POPULATION ECOLOGY - ORIGINAL PAPER Selection on laying date is connected to breeding density in the pied flycatcher Markus P. Ahola • Toni Laaksonen • Tapio Eeva • Esa Lehikoinen Received: 15 October 2008 / Accepted: 8 September 2011 / Published online: 11 October 2011 Ó Springer-Verlag 2011 Abstract 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 cli- matic 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 fly- catchers 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. Keywords Climate Á Ficedula hypoleuca Á Recruitment Á Spring temperature Á Temporal trends Introduction Natural selection obviously favours individuals that pro- duce many high-quality offspring. The timing of breeding and clutch size are two key reproductive traits affecting the productivity and thus the fitness of individual birds (Daan et al. 1990; Lack 1954). Correct timing involves beneficially matching with the availability of resources, whereas an optimal clutch size maximizes both quality and quantity of the offspring produced. Clutch size is often negatively correlated with timing of breeding (Garamszegi et al. 2004; Gladbach et al. 2010; Gooden- ough et al. 2009; von Haartman 1982), and both timing and clutch size are affected by parental condition (Rowe et al. 1994). In addition to the correct timing, the availability of food, nest sites, and other resources are affected by the density of competitors (Gustafsson 1987). For example, breeding density is related to clutch size and fledgling production (Ahola et al. 2009; Both 1998, 2000; Dhondt et al. 1992). Communicated by Ola Olsson. M. P. Ahola (&) Centre for Maritime Studies, Environmental Research and Regional Development, University of Turku, P.O. Box 181, 28101 Pori, Finland e-mail: mapeah@utu.fi M. P. Ahola Á T. Laaksonen Á T. Eeva Á E. Lehikoinen Department of Biology, Section of Ecology, University of Turku, 20014 Turku, Finland T. Laaksonen Finnish Museum of Natural History, University of Helsinki, P.O Box 17, 00014 Helsinki, Finland 123 Oecologia (2012) 168:703–710 DOI 10.1007/s00442-011-2135-5