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An emerging question in animal behaviour is whether and how behavioural plasticity will enable organisms to adjust to human-induced, rapid environmental changes that affect breeding conditions. Adaptive behavioural plasticity in response to changing resource conditions will depend on the sensitivity of the neuroendocrine system to food stimuli and on constraints in the mechanisms mediating the expression of reproductive behaviours. We tested the hypotheses that food availability mediates plasticity in androgen and corticosterone (CORT) responses, and that circulating concentrations of these hormones in turn mediate investment into competing mating and parental behaviours in a migratory songbird. We provided supplemental food to black-throated blue warblers, Setophaga caerulescens , breeding in habitats with natural low and high food abundance. The effects of supplemental feeding were most pronounced in food-poor habitat. During their social mates' fertile stage, fed males sang less near their nest sites, had lower plasma androgen and CORT levels, and had higher residual mass than control males. Fed males benefited indirectly from their mates' access to supplemental food because fed females provided more parental effort than control females. Fed males did not increase their own parental effort. Our results suggest that fed males increased mate-guarding effort over extrapair mate attraction while their social mates were fertile and invested additional resources into territorial defence or self-maintenance during the parental stage. These findings reveal plastic patterns of covariation among endocrine parameters and mating behaviours, but not parental behaviours, in response to food supplementation. These responses enabled individuals to adjust their reproductive effort rapidly to changes in resource conditions. Our findings suggest that adaptive reproductive investment in response to environmental change is unlikely to be constrained by the hormonal mechanisms underlying the mediation of competing mating and parental behaviours. [ABSTRACT FROM AUTHOR]

Numerous studies have correlated the advancement of lay date in birds with warming climate trends, yet the fitness effects associated with this phenological response have been examined in only a small number of species. Most of these species–primarily insectivorous cavity nesters in Europe–exhibit fitness declines associated with increasing asynchrony with prey. Here, we use 25 years of demographic data, collected from 1986 to 2010, to examine the effects of spring temperature on breeding initiation date, double brooding, and annual fecundity in a Nearctic - Neotropical migratory songbird, the black-throated blue warbler (Setophaga caerulescens). Data were collected from birds breeding at the Hubbard Brook Experimental Forest, New Hampshire, USA, where long-term trends toward warmer springs have been recorded. We found that black-throated blue warblers initiated breeding earlier in warmer springs, that early breeders were more likely to attempt a second brood than those starting later in the season, and that double brooding and lay date were linked to higher annual fecundity. Accordingly, we found selection favored earlier breeding in most years. However, in contrast to studies of several other long-distance migratory species in Europe, this selection pressure was not stronger in warmer springs, indicating that these warblers were able to adjust mean lay date appropriately to substantial inter-annual variation in spring temperature. Our results suggest that this North American migratory songbird might not experience the same fecundity declines as songbirds that are unable to adjust their timing of breeding in pace with spring temperatures. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]