Experimental food supplementation reveals habitat-dependent male reproductive investment in a migratory bird

Abstract

Environmental factors can shape reproductive investment strategies and influence the variance in male mating success. Environmental effects on extrapair paternity have traditionally been ascribed to aspects of the social environment, such as breeding density and synchrony. However, social factors are often confounded with habitat quality and are challenging to disentangle. We used both natural variation in habitat quality and a food supplementation experiment to separate the effects of food availability-one key aspect of habitat quality-on extrapair paternity (EPP) and reproductive success in the black-throated blue warbler, Setophaga caerulescens. High natural food availability was associated with higher within-pair paternity (WPP) and fledging two broods late in the breeding season, but lower EPP. Food-supplemented males had higher WPP leading to higher reproductive success relative to controls, and when in low-quality habitat, food-supplemented males were more likely to fledge two broods but less likely to gain EPP. Our results demonstrate that food availability affects trade-offs in reproductive activities. When food constraints are reduced, males invest in WPP at the expense of EPP. These findings imply that environmental change could alter how individuals allocate their resources and affect the selective environment that drives variation in male mating success.

Keywords: environmental constraints; extrapair mating; genetic reproductive success; habitat quality; paternity.

Figure 1.

The differences in the proportions of food-supplemented versus control male black-throated blue warblers that sired any young on their territory (WPP), sired extrapair young (EPP) and fledged two broods (double brooded) by habitat quality. Bars show the differences ±s.e. in the proportions of food-supplemented ‘fed’ males relative to control males (n = males; low quality: fed = 16, control = 58; high quality: fed = 45, control = 226). Significance assessed with binomial GLMMs after accounting for male identity and age (table 2). Unfilled bars denote low quality and filled bars denote high quality.

Figure 2.

(a) Relative contribution of within-pair paternity (WPP), extrapair paternity (EPP) and fledging two broods (double brooding, DB) on total reproductive success for all males pooled across treatments and habitats (mean ± s.e.). Categories include males that were: unsuccessful (none), single brooded, lost WPP and gained EPP (EPP only), single brooded and gained WPP (WPP only), double brooded and gained WPP (DB + WPP), single brooded, gained WPP and EPP (WPP + EPP) and double brooded, gained WPP and EPP (DB + WPP + EPP). (b) Comparison of total reproductive success (mean ± s.e.) for males that gained EPP or not by habitat and treatment. Significance was assessed with Poisson GLMMs after accounting for male identity and age (electronic supplementary material, table S3). The dashed lines indicate the mean total annual reproductive success, 2009–2012. Samples sizes are given at the base of the bars.