Selection on laying date is connected to breeding density in the pied flycatcher
Timing of reproduction and clutch size are important determinants of breeding success, especially in seasonal environments. Several recent bird population studies have shown changes in breeding time and in natural selection on it. These changes have often been linked with climate change, but few studies have investigated how the traits or natural selection are actually connected with climatic factors. Furthermore, the effect of population density on selection has been rarely considered, despite the potential importance of density in demographic processes. We studied variation in natural selection on laying date and on clutch size in relation to measures of spring phenology and population density in a long-term study of pied flycatchers in SW Finland. The phenological stage of the environment at mean egg-laying did not affect the direction of selection on either laying date or on clutch size. There was, however, stronger selection for earlier laying date when the breeding density of the population was high, suggesting that early breeding is not necessarily beneficial as such, but that its importance is emphasized when high population density increases competition. In addition, early breeding was favoured when the pre-breeding period was cool, which may indicate an increased advantage for the fittest individuals in harsher conditions. In the middle of the twentieth century, there was selection for large clutch size, which subsequently ceased, along with an overall decrease in recruit production. Our results indicate that attention should be paid to demographic factors such as breeding density when studying natural selection and temporal changes in it.
KeywordsClimate Ficedula hypoleuca Recruitment Spring temperature Temporal trends
We greatly appreciate the contribution of Professor Lars von Haartman, whose lifelong interest in hole-breeding Passerines and dedication to data collection made our study possible. We thank Risto A. Väisänen and the Finnish Museum of Natural History for trusting the data to us, and a number of people who participated in field work in the Harjavalta area. Constructive comments and suggestions by Christiaan Both, Tore Slagsvold, Elina Koivisto and two anonymous reviewers improved the manuscript. A suggestion of new analysis by the other reviewer was especially helpful. Eric Le Tortorec kindly checked the language. The Finnish Meteorological Institute provided the temperature data. This study was funded by the Academy of Finland (project 8119367 to TE), the Kone Foundation (grants to MA and EL) and the Emil Aaltonen Foundation (a grant to TL). The data were collected in compliance with Finnish legislation and with appropriate permissions.
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