AGE AT FIRST BREEDING AND NATAL DISPERSAL IN A DECLINING POPULATION OF CASSIN'S AUKLET

Age of first breeding (AFB) and natal dispersal distance (NDD) were investigated relative to proximate and ultimate factors in 276 known-age Cassin's Auklets (Ptychoramphus aleuticus) breeding during 1981–1999 on Southeast Farallon Island, California. Breeding density in 446 study boxes declined significantly during the period 1987–1999, confirming other information indicating a long-term decline (since at least the 1970s) in the population. Mean AFB was 3.34 years when sexes were combined, ranging from 2–10 (mean 3.36) years in females and 2–9 (mean 3.36) years in males. AFB showed a significant linear increase with decreasing colony density, suggesting that increased stress on the entire population, such as that caused by decreasing food availability, may have prevented younger birds from attaining breeding condition at an early age. Median NDD was 8.84 m, ranging from 0–448.7 m; two individuals of each sex recruited into their natal boxes. NDD showed a significant linear increase with decreasing breeding density but no significant linear or curvilinear correlations with mate fidelity, annual reproductive success, or lifetime reproductive output. A significant proportion (66.9%) of movements after recruitment (breeding dispersal) resulted in closer proximity to the natal site. Thus, although natal philopatry was strongly developed in the Cassin's Auklet, it did not seem to be adaptive. Those paradoxical results suggest that selective equilibrium may be acting on natal philopatry in response to the population decline: a low-philopatry, low-cost (later breeding and increased survival) strategy has become increasingly adaptive in Cassin's Auklets, explaining increases in AFB and NDD with the declining breeding densities. The lack of sex-specific effects on AFB and NDD suggests that an even sex-ratio and roughly equal roles in reproduction among Cassin's Auklets exists on Southeast Farallon Island, and that inbreeding avoidance may be counter-balanced by relatively low mate fidelity in this species.