8th July.) Only two young fledged, the last being still dependent upon the host species on 18th August.

There were 54 pairs of Reed Warblers and 136 of their nests were found. Of these, 28 were deserted; 25 were cuckolded; 80 were robbed; n produced fledged young (one, a Cuckoo); and 19 were not followed up. Five Sedge Warbler nests were also found, one of which was cuckolded, the others producing fledged young. An unlocated Reed Warbler nest produced a fledged young Cuckoo.

DISCUSSION Most of Chance's assertions on egg-laying by Cuckoos (pages 369-370)

376 Cuckoos and Reed Warblers

were supported by our 1974 observations. In some instances, how­ever, our studies produced no evidence to support or refute his claims. For example, the maximum number of eggs recorded from a single Cuckoo in our study area in 1974 was twelve, whereas Chance recorded up to 25 eggs laid by one bird (though under experimental conditions). Also, while we cannot disprove Chance's conclusion that only the male calls 'cuckoo', we believe that the female may also do so on occasions. In the case of four of his points, our evidence pointed to contrary conclusions, all of which, however, may be related to the different host species (and very different habitats) which were utilised in the different study areas. In par­ticular, in our study area the Cuckoos did not have to cross large open areas and may have had less difficulty in locating the nests of their hosts. Unlike Chance's findings, mostly on Meadow Pipit-Cuckoos, our observations of Reed Warbler-Cuckoos were that (i) the male sometimes accompanied the female when egg-laying; (ii) the female did not 'glide' to the nest but usually approached through the reeds; (iii) the female did not visit the nest immediately before laying to determine its precise location; (iv) cuckolded nests were sometimes revisited and repeat-nests of cuckolded hosts were occasionally cuckolded again.

If a successful breeding season at the site in 1973 was reflected over the country as a whole, it may not be surprising that there were as many as six female and at least two male Cuckoos in an area of about 150 hectares in 1974. A full description of the eggs found in 1973 was not recorded but it was thought that they most resembled the 1974 eggs of Cuckoo p. Since there was a tendency for her 1974 eggs to be laid in almost identical sites to those in 1973, this bird may have been using the territory in both years. Alternatively, but less likely, if the eggs of the female offspring resemble those of the parent, Cuckoo P may have been a one-year old offspring of the 1973 female. A male Cuckoo in 1973 °ften called from exposed telegraph poles, but this behaviour was not noted in 1974 and it seemed reasonable to believe that the dominant male of the previous year had not returned.

Egg-laying by Cuckoos began on virtually the earliest possible date (22nd May) for Reed Warbler nests and continued as late as n t h July, after the males had apparently left the territory. Lack (1963), using data recorded by egg-collectors G. Charteris and A. E. Lees, indicated that Reed Warbler-Cuckoos did not apparently lay after 8th July, despite the availability of nests. He suggested that cessation of laying may be primarily determined by the available food supply, although more research was necessary for this hypothesis to be established. At the Cambridgeshire site in 1974 there was an abundant supply of Peacock Nymphalis io butter-

Cuckoos and Reed Warblers 377 fly larvae until the end of July, and the Cuckoos were seen feeding on these from mid-June to 14th July. From records kept at Monks Wood Nature Reserve, the peak numbers of adult Peacocks occurred about one week later in 1974 compared with 1973. The Cuckoos' departure from the site was also about one week later in 1974 and the two facts may be correlated.

In 1973 independent young Cuckoos were seen at the site from the end of June onwards. In 1974, after much predation, the first independent juvenile was not seen until 12th July, one day after the laying of the last egg. Clearly, more research is required, but it seems possible that the cessation of laying may be linked with the appearance of fledged young Cuckoos within the breeding territory.

Cuckoos were known to rob nests on nine occasions extending from 7th June until 8th July. Of the eight nests involved, 15 fresh eggs, four deserted fresh eggs, and two four-day old nestlings were taken. Circumstantial evidence, such as bubbling calls heard or birds seen near the nests, suggested that 15% of the remaining 72 robbed nests were predated by Cuckoos.

There are three main reasons why adult Cuckoos may take eggs and young from the nests of their hosts: ii) for additional food; (ii) to force the hosts to re-nest, thus bringing their breeding cycle in line with that of the Cuckoo; (iii) to prevent other female Cuckoos laying eggs in suitable nests within the same territory. The predation of nesdings supports the second of these reasons. Cuckoo B probably took the young so that the warblers would build a repeat-nest suitable for her own egg which was laid on 29th June. The fact that two female Cuckoos were heard at the nest-site suggests that both had designs on this particular pair of Reed Warblers, especially as we know that a young Cuckoo was reared here in 1973. Again, further research is needed to elucidate the reasons for nest-predation by Cuckoos.

Future research One of the greatest disadvantages in the field study of Cuckoos is that the observer cannot positively distinguish between one bird and another. Their secretive habits enable the birds to evade observation for long periods and the observer can rarely be certain of the sex of a particular individual. Although there is much evidence to suggest that only the male calls 'cuckoo' and that only the female 'bubbles', it is by no means conclusive.

By marking Cuckoos from our breeding site over a period of years and in such a way that an individual could be recognised in the field, it is hoped to investigate the following: (i) the calls used by the sexes; (ii) the incidence of polygamy, polyandry or promiscuity; (iii) the size and maintenance of territories for each sex; (iv) egg-

378 Cuckoos and Reed Warblers

laying by the female unaccompanied by the male, at similar sites each season; (v) the sex involved in egg-robbing and the reasons behind such behaviour; (vi) adult Cuckoos' interest in their own offspring; (vii) possible return by first-year birds to breed where they were born and to parasitise the host species by which they were reared; (viii) the longevity and breeding capacity of individual Cuckoos; (ix) the duration of time spent at the breeding site, especially in relation to food supply and egg-laying; (x) inheritance of egg-colour; (xi) survival rates of young Cuckoos; (xii) the length of time that the young remain dependent upon the host species and the food taken by the young after reaching independence.

ACKNOWLEDGEMENTS The work would almost certainly not have been undertaken but for the enthusiasm of M, R. Tibbies and I am grateful to him for keeping notes on his observations at crucial times when I was unable to visit the site. I am also indebted to J . L. F. Parslow for his constructive criticism of an earlier draft of this paper.

SUMMARY The paper reports observations on breeding Cuckoos Cuculus canorus in 1974 during the making of a wildlife film on the species. The study site of about 150 hectares in Cambridgeshire consists of a number of small reed-beds where 22 of 54 Reed Warbler Acrocephalus scirpaceus pairs proved to be the main hosts. At least two male and six female Cuckoos were found in the area between 28th April and 14th July, and 27 eggs were laid between 22nd May and n t h July. The maximum number of eggs laid by one female was twelve, the distance between the sites of laying of first and last eggs being a little under 1 km. The eggs tended to be laid at two-day intervals in the afternoons in nests with one, two or three eggs but occasionally in nests with no eggs and once in a nest containing four incubated eggs. Only one Cuckoo egg was laid in the nest of a different host species—a Sedge Warbler A. schoenobaenus; this was possibly a forced departure from the preferred host, resulting from territorial interference by another female Cuckoo.

Of 136 Reed Warbler nests found, 25 were cuckolded and at least 25% were predated by Cuckoos, including one nest containing young Reed Warblers. I t was thought that the main reason for host-nest predation was to provide a con­tinuity of suitable nests for the Cuckoos to use, although repeat-nests of warblers known to have been previously predated by Cuckoos were not always cuckolded.

Cuckoos were filmed at the nest on three occasions, all of which can be seen in the film The Private Life of the Cuckoo. Several still photographs were also obtained of Cuckoos at Reed Warbler nests. The making of the film has stimulated further research at the site and it is hoped that more information can be obtained by studying experimentally marked Cuckoos over a number of years.

REFERENCES CHANCE, E. P. 1922. The Cuckoo's Secret. London.

1940. The Truth about the Cuckoo. London. LACK, D. 1963. 'Cuckoo hosts in England'. Bird Study, 10: 185-202. SEEL, D. C. 1973. 'Egg-laying by the Cuckoo'. Brit. Birds, 66: 528-535.

Ian Wyllie, The Institute of Terrestrial Ecology, Monks Wood Experi­mental Station, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS

P L A T E 48. Above, completed Reed Warbler Acrocephalus scirpaceus clutch con­taining in foreground slightly larger but similarly marked egg of Cuckoo Cuculas canorus; below, using powerful leg muscles and hollow back, the young Cuckoo ejects hosts' eggs eight to 36 hours after hatching (photos: Ian Wyllie)

P L A T E 49. Above, adult Cuckoo Cuculus canorus approaching nest of Reed Warbler Acrocephalus scirpaceus through reeds; below, Cuckoo causes no damage to hosts' nest as she clings to side in order to pick out an egg (photos: Jan Wyllie)

P L A T E 50. Above, a Cuckoo Cuculus canorus occasionally destroys the nest of a Reed Warbler Acrocephalus scirpaceus by devouring the contents, causing hosts to rebuild; below, when egg laying, a female Cuckoo holds egg of host in her bill before quickly moving across open nest to drop own egg {photos: Ian Wyllie)

P L A T E 51- At 16 days old a young Cuckoo Cuculus canorus in nest of Reed Warbler Acrocephalus scirpaceus already dwarfs its foster parent {photo: Ian Wyllie)