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Life-history theory suggests that delayed maturation is beneficial to birds when costs of breeding early in life are high. We compared selected aspects of the biology of male Cooper's Hawks (Accipiter cooperii) that began breeding at 1 year of age (SY males) vs. males that began breeding ≥2 years of age (ASY males) in an effort to elucidate what renders breeding of SY males rare. Of the 732 males of known age whose breeding we studied over 32 years (1980-2011) 13 (2%) were SY and 719 (98%) were ASY. During this period, the incidence of breeding of SY males changed markedly, as 12 of the instances of breeding of SY males were recorded in the first half of our study, only 1 in the latter half. We suggest that SY males could acquire nest sites more readily in the first half of the study when the breeding population of ASY males was apparently lower. Males breeding in their second year were on average similar in size to ASY males but were molting more extensively than were ASY males. ASY males lived longer than males breeding in their second year, and maximum lifetime production of young was greater, so SY males may have sacrificed greater longevity for breeding early. A strategy of facultative breeding in response to the constraints of higher numbers of older breeding males and restraints of trade-offs in energy allocation may explain why SY male Cooper's Hawks rarely breed. [ABSTRACT FROM AUTHOR]

The article focuses on a study that compares morphological data on breeding Cooper's Hawks caught in southern Vancouver Island, British Columbia, central and southeastern Wisconsin, and northwestern and north-central North Dakota from 1999 to 2003. It measures the woodland hawks' body mass, tail length, wing chord, hallux length, and culmen length. Results show differences in breeding Cooper's Hawks' body mass and body structure. It adds that geographic segregation affect regional variations.