Energetic consequences of sex-related habitat segregation in wintering American kestrels (Falco sparverius)

Abstract

Behavioural dominance can cause individuals to use less-preferred habitats, with potentially important life-history consequences. In the American kestrel, Falco sparverius, females exclude males from preferred open areas; I hypothesized that this sex-related habitat segregation leads to energetic consequences for males. I predicted that males would show decreased body condition over winter, while females would not, and that females would maintain larger net-energy surpluses than would males. Working in southeastern Pennsylvania, between 1991 and 1995, I conducted 150 behavioural observations and measured body condition of 235 wintering kestrels. Male kestrels maintained a lower body condition (residuals of a regression of mass vs. size) than did females and showed a decrease in mean body condition over winter, which females did not. I estimated that females had larger daily energy expenditures than did males (248 vs. 195 kJ per bird per day, ratio 1.27) but also had larger daily energy intakes (537 vs. 322 kJ/day, ratio 1.67). Females had larger net energy surpluses than did males (288 vs. 126 kJ/day, ratio 2.27), particularly when temperatures were <0°C. I conclude that differences in energetics are due in part to habitat differences and that males may suffer consequences by being forced to use less-preferred habitats.