New support for an old hypothesis: density affects extra-pair paternity

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.

Keywords: Birds; density; extra-pair parentage; microsatellites; population.

Figure 1

Location of the subpopulations studied in northeastern Switzerland. Red circles indicate the three large subpopulations, green circles the small subpopulations. Letters inside circles represent the first two letters of the subpopulation names shown in Table 1. Source: Federal Office of Topography.

Figure 2

Reed bunting nestlings approx. 8 days old in northeastern Switzerland, 19 May 2005. Picture: G. Pasinelli.

Figure 3

Extra-pair paternity rate per subpopulation and year in relation to a) the median number of neighbors and b) the median nearest neighbor distance (m) per subpopulation and year. Each filled circle represents the median of the EPP rate within a subpopulation in a specific year. Lines (interquartile range) show the variation in EPP rates among territories within subpopulations. N = 181 broods from 13 subpopulations collected over 4 years.