Selection on laying date is connected to breeding density in the pied flycatcher

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 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.

Current spring warming as a driver of selection on reproductive timing in a wild passerine

“…Population density has previously been shown to increase the strength of selection acting on laying date (e.g. in pied flycatchers, Ficedula hypoleuca , Ahola, Laaksonen, Eeva, & Lehikoinen, ). These variables were taken into account because they fluctuated across years, potentially shaping the climatic signal on selection acting on laying date.…”

Long-term recovery of clutch size and egg shell quality of the pied flycatcher (Ficedula hypoleuca) in a metal polluted area

“…The latter change does not, however, fully explain the increase in density because also occupation rates (nests/nest boxes) doubled (from 30% to 59%) in the polluted zone during the study period. Advanced breeding times in our study population likely reflect increased spring temperatures during the study period, but Ahola et al (2012) also noticed that there was stronger selection on earlier breeding in dense F. hypoleuca population, which could further promote early onset of laying.…”

The early birds and the rest: do first nesters represent the entire colony?

“…Although such changes can cause mis-timings with availability of prey for some species (Both et al 2006;Saino et al 2009;Watanuki et al 2009), some species or populations seem to benefit from climate change or have altered their behavior or otherwise adapted to environmental changes (Mace et al 2010;Jonker et al 2011;2012). However, spring arrival dates can depend on factors besides climate, most notably cohort sizes and annual changes in the sizes of migratory populations (Miller-Rushing et al 2008;Ahola et al 2012). Findings on spring arrival date also depend on the exact indices used; in a study of 32 migratory species, MillerRushing et al (2008) found that earliest arrivals were delayed 0.20 days each decade whereas conversely, mean arrival dates for each species were advanced 0.78 days each decade.…”

“…There should be a strong biological selection for earlier laying dates because the nesting density has increased since 1992 (Jónsson and Lúðvíksson 2013). Thus, early breeding may not be advantageous on its own but a necessity under increased population density which increases competition for preferred nest sites (Ahola et al 2012). The main goal of this study was to identify and compare potential sources of variation in first nest date with that of the median nest date.…”

“…Individual birds make different adjustments to environmental changes that facilitate earlier nest initation (Ahola et al 2012;Arzel et al 2014). Early nesting individuals generally are in optimal physical condition, often lay large clutches or attain a high nesting success (Erikstad et al 1993;Arnold et al 2004;Bêty et al 2004).…”