ecology and conservation of european forest

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The osprey (Pandion haliaetus) has a wide distribution ... populations, timing of breeding in the osprey is known to depend ..... but is not free from some pitfalls.
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Ecology and Conservation of European Forest-Dwelling Raptors Iñigo Zuberogoitia & José Enrique Martínez (Eds.) Estudios Medioambientales Icarus S.L.

Marc Galvez.

Published by: Departamento de Agricultura de la Diputación Foral de Bizkaia © Diputación Foral de Bizkaia All rights reserved. No part of this publication may be reproduced in any form or by any means without the prior permission of the copyright owner. Edited by: Iñigo Zuberogoitia & José Enrique Martínez Estudios Medioambientales Icarus S.L. Project Supervision: Antonio Galera Coleto, Director of the Department of Conservation, Natura 2000 Network and Biodiversity. Graphic and maps Design: Antonio Secilla General Design: ITZEL Digital, S.L. Printer: Berekintza, S.L. Cover photo by: Sparrowhawk. Juán Sagardía. Translators: Phillip Thomas José Delgado Maria Francia Iñiguez Puebla Mikel John O´Rourke Rally Haigh

I.S.B.N.: 978-84-7752-489-2 D.L.: 1730-2011

Large-scale climatic phenomena affect timing of breeding in the osprey Pandion haliaetus Tapio Solonen Solonen, T. Luontotutkimus Solonen Oy, Neitsytsaarentie 7b B 147, FI-00960 Helsinki, Finland

al., 2005). In any case, many factors may be involved, and detecting and interpreting real causal relationships may be complicated (e.g., Svensson, 2004).

Introduction The onset of breeding in birds is largely determined by the prevailing food and weather conditions (Perrins, 1970; Drent & Daan, 1980). The effects of weather are often indirect and connected with other factors, including the food supply (Newton, 1998). The rising of spring temperatures to a certain level often seems to be a key factor triggering the commencement of egg laying (von Haartman, 1963; Crick et al., 1997; Svens­ son, 2004; Both et al., 2005). This implies that the recent warming of climate can be expected to bring about earlier onset of breeding in birds (Coppack & Both, 2002; Sergio, 2003; Both et al., 2004). In migratory species, the preceding advanced spring arrival (Hüppop & Hüppop, 2003; Lehikoinen et al., 2004; Stervander et al., 2005) should reinforce this effect. However, long-distance migrants are suggested to be controlled primarily by endogenous factors which are independent of temperature (Berthold, 1996; Coppack & Both, 2002; Palm et al., 2009). Correlations found between the spring arrival and conditions at the wintering grounds in long-distance migrants (Cotton, 2003; Saino et al., 2004) suggest that to start their spring migration they rely not only on photoperiodic cues but also local environmental conditions (Both et

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The osprey (Pandion haliaetus) has a wide distribution range around the northern hemisphere and it overwinters mainly in tropical areas worldwide (Cramp & Simmons, 1980; Poole, 1994; Saurola, 1994). As a migratory bird it must adapt to the environmental conditions in all parts of the annual range, making the constraints to adaptation even more challenging than those for resident species that spend their entire annual cycle within a specific area (Møller et al., 2006). The osprey is dependent on ample supplies of medium-sized fish obtainable near surface of clear water (Cramp & Simmons, 1980). Earlier findings suggest that local conditions in the vicinity of the breeding grounds are of decisive importance for the onset of egg laying in the osprey (Poole, 1994; Saurola, 2005). In the northern populations, timing of breeding in the osprey is known to depend largely on the spring ice melting and consequent opening of shallow waters for fishing near the breeding grounds. Apart from local circumstances, larger-scale or global climatic phenomena seemed to concern closely local populations of the species (Solonen, 2008). On the other hand, variable local conditions may more or less mask the effects of large-scale factors (e.g., Svensson, 2004; Saurola, 2005). Therefore, both local and large-scale approaches are needed to reveal thoroughly the causal relationships involved.

Timing of breeding in relation to climatic variables How are the recent large-scale climatic fluctuations and the proposed global warming reflected in the timing of breeding in the osprey, and what is the role of the local weather conditions? I studied these questions in connection with the nationwide monitoring programme of the Finnish ospreys (Saurola, 2008) in a small local population of about 20 pairs in Eastern Uusimaa (60° N, 25° E), near the southern coast of Finland in 1981–2006.

to March) indicate milder and wetter winter weather in the North. Local weather conditions were described by the mean temperatures of four winter and spring months in Helsinki (Finnish Meteorological Institute), in the immediate vicinity of the study area.

By confining the study to a small and compact fraction of the unevenly distributed and environmentally heterogeneous total Finnish population (Saurola 2005), the confusing effects of variable local conditions were minimized, and the large-scale effects might be easier to detect (cf. Both et al. 2004; Svensson 2004). The timing of breeding was determined on the basis of the developmental stage of young found and ringed in the successful nests. The data included 379 young from 180 broods. For characterizing the weather conditions at large, I used the winter and monthly indices of the North Atlantic Oscillation (NAO). NAO is the most prominent pattern of atmospheric variability over the northern hemisphere and has a marked effect on the European weather conditions (Jones et al., 1997; Hurrell et al., 2001). The higher positive values of the winter NAO indices (averaging the monthly values from December

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4 January NAO index

Pertti Saurola climbing a pine in order to reach an osprey nest. Iñigo Zuberogoitia.

The widely fluctuating winter NAO index showed no evident increase during the last hundred years (linear regression, t98 = 0.41, P = 0.69), but during the last 50 years there was a significant increasing trend (t48 = 2.88, P = 0.006). During the study period, the winter NAO index fluctuated considerably, but there was no consistent trend (t24 = -1.20, P = 0.24) to parallel with the concurrent global warming (see also Svensson, 2004; Stervander et al., 2005). In the local weather variables studied, there were no significant trends either (P > 0.10). However, according to the observations of amateur ornithologists collected by the local ornithological society of Uusimaa and published in the journal Tringa, the spring arrival of the first ospreys near the southern coast of Finland advanced significantly during 25 years (t24 = -2.71, P = 0.005). The date of arrival could be best predicted (adj R2 = 0.517) from a linear combination of the independent variables February NAO (P = 0.007), mean temperature of April (P = 0.035) and year (P = 0.015). Timing of breeding in the osprey population studied fluctuated annually but there was no significant trend (Solonen, 2008). The annual mean in the first half of the study period did not differ significantly from that of the second one either.

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-4

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Wing length (cm)

Figure 44. Relationship between the mildness of winter (January NAO index) and onset of egg laying (expressed by the wing length of the largest nestling in the earliest broods of the year on the 20th July) in the local population of the osprey studied in 1981–2006 in southern Finland (r = 0.660, P < 0.001, df = 24).

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Table 30. Variables explaining significantly timing of egg laying (indicated by the estimated wing length of the largest nestling in the broods on the 20th July) in the annually earliest, mean, and median broods of the osprey population studied in 1981–2006. Generalized linear models were applied using the independent NAO indices and local mean temperatures of the winter and spring months preceding the breeding season as well as the annual first arrival dates of spring migrants as explanatory variables. F/t

P

AIC

Adj R2

12.250 4.704 9.938 4.297 9.600 4.216 8.914 3.371 3.209 5.568 4.237 2.665

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 0.001 0.003 0.004 0.011 0.027 0.014

91.31 91.18 93.98 93.33 94.39 93.76 95.25 98.09 98.89 99.90 102.01 101.43

0.474 0.458 0.417 0.411 0.408 0.402 0.388 0.293 0.271 0.268 0.206 0.196

8.426 3.756 7.460 4.648 2.876 2.500

0.002 < 0.001 0.003 0.020 0.008 0.020

96.14 96.41 97.44 101.61 100.73 102.42

0.373 0.344 0.341 0.226 0.225 0.174

2.917 4.092

0.008 0.030

106.95 108.92

0.231 0.198

Earliest broods March temperature + arrival date March temperature January temperature + arrival date January NAO April temperature + arrival date January temperature January NAO + arrival date Winter NAO April temperature Winter NAO + arrival date February temperature + arrival date February temperature Mean broods April temperature + arrival date January NAO January NAO + arrival date Winter NAO + arrival date Winter NAO April mean temperature Median broods January NAO January NAO + arrival date

The breeding of ospreys near the southern coast of Finland seemed to be significantly connected with recent large-scale climatic phenomena (Table 30). Timing of breeding could be predicted from the NAO index of the preceding winter, in particular that of January: the higher the indices, the earlier breeding (Fig. 44). In addition to large-scale climatic phenomena, local weather conditions were reflected in the timing of egg laying, particularly in the earliest breeders of the population (Table 30). The commencement of egg laying in the earliest broods of the season was best explained by the mean temperature of March. Contributions of temperatures of other months examined as well as those of the January and winter NAO indices were also significant. The first arrival dates, in general, enhanced the models based on the climatic variables only slightly, if at all. The positive effect of the first arrival date was pronounced only in the models using the April temperature as the climatic explanatory variable. For the mean commencement of egg laying of the season, the role of large-scale climatic phenom-

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ena was the most pronounced. Of the local weather variables, only the effect of the April temperature was significant in this case. This effect was considerably enhanced when the first arrival date was included in the model. Significant relationships between climatic variables and timing of breeding in the osprey were probably more or less linked with timing of ice melting (Saurola, 2005). Variation in ice conditions has strong effects on the breeding of various piscivorous birds (Gaston et al., 2005). The ice melting in the North is known to be dependent besides on local spring temperatures also on the strength of the preceding winter influenced by large-scale climatic fluctuations and changes (Blenckner et al., 2004; Schwartz et al., 2006).

Causes and consequences The weather variables showed no significant trends for the study period (cf. Schwartz et al., 2006), but in the long run the 1990s were characterized by remarkably

After mild winters, the ospreys bred early and produced large broods: nestlings in a nest in Pernaja, southern Finland, 13 July 2004. Tapio Solonen.

mild winters (Drebs et al., 2002). Their effects were probably reflected in the spring arrival dates of the osprey but there was no advancement in the breeding season. The relationship between the climatic oscillations of the Northern Atlantic and timing of breeding in the osprey near the southern coast of Finland was, however, clear. In migratory birds, there may be various species-specific responses to climate variables (Palm et al., 2009). In the osprey, it can be expected that large-scale climatic processes induce the spring migration and adjust it so that the arrival to the breeding grounds in the North is timed according to ice melting (cf., Saino et al., 2004). Variations in the timing of breeding can be interpreted as responses of birds in order to guarantee that breeding conditions will be as suitable as possible. The early onset of breeding in birds has been suggested to be

a favourable trait evolved to ensure the best feeding conditions for forthcoming young (Lack, 1968; Perrins, 1970; Drent & Daan, 1980; Daan et al., 1989). It may also offer possibilities to produce more clutches and young within a season, or, typically in various long-lived species of northern latitudes, to ensure that the time-taking process of reproduction can be conducted within the strict limits of tolerable environmental conditions. The latter is particularly important for the successful reaching of independence and, in migratory species, for sufficient preparation of young birds for migration. The above suggests that the ability to respond quickly to climatic conditions is profitable (Prop & Deerenberg, 1991; Nolet & Drent, 1998; Stervander et al., 2005). However, sudden shifts to earlier breeding may be detrimental as well if the critical periods for successful reproduction are changed to less suitable ones (Visser et al., 1998; Sanz, 2003;

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Both & Visser, 2005; Both et al., 2009; Lehikoinen et al., 2009). In the osprey, early breeders are known to produce more young than later ones (Saurola & Koivu, 1987; Poole, 1989). The seasonal decrease in clutch size in birds (Klomp, 1970; Murphy & Haukioja, 1986) is thought to form a reaction norm in response to variation in the seasonal gain of parental condition and a seasonal decrease in offspring’s reproductive value (Drent & Daan, 1980; Daan et al., 1990; Rowe et al., 1994; Brommer et al., 2003). Also in the present population, the annual brood size (range 1–4) was the higher the earlier was the onset of breeding (r = 0.339, P < 0.001, df = 178). This suggests that – at least during the study period – early breeding was advantageous for the population. However, there may be temporal differences, for instance in vulnerability of nestlings and fledglings to predation by the eagle owl (Bubo bubo) and northern goshawk (Accipiter gentilis), common causes of death of young ospreys in the study area. The later fate of the most of the fledged young was left unknown due to the small number of ringed birds and consequent lack of recoveries. In a long-lived species such as the osprey, some effects of environmental changes might be seen only after a considerable time lag. Thus, detectable longterm responses of breeding ospreys to the climatic variables studied during the relatively short study period do not seem very probable (cf. Sergio, 2003; Svensson, 2004). The interpretation of results is complicated by the fact that various causes and consequences may be temporally separated, and they can vary considerably both temporally and spatially (Svensson, 2004; Both & Visser, 2005). This means that the outcome obtained may depend on the exact location and length of the period of time considered (Stervander et al., 2005; Gordo & Sanz, 2006; Solonen, 2008). Latitudinal gradients in phenotypes may provide model systems that can be used to achieve a better understanding of the effects of large-scale climatic phenomena (Møller et al., 2006). Organisms with a large latitudinal range of distribution, such as the osprey, are particularly well suited for studies of the effects of climate change on ecological and evolutionary processes. At the population level, different mechanisms may explain changes in seasonally labile traits such as breeding time due to variation in environmental conditions among years. In the present case of the

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osprey, the mechanism of the response to changing environmental conditions seemed to be phenotypic plasticity rather than microevolution. Further, the onset of breeding seemed to be a flexible response to the consequences of large-scale climatic phenomena occurring months before the potential laying season rather than simply a reaction to the general level of prevailing local spring temperatures. Shorter spells of suitable temperatures may still be responsible for the actual commencement of laying. The effects of global climatic changes may depend on the details of changes in local weather conditions within each season (Jónsson et al., 2009). The present indicator of timing of breeding (the estimated wing length of the largest nestling within the brood in the reference day) seems quite reasonable but is not free from some pitfalls. In particular, the annual sample size should be large enough to minimize the effects of between-brood variation that may be considerable due to various reasons. For instance, older and more experienced pairs arrive earlier and lay eggs more quickly than younger pairs (Poole, 1985). Local prerequisites for nesting may also be involved. For instance, after the spring arrival, it takes much less time to prepare a ready nesting platform (an old nest or an artificial nest site) than to build a largely or completely new nest. So, the scarcity and decaying of suitable nesting trees and nests cause plenty of rebuilding and consequent delayed onset of egg laying while the construction of steady artificial nests by humans increases chances for early clutches. Some additional variation is due to, among others, the sex differences between nestlings. Responses to climatic phenomena have generally been assessed as the mean response at the population level (Møller et al., 2006). Previous studies of the effects of climate change on bird migration have, in general, investigated only the first arrival dates of birds. Such approaches invariably disregard heterogeneity in response, for example, due to age and sex differences. So, the investigations should increasingly include groups of various kinds of individuals to reveal more comprehensively characteristics of responses of total populations.

Conclusions The results indicate prominent effects of large-scale climatic phenomena on timing of breeding in the osprey. The commencement of egg laying seemed to be

a flexible response to the consequences of large-scale climatic phenomena occurring months before the potential laying season rather than merely a reaction to the general level of prevailing local spring temperatures. However, the local environmental conditions both in winter and in the following spring seemed to be significantly linked with and thus predictable from the wintertime large-scale climatic phenomena. The logic interpretation is that after mild winters the ice cover is thinner, and ice melting and consequent availability of fish for the osprey earlier than after hard winters. Additional variation in timing of breeding is probably largely due to local factors such as the distance between the nest site and fishing waters as well as individual characteristics such as the age and basic condition of the birds. The present study did not reveal any evidence of global warming but the results sug-

gest that the response of ospreys to increasing temperatures should be a clear advancement of breeding. Early breeding seemed to be advantageous in terms of the number of fledglings produced. A larger set of ringed nestlings should, however, be available for examining possible differences in fitness between early and late broods. The monitoring programme of the Finnish ospreys offers an outstanding opportunity for such analyses.

Acknowledgements Many people have helped in various ways in the field work. Pertti Saurola is acknowledged for inspiration. Hannu Pietiäinen read a draft of the manuscript and made useful suggestions. I dedicate this contribution to the memory of my mother Anna-Liisa.

Osprey carrying a fish. Carlos González.

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