Seasonal reproductive pattern of white-winged Crossbills in interior Alaska

It has been proposed that crossbills (Loxia spp.) are opportunistic breeders that time the onset of their reproductive cycle based on food availability rather than photoperiod, as is the case in most other bird species. Crossbills are able to nest throughout most of the year at middle latitudes, but the reproductive biology of these birds at high latitudes remains poorly known. A total of 469 White-winged Crossbills (Loxia leucoptera) was caught in Fairbanks, Alaska during a 2.5-yr period. Based on the proportion of juvenile birds in the sampled population, the proportion of adult females with a brood patch, and seasonal changes in adult male cloacal protuberance size, it appears that interior Alaska White-winged Crossbills breed only in spring and early summer. Timing of reproduction of crossbills breeding at high latitudes may be limited by environmental factors (potentially, ambient temperature and/or photoperiod) rather than by food availability.