The relationship between female brooding and male nestling provisioning: does climate underlie geographic variation in sex roles?

Abstract

Comparative studies of populations occupying different environments can provide insights into the ecological conditions affecting differences in parental strategies, including the relative contributions of males and females. Male and female parental strategies reflect the interplay between ecological conditions, the contributions of the social mate, and the needs of offspring. Climate is expected to underlie geographic variation in incubation and brooding behavior, and can thereby affect both the absolute and relative contributions of each sex to other aspects of parental care such as offspring provisioning. However, geographic variation in brooding behavior has received much less attention than variation in incubation attentiveness or provisioning rates. We compared parental behavior during the nestling period in populations of orangecrowned warblers Oreothlypis celata near the northern (64°N) and southern (33°N) boundaries of the breeding range. In Alaska, we found that males were responsible for the majority of food delivery whereas the sexes contributed equally to provisioning in California. Higher male provisioning in Alaska appeared to facilitate a higher proportion of time females spent brooding the nestlings. Surprisingly, differences in brooding between populations could not be explained by variation in ambient temperature, which was similar between populations during the nestling period. While these results represent a single population contrast, they suggest additional hypotheses for the ecological correlates and evolutionary drivers of geographic variation in brooding behavior, and the factors that shape the contributions of each sex.

Offspring growth and mobility in response to variation in parental care: a comparison between populations

“…Potential factors driving differences in the relationship between food delivery and growth between populations include delayed growth responses, higher maintenance costs, higher thermoregulatory costs, and/or lower food quality. Hatchling size, asymptotic mass (Sofaer et al 2013a), and mean ambient temperature (Yoon et al 2017) were similar between our study populations. However, nestlings' thermal environments differed, as females in California spent much less time brooding (Table 1).…”

“…4b). Time spent brooding by females was far higher in Alaska (Table 1), results that do not simply reflect variation in temperature (Yoon et al 2017). Consequences for nestlings also differed: nestlings in Alaska that received more food grew more quickly, whereas those in California did not (Fig.…”

“…A1). Ambient temperature during nest videos did not differ between populations (Yoon et al 2017). The timing of our video and nestling growth data collection during the late nestling stage was designed to control for both nestling age and nestling position along the growth curve (Supplementary material Appendix 1 Methods).…”

“…Our models of growth did not support a brooding effect within populations (Table 2), but the much lower brooding rate in California may mean nestlings there invest more energy in thermoregulation at the expense of growth. Brooding represents an underappreciated dimension of variation in parental expenditure, as providing thermal energy is energetically costly (as has been shown during incubation; Nord and Williams 2015), and differences in brooding did not simply reflect weather conditions (Yoon et al 2017). Quantifying variation in nestlings' thermal maintenance costs across geographic gradients is critical, as intrinsically slow growth rates may promote offspring quality, while slow growth arising from the thermal environment should not.…”

Nest predation risk influences a cavity-nesting passerine during the post-hatching care period

“…We also included ambient temperature (°C) and brood size (the number of nestlings per nest) as covariates in the model, which were likely to influence parental behaviours such as feeding and brooding nestlings54. Nest ID was included as a random effect to control for repeated measurements of the same nest boxes for the short-term risk.…”