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 investi gated relative to proximate and ultimate factors in 276 known-age Cassin's Auklets (Ptychoramphus aleuticus) breeding during 1981-1999 on Southeast Fa rallon Island, California. Breeding density in 446 study boxes declined sig nificantly during the period 1987-1999, confirming other information indica ting 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 signific ant linear increase with decreasing colony density, suggesting that increas ed stress on the entire population, such as that caused by decreasing food availability, may have prevented younger birds from attaining breeding cond ition at an early age. Median NDD was 8.84 m, ranging from 0-448.7 ms; two individuals of each sex recruited into their natal boxes. NDD showed a sign ificant 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 pro ximity to the natal site. Thus, although natal philopatry was strongly deve loped in the Cassin's Auklet, it did not seem to be adaptive. Those paradox ical results suggest that selective equilibrium may be acting on natal phil opatry in response to the population decline: a low-philopatry, low-cost (l ater breeding and increased survival) strategy has become increasingly adap tive in Cassin's Auklets, explaining increases in AFB and NDD with the decl ining breeding densities. The lack of sex-specific effect's on AFB and NDD suggests that an even sex-ratio and roughly equal roles in reproduction amo ng Cassin's Auklets exists on Southeast Farallon Island, and that inbreedin g avoidance may be counter-balanced by relatively low mate fidelity in this species.