A two-sex integrated population model reveals intersexual differences in life history strategies in Cooper's hawks

Integrated population models (IPMs) combine population counts with demographic data to improve the precision of estimates of population size and demographic rates. IPMs can also provide information on demographic parameters for which data are lacking (e.g., immigration) if other sufficient information is available. IPMs often use demographic data from one sex, usually females, implicitly assuming the chosen sex adequately represents the species' life history; for highly size-dimorphic raptors, that assumption may not hold true. We developed a two-sex IPM for a New Mexico population of the highly size-dimorphic Cooper's hawk (Accipiter cooperii) from 2011 to 2020 and compared estimates of life history characteristics between sexes. Because we had data to directly estimate sex- and age-specific probabilities of breeding, fecundity, survival, brood sex ratios, and emigration rates, we could indirectly estimate age-specific immigration rates for both sexes. Our two-sex IPM revealed that population growth was most strongly associated with increased immigration and decreased first-year (FY) emigration in females and with after-first-year (AFY) survival in males. Most males that were recruited as new breeders in our study area were AFY residents, whereas most female recruits were AFY immigrants. All females that fledged in our study area and survived until their first breeding season bred in their first year, whereas only 3% of surviving FY males bred. We found evidence of density dependence in the survival of AFY males (r = -0.11, 95% credible interval = -0.36 to -0.03) and in AFY female immigration (r = -0.17, 95% credible interval = -0.27 to -0.12). Our findings reveal that male and female Cooper's hawks differ in their response to population density, and in how they contribute to population growth. Thus, sexual differences in vital rates can be an important consideration in raptor population models.