Abstract
Density has been suggested to affect variation in extra-pair paternity (EPP) in avian mating systems, because increasing density promotes encounter rates and thus mating opportunities. However, the significance of density affecting EPP variation in intra- and interspecific comparisons has remained controversial, with more support from intraspecific comparisons. Neither experimental nor empirical studies have consistently provided support for the density hypothesis. Testing the density hypothesis is challenging because density measures may not necessarily reflect extra-pair mating opportunities, mate guarding efforts may covary with density, populations studied may differ in migratory behavior and/or climatic conditions, and variation in density may be insufficient. Accounting for these potentially confounding factors, we tested whether EPP rates within and among subpopulations of the reed bunting (Emberiza schoeniclus) were related to density. Our analyses were based on data from 13 subpopulations studied over 4 years. Overall, 56.4% of totally 181 broods contained at least one extra-pair young (EPY) and 37.1% of totally 669 young were of extra-pair origin. Roughly 90% of the extra-pair fathers were from the adjacent territory or from the territory after the next one. Within subpopulations, the proportion of EPY in broods was positively related to local breeding density. Similarly, among subpopulations, proportion of EPY was positively associated with population density. EPP was absent in subpopulations consisting of single breeding pairs, that is, without extra-pair mating opportunities. Our study confirms that density is an important biological factor, which significantly influences the amount of EPP within and among subpopulations, but also suggests that other mechanisms influence EPP beyond the variation explained by density.
Highlights
Despite considerable efforts, the underlying factors determining variation in levels of extra-pair paternity (EPP) among species and among populations of the same species are still not fully understood
Three of them were banded after the year in which they sired extra-pair young, so that we were not able to locate their territory in the relevant year
We showed that density explains variation in levels of EPP
Summary
The underlying factors determining variation in levels of extra-pair paternity (EPP) among species and among populations of the same species are still not fully understood. There is still debate about the influence of population-specific ecological factors (e.g., density and synchrony) on levels of EPP. Variation in population density is one of the classic factors proposed to explain inter- and intraspecific variation in EPP in avian mating systems. The density hypothesis states that increased proximity among individuals increases encounter rates and mating opportunities when searching for potential extra-pair mates, thereby reducing the costs of extrapair matings. If density increases, the rate of EPP should increase as well (Westneat et al 1990). An effect of density on EPP has been shown in experimental studies (Gowaty and Bridges 1991; Charmantier and Perret 2004; Stewart et al 2010) and in some observational studies (e.g., Gibbs et al 1990; Yezerinac et al 1999; Ryder et al 2012), but not in others (see Appendix S2)
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