Age distribution and longevity in a breeding population of Swainson’s Hawks, Buteo swainsoni

Age structure of breeding populations may be important in the evaluation of population health and may indicate when populations are susceptible to decline. However, few long-lived species have well understood age distributions. We investigated the breeding age distribution in a long-lived species, the Swainson’s Hawk, in a population that was monitored for 40 years and whose population has grown fourfold over the course of the study. We observed 279 known-aged Swainson’s Hawks 1315 times during our study. The average age of breeding Swainson’s Hawks in the Butte Valley across all surveyed years was 8.3 ± 4.3 years. The oldest known-aged individual in our population was 26 years old and the average age at death (individuals presumed dead) was 9.2 ± 5.5 years old (n = 160). There was no difference in longevity between males (mean ± SE; 9.7 ± 0.6 years old, n = 85) and females (9.0 ± 0.7 years old, n = 69). The proportion of subadults breeding (n = 56) was related to population growth, with a greater proportion of subadults breeding when the growth rate was higher.