latitudinal variation in breeding parameters of the common kestrel falco tinnunculus · 2018. 6....

LATITUDINAL VARIATION IN BREEDING PARAMETERS OF THE COMMON KESTREL FALCO TINNUNCULUS

VARIACIONES LATITUDINALES EN LOS PARÁMETROS REPRODUCTIVOS DEL CERNÍCALO VULGAR FALCO TINNUNCULUS

José CARRILLO* 1 and Enrique GONZÁLEZ-DÁVILA**

SUMMARY.—Latitudinal variation in breeding parameters of the common kestrel Falco tinnunculus.We analyzed geographic variation in laying date, clutch size, and number of fledglings in the com-

mon kestrel Falco tinnunculus in the Western Palearctic. We also examined whether breeding parame-ters of some island-dwelling kestrels differed in latitude, with special reference to Tenerife Island. Gen-eral Linear Models were applied. The mean laying date (LD, mean 30 April) correlated positively withlatitude, with a delay of 6 days for every 10 ºN. The mean clutch size (CS, mean 4.95 ± 0.26) increasedsignificantly with latitude and was affected by nest-type (nest-box vs natural nests). The mean number offledglings showed no correlation. Tenerife populations nesting below 1,000 m.a.s.l. showed the earliestLD in the Western Palearctic while those nesting above 1,000 m.a.s.l. showed a similar nesting pattern toEuropean populations, although the CS is lower. The latitudinal variation in LD and CS coincides with theclassical postulates (delay and increase respectively, towards the North) but does not explain the varia-tions presented here. In agreement with recent studies, nest-type (nest-box vs natural nests) and climaticfactors such as temperature (winter, spring) and spring rainfall influence latitudinal variations. Datafrom Tenerife are consistent with the hypotheses of delayed breeding associated with altitude and reducedCS in island birds.

Keywords: clutch size, common kestrel, Falco tinnunculus, fledglings, islands, latitude, laying date,longitude, Palearctic, Tenerife.

RESUMEN.—Variaciones latitudinales en los parámetros reproductivos del cernícalo vulgar Falco tin-nunculus.

Analizamos las variaciones geográficas en la fecha de puesta, tamaño de puesta y número de pollosque vuelan de cernícalo vulgar en el Paleártico occidental. También examinamos si los parámetros re-productivos de las poblaciones insulares difieren según la latitud, en especial en la isla de Tenerife. Se apli-caron Modelos Lineales Generalizados. La fecha media de puesta (LD, 30 de abril) se correlacionó posi-tivamente con la latitud con un retraso de 6 días por cada 10º N. El tamaño medio de puesta (CS, media4,95 ± 0,26) se incrementa con la latitud significativamente, estando afectado por el tipo de nido (cajas-nido

* Departamento de Biología Animal (Zoología). Universidad de La Laguna, E-38206 La Laguna,Tenerife, Spain. Present address: Departamento de Ecología, Universidad de La Laguna, E-38206 La Laguna,Tenerife, Spain.

** Departamento de Estadística. Investigación Operativa y Computación, Universidad de LaLaguna, E-38206 La Laguna, Tenerife, Spain.

1 Corresponding author: [email protected]

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INTRODUCTION

Reproduction in birds is influenced by ge-ographic variations and environmental condi-tions associated with different degrees of lat-itude and longitude, but also with altitudeand insularity (Sanz, 1998; Newton, 2003;Cooper et al., 2005a).

The timing of breeding seasons may varywith proximate as well as ultimate factors(Sanz, 1998). Seasonally reproducing birdshave evolved responses to environmental stim-uli (e.g. food, climate, photoperiod) for raisingoffspring during a brief and favourablebreeding time (Greives et al., 2008). Food avail-ability, considered as a proximate or ultimatefactor, is one of the main determinants of re-productive strategies in birds (Martin, 1987)and may regulate the onset of egg laying (Apari-cio, 1998). However, factors such as climate(Meijer et al., 1988; Carrillo and González-Dávila, in press), habitat (Sanz, 1995; Fargal-lo, 2004), photoperiod (Greives et al., 2008;but see Visser et al., 2009), and risk of preda-tion (Lima, 1987; Fargallo et al., 2001) mayinfluence the onset of egg laying to a greateror lesser extent depending on geographic lo-cation (Sanz, 1998).

Geographic variation in clutch size is a wellknown phenomenon in birds, with the most fre-quent trend being an increase in clutch sizewith increasing latitude and less markedly withlongitude (Lack, 1947; Hendricks, 1997; butsee Ricklefs, 2000). The general explanation

for latitudinal trends in clutch size is that thedays are longer in summer towards the north(Lack, 1947; Perrins and Birkhead, 1983).Clutch size fluctuates more at northern lati-tudes than in southern ones because many birdslay more eggs in years of abundant food andfew or none in years of scarcity (Newton, 1979).Several environmental conditions such as habi-tat (Sanz, 1998; Valkama and Korpimäki,1999), climate (Sanz, 1995; Carrillo andGonzález-Dávila, in press), nest-type (Coop-er et al., 2005b), or risk of predation (Eggerset al., 2006) could influence number of broodsor clutch size.

Geographic tendencies in breeding param-eters of birds have given rise to a large numberof hypotheses; the main ones being: (i) foodresource limitation (Lack, 1947); (ii) Ashmole´shypothesis (Winter food scarcity in temper-ate regions causes a decline in bird populationsto levels below the carrying capacity of the sys-tem; surviving pairs have enough food in springto produce greater clutches and raise more off-spring; Ashmole, 1961); (iii) nest predation(Cody, 1966), which have been subsequentlyimproved or modified (see reviews in Sanz,1998; Fargallo, 2004).

Islands are ecosystems whose dynamics dif-fer from those of continents (Nunn, 1994). Thismay induce variations in breeding ecology of is-land birds compared with those of continentalpopulations (Wiggins et al., 1998). The follow-ing factors have been postulated as determinantsin the breeding ecology of island birds: habitat

vs nido natural). El número de pollos que vuelan no presentó correlación. La población tinerfeña que habitael matorral xérico en el sur tiene la LD más temprana del Paleártico y los que habitan a más de 1.000 m.s.n.m.la presentan similar a los europeos, mientras que el CS es inferior. La variación latitudinal en la LD y CScoincide con los postulados clásicos (retraso e incremento, respectivamente, hacia el norte), pero no ex-plica las relaciones aquí presentadas. De acuerdo con estudios recientes, el tipo de nido (cajas-nido vsnidos naturales) y los factores climáticos como la temperatura (invierno, primavera) y la lluvia primave-ral, son los que afectan verdaderamente a estas variaciones latitudinales. Los registros de Tenerife son con-sistentes con las hipótesis del retraso reproductivo con la altitud y la reducción del CS en aves insulares.

Palabras clave: cernícalo vulgar, Falco tinnunculus, fecha de puesta, islas, latitud, longitud, número depollos que vuelan, Paleártico, Tenerife, tamaño de puesta.

CARRILLO, J. and GONZÁLEZ-DÁVILA, E.

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type (Grant, 1965), food scarcity (Lack, 1968but see Cody, 1971), taxonomic divergences(Cody, 1971), differential predation pressure(Loiselle and Hoppes, 1983), climatic conditions(Blondel, 1985), intra-specific competition(Blondel, 1985), genetic factors (Frankham,1997), differences in the local timing of maxi-mum food abundance (Lambrechts and Dias,1993), island size and isolation, especially thesmaller ones and those farthest from the conti-nent (Wiggins et al., 1998), and parasitic infes-tations (Carrete et al., 2009). Compared withmainland populations, island birds are charac-terized by later laying dates, smaller clutch sizesand prolonged nestling development (Lambrechtsand Dias, 1993; Wiggins et al., 1998).

Although avian life history strategies havebeen studied for over a half a century, hypothe-ses about the basic patterns of variation in life-history traits do not seem sufficiently tested(Cooper et al. 2005a). Life-history traits of thecommon kestrel Falco tinnunculus (hereafterkestrel) have been studied in many parts of theworld (Village 1990). This species is a smallopen-country raptor that breeds duringspring in a great variety of nesting sites. Foodabundance and nest site scarcity favour theirreproduction in nest-boxes, which has facili-tated the study of this species in Europe. Thekestrel is territorial, single brooded, and occu-pies a wide geographic range in the WesternPalearctic from about 70º N to 15º S and 65º Eto 25º W (Village, 1990). Kestrels also inhab-it islands but there are few studies about theirbreeding ecology in these ecosystems (Carri-llo and González-Dávila, 2005). Kestrel breed-ing parameters (namely laying date, clutch sizeand number of fledglings) have been reportedfrom many parts of the Palearctic (Village,1990) but the resulting data have not been sub-jected to geographic comparisons.

Our aims were to: (i) document geograph-ic variation in breeding parameters related tolatitude, longitude and altitude, using a large-scale data set from the Western Palearctic;(ii) evaluate the results in the light of the pre-

viously mentioned hypotheses; (iii) examinewhether breeding parameters of some island-dwelling kestrels differed in latitude, with spe-cial reference to Tenerife.

MATERIAL AND METHODS

We obtained data for mean laying date (LD),mean clutch size (CS) and mean number offledglings (NF) from studies covering widelyscattered areas over the Western Palearctic. Ourcriteria for inclusion were: (i) only control groupdata (unmanipulated nests) were taken from ex-perimental studies, (ii) data from kestrel nest-box populations were recorded to test the effectof nest type, but controlling for lower predationrisks than natural-cavity nesting populations(Fargallo et al., 2001), and (iii) at least 30 nestscontrolled during more than four years, to min-imize the bias produced by local and inter-an-nual differences in productivity and prey sup-ply, especially relevant in unstable environments(Korpimäki and Norrdhal, 1991; Fargallo et al.,2009). The data corresponded to 25 differentareas ranging from latitude 28º N to 65º N andlongitude 17º W to 28º E (fig. 1, table 1). Theeffects of climate change over the last twodecades (i.e. increase in spring temperatures)may affect the breeding biology of birds by ad-vancing laying date or modifying nesting suc-cess (Crick, 2004). So we considered the meanyear of the study period in each population.

Laying date was defined as the mean layingdate (LD) of a given population for all studyyears, recorded in Julian dates. LD in Tenerifeis represented by two different sets of databecause there is a delay of one month in lay-ing above 1,000 m altitude (Carrillo andGonzález-Dávila, 2005). Therefore, we con-sidered “lowland populations” (below 1,000m.a.s.l.) and “highland” ones (above 1000 m).Clutch size was taken as the weighted averageof the annual means per number of clutches(CS). We included data from two studies thatdid not meet our nest-criterion (for LD, num-

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TENDENCIES IN BREEDING PARAMETERS OF FALCO TINNUNCULUS 217

ber of nests = 13, Salvati, 2002; for CS, num-ber of nests = 29, Martínez and Calvo, 2006)to supplement the lack of information for ar-eas between 30º N and 50º N latitude. Numberof fledglings of a given population was themean for the study period (NF). However, NFis probably the least reliable parameter becauseit may be calculated by different observers us-ing different methods (Village, 1990). Given

that the study by Korpimäki and Norrdahl(1991) was carried out on two kinds of nests,we distinguished natural nests from nests-box-es (see table 1). CS and NF for Tenerife weretaken as the Tenerife mean, since no inter-habi-tat differences exist (Carrillo and González-Dávila, 2005). Regarding the low sample sizesin three cases (LD in Italy, n = 13; LD in Teneri-fe, n = 17 and NF in Morocco, n = 27), we per-


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Co. Lat. Long. Alt. LD n CS n N F n (nº nests)Nest-type Study period References

Finland 65º 28º - - - 5.14 194 3.94 139 NN 1974-77/1963-66 Kuusela (1983)Finland 63º 23º 300 127 287 5.46 287 - - NB 1985-96 Korpimäki & Wiehn (1998)Finland 63º 22º < 200 - - - - 2.95 88 NN 1977-82 Korpimäki & Norrdahl (1991)Finland 63º 22º < 200 - - - - 2.99 88 NB 1983-87 Korpimäki & Norrdahl (1991)Finland 61º 24º - - - 4.84 32 * * NN 1974-77/1963-66 Kuusela (1983)Norway 61º 9º 511 - - 4.93 31 3.06 31 NN 1942-46Scotland 55º -3º 200-540 122 129 5.00 120 4.00 79 NN 1976-79 Village (1986)Scotland 55º -3º 200-540 123 127 5.00 139 4.00 81 NN 1976-79 Village (1990)The Netherlands 53º 5º 0 122 213 5.12 213 - - NB 1977-86 Meijer England 53º -1º 50 132 263 4.50 247 3.45 165 NN 1981-87 Village (1990)The Netherlands 52º 6º 0 118 397 5.29 397 - - NB 1960-65 Cavé (1968)Germany 52º 9º 55-160 121 57 5.41 32 3.78 65 NN 1982-89 Kostrzewa (1989)Czech Republic 50º 16º 200 117 217 4.91 224 4.76 215 NB 1986-89 Plesník & Dusík (1994)Germany 48º 9º 820 - - 4.94 46 4.30 76 NN 1963-67 Rockenbauch (1968)France 47º 4º 400 116 82 4.72 82 3.90 82 NN 1973-80 Bonin & Strenna (1986) Switzerland 46º 8º - - - 4.98 100 - - NN - Géroudet (1978)France 45º 1º 488 - - 4.77 45 2.99 59 NN 1976-79 Nore (1979)Italy 41º 12º 43-139 112 13 * * * * NN 1996-2000 Salvati (2002)Spain (Iberian P.) 40º -4º 1300 127 116 5.00 84 3.50 116 NB 1993-98 Fargallo et al. (2001)Spain (Iberian P.) 39º 0º 300-1400 * * * * 4.00 47 NN 1982-87 Gil-Delgado Majorca Island 39º 3º 10-250 116 101 4.47 83 - - NN 1988-93/2000-03 Mestre & Vidal (pers. obs.)Spain (Iberian P.) 38º -1º - - - 4.70 29 - - NN 1990-2005 Martínez & Calvo (2006)Algeria-Tunisia 32º 6º - - - 4.53 32 - - NN - Heim de Balsac & Mayaud (1962)Morocco 31º -4º - 104 40 4.80 40 2.40 27 NN 1979-82 Bergier (1987)Tenerife Island2 28º -17º 0-2400 80 120 4.41 133 3.00 124 NN 1985-94 Carrillo & González-Dávila (2005)Tenerife xerophytic scrub3 28º -17º 75-500 77 103 4.45 112 2.95 107 NN 1985-94 Carrillo & González-Dávila (2005)Tenerife subalpine scrub3 28º -17º 1200-2400 107 17 * * * * NN 1985-94 Carrillo & González-Dávila (2005)

TABLE 1

Country (Co.), latitude (Lat.), longitude (Long.), altitude of the study area (Alt., m.a.s.l), mean laying date(LD, 1 = 1 January), mean clutch size (CS) and mean number of fledglings (NF) of the common kestrelpopulations in the Western Palearctic. NN natural nests; NB nest-boxes; n sample size; – data not avail-able; * data not used (see Material and Methods); 1 we did not consider the data for 1943 because this pub-lication only reported one breeding pair for that year; 2 mean data for Tenerife Island; 3 data from twohabitats in Tenerife.

formed another analysis using weighted data,but the results were not different.

Latitude and longitude (in degrees, rela-tive to Greenwich meridian) and altitude(m.a.s.l.) were either obtained from the litera-ture or determined from topographical maps.For very extensive areas we considered the cen-tral point of the region as our reference point.The altitude was the mean value of the range

when a range of altitudes was reported. Of 25areas studied, 21 % were situated between70º N and 60º N, 29 % between 60º N and 50ºN, 25 % between 50º N and 40º N, 21 % be-tween 40º N and 30º N and 4 % between 30º Nand 20º N. The northernmost areas included inour study were Norway (between 71º N and 58ºN; Hagen, 1969) and Finland (between 70º Nand 60º N; Kuusela, 1983), while the southern-

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N F n (nº nests)Nest-type Study period References

3.94 139 NN 1974-77/1963-66 Kuusela (1983)- - NB 1985-96 Korpimäki & Wiehn (1998)

2.95 88 NN 1977-82 Korpimäki & Norrdahl (1991)2.99 88 NB 1983-87 Korpimäki & Norrdahl (1991)

* * NN 1974-77/1963-66 Kuusela (1983)3.06 31 NN 1942-461 Hagen (1969)4.00 79 NN 1976-79 Village (1986)4.00 81 NN 1976-79 Village (1990)

- - NB 1977-86 Meijer et al. (1988)3.45 165 NN 1981-87 Village (1990)

- - NB 1960-65 Cavé (1968)3.78 65 NN 1982-89 Kostrzewa (1989)4.76 215 NB 1986-89 Plesník & Dusík (1994)4.30 76 NN 1963-67 Rockenbauch (1968)3.90 82 NN 1973-80 Bonin & Strenna (1986)

- - NN - Géroudet (1978)2.99 59 NN 1976-79 Nore (1979)

* * NN 1996-2000 Salvati (2002)3.50 116 NB 1993-98 Fargallo et al. (2001)4.00 47 NN 1982-87 Gil-Delgado et al. (1995)

- - NN 1988-93/2000-03 Mestre & Vidal (pers. obs.)- - NN 1990-2005 Martínez & Calvo (2006)- - NN - Heim de Balsac & Mayaud (1962)

2.40 27 NN 1979-82 Bergier (1987)3.00 124 NN 1985-94 Carrillo & González-Dávila (2005)2.95 107 NN 1985-94 Carrillo & González-Dávila (2005)

* * NN 1985-94 Carrillo & González-Dávila (2005)

[País (Co), latitud (Lat.), longitud (Long.), altitud del área de estudio (Alt., m.s.n.m), fecha media de pues-ta (LD, 1 =1 de enero), tamaño medio de puesta (CS) y número medio de volantones (NF) de las pobla-ciones de cernícalo vulgar en el Paleártico occidental. NN nidos naturales; NB cajas nido; n = tamañode muestra; –registros no disponibles; * datos no utilizados (ver Material y Métodos); 1 no consideramos1943 porque en esa publicación sólo se cita una pareja ese año; 2 medias para Tenerife; 3 registros dedos hábitats en Tenerife.]

most area was Tenerife Island (28º N; Carril-lo and González-Dávila, 2005).

Sample size of LD, CS and NF may differ inthe analysis because some of the studies con-sidered do not provide all the breeding param-eters (see table 1). Pearson’s correlation coef-ficient was used to determine relationshipsbetween any two continuous variables (e. g. lat-itude vs longitude, CS vs longitude, etc). Gen-eralized Linear Models with stepwise methodfor selecting variables were used to analyze therelations between variables of interest and pre-dictor variables (e.g. latitude, longitude and

nest-type), after checking the normality of thevariables using Kolmogorov-Smirnov test. Wealso performed statistical analysis using a quad-ratic relationship method (squared latitude) butresults were not better than those obtained witha simple linear model. We used Student’s t testto compare CS in Tenerife with Morocco andMajorca with Murcia. All tests are two tailedand a P value of < 0.05 was considered as sta-tistically significant. All values are presentedas means ± 1 SD. Data were analyzed usingSPSS 14.0 (SPSS Inc. 2005) and STATISTI-CA 6.0 (STATISTICA 2001).


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FIG. 1.—Geographic variations in mean clutch size of the common kestrel in the Western Palearctic. (Seetable 1 for sources). [Variaciones geográficas en el tamaño medio de puesta del cernícalo vulgar en el Paleártico occidental.(Consultar la tabla 1 para las referencias).]

RESULTS

LD in the Western Palearctic excluding is-land data (Majorca and Tenerife) was 120.01(SD = 7.49, n = 12, min. 103.92 days, max.132). Stepwise multiple regression analysis in-cluding all three geographical variables (lati-tude, longitude, altitude) and mean year of studyin each population left latitude in the model asthe only variable affecting LD (F1,10 = 8.74, P= 0.014, R2 = 0.47; LD = 90.243 + 0.603 lati-tude; fig. 2). The tendency was unchangedwhen Tenerife (highland populations) and Ma-jorca island data were included (island effect:P = 0.827; latitude * island interaction: P =0.782). Latitudinal variation in LD was ex-

plained better by a linear model (R2-adj =0.413) than by a quadratic model (R2-adj =0.374). According to this model, consideringlatitude 28º, where Tenerife is located, LDwould be 107.13 days ± 7.03 (for comparisonswith observed data, see table 1 and LD in Car-rillo and González-Dávila, 2005) and consid-ering latitude 39º, where Majorca is located,LD would be 113.76 days ± 4.50 (for com-parisons with observed data, see table 1, fig.2). LD in lowland populations in Tenerifewas significantly lower than theoretical LD ac-cording to latitude. LD of kestrel populationsinhabiting xerophytic scrub in the south ofTenerife (lowland populations) was the earli-est ever reported for this species in the West-

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FIG. 2.—Latitudinal variation (ºN) in mean laying date (1 = 1 January) of the common kestrel populations.Regression line: LD = 90.243 + 0.603 latitude. Areas considered: • CZ (Czech Republic), FI (Finland),FR (Auxois, France), GB (Great Britain), GE (Germany), IT (Roma, Italy), MO (Morocco), NE (TheNetherlands), SC (Scotland), SG (Segovia, Spain). Island data (∆) are included in the graph: MA (Ma-jorca Island), SS (subalpine scrub in Tenerife Island), XS (xerophytic scrub in Tenerife Island). [Variación latitudinal (ºN) en la fecha media de puesta (1 = 1 de enero) de las poblaciones de cernícalovulgar. Línea de regresión: LD = 90.243 + 0.603 latitud). Áreas consideradas: • CZ (República Checa),FI (Finlandia), FR (Auxois, Francia), GB (Gran Bretaña), GE (Alemania), IT (Roma, Italia), MO (Ma-rruecos), NE (Holanda), SC (Escocia), SG (Segovia, España). Los registros insulares (∆) están inclui-dos en la figura: MA (Mallorca), SS (matorral subalpino de Tenerife), XS (matorral xérico de Tenerife).]

ern Palearctic. It corresponded to March 18;44 days earlier than anywhere else in the West-ern Palearctic.

CS for all areas excluding island data (Ma-jorca and Tenerife) was 4.95 ± 0.26 eggs (n =19, min. 4.50, max. 5.46). Stepwise multipleregression analysis including all three geo-graphical variables (latitude, longitude, alti-tude) and the dichotomous variable “nest-type”(1 = natural nest, 0 = nest-box) left latitude andnest-type in the model as the only variables af-fecting CS (F2,16 = 6.52, P = 0.008, R2 = 0.45;CS = 4.533 + 0.012 latitude – 0.254 nest-type;fig. 3). After including the interaction between

latitude and nest-type, we found that the mod-el improved (F2,16 = 6.94, P = 0.007, R2 = 0.46;CS = 4.317 + 0.016 latitude – 0.005 nest-type* latitude). The latitudinal tendency was un-changed when Tenerife (CS for all island, seemethods) and Majorca Island data were includ-ed (island effect: P = 0.353; latitude * islandinteraction: P = 0.829). We did not include LDin the model because not all studies report thisparameter. Considering those studies that re-ported both LD and CS, Pearson´s correla-tion was 0.461 (P = 0.113).

We found significantly reduced CS in Teneri-fe as compared to that of neighbouring Moroc-


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FIG. 3.—Latitudinal variation (ºN) in mean clutch size of the common kestrel populations for differentnest-types. Solid line (natural nests): CS = 4.359 + 0.010 latitude; dotted line (nest-boxes): CS = 4.061 +0.021 latitude. Areas considered: CZ (Czech Republic), FI (Finland), FR (France), GB (Great Britain),GE (Germany), MO (Morocco), MR (Murcia, Spain), NE (The Netherlands), NW (Norway), SC (Scot-land), SG (Segovia, Spain), SW (Switzerland), TU (Tunisia-Algeria). Island data (∆) are included in thegraph: CI (Tenerife Island, Canary Islands), MA (Majorca Island). [Variación latitudinal (ºN) en el tamaño medio de puesta de las poblaciones de cernícalo vulgar para di-ferentes tipos de nido. Línea continua (nidos naturales): CS = 4.359 + 0.010 latitud; línea discontinua(cajas nido): CS = 4.061 + 0.021 latitud. Áreas consideradas: CZ (República Checa), FI (Finlandia), FR(Francia), GB (Gran Bretaña), GE (Alemania), MO (Marruecos), MR (Murcia), NE (Holanda), NW (No-ruega), SC (Escocia), SG (Segovia), SW (Suiza), TU (Túnez-Argelia). Los registros insulares (∆) están in-cluidos en la figura: CI (Tenerife), MA (Mallorca).]

co (t = 2.51, df = 176, P = 0.013). We did notfind significant differences in Majorca IslandCS as compared to that of Tenerife (t = 0.48,df = 214, P = 0.635), or that of Murcia(Martínez and Calvo, 2006) - the only Iberianregion close to this Balearic island with knowndata on CS (t = 1.15, df = 110, P = 0.254).

NF for all areas excluding island data (Tene-rife) was 3.65 ± 0.68 (n = 15, min 2.40, max.4.76). NF showed no correlation with any ofthe following factors: LD (r = 0.36, P = 0.39,n = 8), CS (r = 0.28, P = 0.38, n = 12), lati-tude (r = -0.04, P = 0.89, n = 15) or longitude(r = -0.04, P = 0.89, n = 15). The percentageof NF with respect to CS was 74 % ± 12.6 %(n = 12, min 50 %, max. 97 %). NF and CS foreach population studied were not correlated af-ter controlling for the effect of latitude (partialr = 0.19, df = 9, P = 0.57).

DISCUSSION

Kestrel LD was affected by latitude, with adelay of about 6 days for every 10º towards thenorth. This species showed later laying date inthe northern latitudes above 53º N and earlierin north-west Africa and lowland Tenerife Is-land. The avian breeding season is adjusted tocoincide with the period when food availabil-ity is sufficient to successfully rear offspring(Greives et al., 2008). Studies performed inseveral environments have reported that thepeak in kestrel nestling energy demand coin-cides with the time of maximum food avail-ability (Village, 1990; Carrillo, 2005). Perhapsfor this reason, survival rates for nestlings andfledglings diminish as the breeding season ad-vances (Aparicio, 1998). Food availability, con-sidered as a proximate or ultimate factor, is oneof the main determinants of reproductive strate-gies in birds (Martin, 1987) and may regulatethe onset of egg laying (Aparicio, 1998). Ex-perimental evidence in the kestrel shows thatsupplementary food offered prior to laying ad-vanced LD (Aparicio, 1994). In addition, tim-

ing of breeding is not only controlled by foodsupply (Meijer et al., 1988; Aparicio 1994).Other factors such as climate (Meijer et al.,1988; Carrillo and González-Dávila, in press)or photoperiod (Meijer et al., 1992) may influ-ence the onset of egg laying to a greater or less-er extent depending on geographic location(Sanz, 1998). Photoperiod seems to be inverse-ly related with the onset of laying dates in thekestrel (Meijer et al., 1992), i.e. increased pho-toperiod corresponds to earlier laying dates. InWestern Palearctic kestrel populations, pho-toperiod was the main factor, but not theonly one, associated with variation in layingdates. For instance, kestrels bred earlier wherewinter and spring temperature were warmerand spring rainfall was lower (Carrillo andGonzález-Dávila, in press). Climatic varia-tions may influence not only laying date butalso clutch size in kestrels (Cavé, 1968).Kestrels breed early and lay large clutches inyears of high prey density (Cavé 1968; Kor-pimäki and Norrdhal 1991). Likewise, kestrelpairs receiving food supplements laid earlierand had larger clutches than control pairs(Aparicio 1994).

Birds inhabiting areas with high seasonalvariation in resources should produce largerclutches. Seasonality probably increases lin-early with latitude (Sanz, 1998) and thereforeaccounts for the linear relationship observedbetween CS and latitude in kestrels (Carrillo,2005; present study). CS increases with lati-tude and daylight hours; these extra hours avail-able for hunting in altricial species, such as thekestrel, could mean increased food supply(Lack, 1947). However, the kestrel begins andends its hunting activity at practically the sametime throughout the year (Masman et al., 1988).In addition, the time devoted by parents toflight-hunting during the breeding season is nomore than three hours a day, even when thereare 17 hours of daylight available (Masman etal., 1988). On the other hand, for the noctur-nal Strigiform species, the number of hoursavailable for hunting activity during the breed-

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ing season diminishes with increased latitude,yet CS of various species actually increases(Mikkola, 1983). This might give support tothe idea that photoperiod per se does not ex-plain why kestrel CS increases with latitude(Meijer et al., 1990) and it seems inconsistentwith Lack’s (1947) hypothesis.

Our study shows that geographic variationsin kestrel CS are influenced by nest-type, andthe increase of CS with latitude is more pro-nounced for those birds breeding in nest-box-es. Variations in CS may be attributable to nest-box breeding because of higher predation innatural nests (Fargallo et al., 2001). In kestrelsbreeding in nest-boxes, those at higher lati-tudes showed higher CS. This increase may becaused by lower nest predation (as shown byValkama and Korpimäki, 1999, in Finland)compared with central or south European pop-ulations (Kostrzewa, 1989; Fargallo et al.,2001). After provisioning nest-boxes, somepopulations increased breeding success, i.e.number of fledglings, due to lower nest pre-dation (Fargallo et al., 2001). Unexpectedly,CS pattern did not correspond with NF ob-served in the Western Palearctic. Apart fromhatching failure, nestling mortality probablyexplains this non-correspondence. Birds adaptclutch size to unusual conditions; the nestlingsof larger broods (i.e. at northern latitudes) un-der unfavourable conditions may suffer under-nourishment, leading to lower survival, andhigher variance in fitness (Boyce and Perrins,1987). Likewise, brood enlargement experi-ments in kestrels and other altricial birds havereported increased nestling mortality (Dijk-stra et al., 1990).

Laying date data of kestrels in islands arescarce, imprecise and there is some inter-is-land variability unexplained by latitude. De-spite great differences in surface area, cli-mate, topography and minimum distance fromthe nearest continental coast, available datafrom some Mediterranean islands seems tofollow patterns similar to those of temperateEurope, i.e. from mid to late April (Menor-

ca, Muntaner and Congost, 1979; Sicily, Sir-acusa, 1985; Corsica, Thibault et al., 1992;Majorca, A. Mestre and S. Vidal, pers. obs.).However, neither Siracusa (1985) nor Thibaultet al. (1992) considered altitudinal gradientin islands as abrupt as Sicily and Corsica, re-spectively. LD in Tenerife that best fits ourmodel, and which most closely resemblesthose of more northern countries (situated tothe north of 45º latitude and with an alti-tude of approximately 500 m.a.s.l.) is that ofhighland kestrels (Carrillo and González-Dávila, 2005). This suggests that increasedaltitude seems to offset the island effect ob-served in lowland kestrels (earlier LD). Al-titudinal variation in laying date in Tenerifecould corroborate the hypothesis of the de-lay in breeding seasons according to altitude(Perrins and Birkhead, 1983). Unfortunate-ly, most studies on reproductive ecology inkestrels are made in specific localities anddo not address the effects of altitude on re-productive parameters. Timing of breedingin Morocco shows little delay compared withthat of temperate Europe, but LD is delayedby about one month with respect to lowlandTenerife kestrels. However, mountainousaltitudes in Morocco may act to delay layingdate, as is suggested by certain data from ageneral study of raptors (Bergier, 1987).

We found significantly reduced CS in theTenerife kestrel population compared to thatof the neighbouring northwest African kestrel,which may corroborate the hypothesis of re-duced CS in islands (Lack, 1947; Hutchison,1981). However, there is no difference in CSbetween the Majorca kestrel population com-pared with the continental region of Murcia.Our findings on the Tenerife kestrel are con-sistent with reduced CS found in bird speciesin the Canary Islands compared with continen-tal populations (Delgado et al., 1987; Garcíadel Rey et al., 2007). However, CS of islandkestrel populations at different latitudes showsdifferent patterns (Finnish islands, Kuusela,1983; Corsica, Thibault et al., 1992; Tenerife,


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Carrillo and González-Dávila, 2005; Majorca,A. Mestre and S. Vidal, pers. obs.). Probably,reduced CS in Tenerife kestrel populations isnot attributable to food scarcity, contrary towhat Lack (1968) suggested about island birds.The main source of kestrel food in Tenerife islargely insects and lizards (Carrillo, pers. obs.)which are relatively abundant and have fewother predators (Martín and Lorenzo, 2001). Itis interesting to consider alternative postulatesto explain low CS in islands; for instance, clutchsize is genetically determined and the Tenerifekestrel is a subspecies (canariensis) which spec-ulatively has undergone mutations giving riseto fewer but successful offspring. Underfavourable conditions, this genetic trait mayhave prospered over time (Postma and van No-ordwijk, 2005).

Latitudinal variation in reproductive suc-cess is a consequence of the interaction be-tween numerous factors including diet duringthe breeding season, hatching failure, nestlingmortality, the different costs of thermoregu-lation and immune response of nestlings at dif-ferent latitudes, recruitment index and adultsurvival. Further studies would be required toaccount for the unexpected similarities in NF,despite the wide variety of factors in stable andunstable environments at different latitudes.Moreover, the mechanisms underlying thebreeding ecology of island kestrels at differentlatitudes remain to be elucidated.

ACKNOWLEDGEMENTS.—We especially wish tothank J. M. Aparicio for advice and encouragementthroughout the project; J. Barquín for assistance inelaborating the map; Jordi Muntaner for facilitat-ing contact with A. Mestre and S. Vidal who pro-vided unpublished data on Majorcan Island kestrels,as well as M. L. McLean for useful comments andmeticulous revision of the English version. J. M.Aparicio, J. Mª. Fernández-Palacios, J.-M. Thiol-lay, C. B. Cooper, and L. Strenna reviewed theprevious versions of the manuscript. The sugges-tions from F. de Lope and two anonymous review-ers improved the manuscript.

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[Recibido: 03-09-2008][Aceptado: 03-07-2009]


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