Abstract
In most bird species timing of breeding affects reproductive success whereby early breeding is favoured. In migratory species migration time, especially arrival at the breeding grounds, and breeding time are expected to be correlated. Consequently, migration time should also have fitness consequences. However, in contrast to breeding time, evidence for fitness consequences of migration time is much more limited. Climate change has been shown to negatively affect the synchrony between trophic levels thereby leading to directional selection on timing but again direct evidence in avian migration time is scarce. We here analysed fitness consequences of migration and breeding time in great cormorants and tested whether climate change has led to increased selection on timing using a long-term data set from a breeding colony on the island of Vorsø (Denmark). Reproductive success, measured as number of fledglings, correlated with breeding time and arrival time at the colony and declined during the season. This seasonal decline became steeper during the study period for both migration and breeding time and was positively correlated to winter/spring climate, i.e. selection was stronger after warmer winters/springs. However, the increasing selection pressure on timing seems to be unrelated to climate change as the climatic variables that were related to selection strength did not increase during the study period. There is indirect evidence that phenology or abundances of preferred prey species have changed which could have altered selection on timing of migration and breeding.
Highlights
In seasonal environments the timing of life-cycle events generally has strong consequences for reproductive success or survival as environmental conditions, are favourable only for a limited period
Arrival time at the colony and breeding time, i.e. the date when the first egg of the clutch was laid, were positively correlated in both males and females
It is generally assumed that timing of migration in birds is important for individual fitness, documented evidence for this is relatively scarce [37]
Summary
In seasonal environments the timing of life-cycle events generally has strong consequences for reproductive success or survival as environmental conditions (e.g. food supply), are favourable only for a limited period. On the one hand migrating birds should not arrive too early at their breeding grounds because harsh environmental conditions, such as cold spells, may pose a mortality risk [1]. While a number of studies reported a relationship between breeding time and reproductive success in birds, e.g.[5,6,7,8], the evidence in migration time is more limited. In black kites early arriving individuals had higher reproductive success, partly because they were able to settle in high quality territories [3]. Accurately recording individual migration time has been difficult in many species, e.g. woodland living passerines
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