Effects of age, breeding experience, mate fidelity and site fidelity on breeding performance in a declining population of Cassin’s auklets

Summary 1. We examined how mate and site fidelity varied with age, experience and sex, and how age, breeding experience, mate experience, site experience and sex affected annual reproductive success and lifetime reproductive output in a declining population of Cassin’s auklets ( Ptychoramphus aleuticus ). Our 276 study birds were 2‐14 years of age, recruited at age 2‐12 years, and had 0‐11 years’ breeding experience, 0‐8 years’ experience with the same mate and 0‐11 years’ experience in the same nest box. 2. Mate fidelity was significantly greater with increasing age in males but not females. There was also a significant negative relationship between mate fidelity and breeding density (as measured by proportion of box occupancy); i.e. the lower the breeding density the higher the incidence of breeding with the same mate. 3. Site fidelity showed significant linear and curvilinear increases with age that were significant in females but not males. There was also a significant negative relationship between site fidelity and breeding density; i.e. the lower the breeding density the higher the incidence of breeding at the same site. 4. Previous breeding experience had no effect on either mate fidelity or site fidelity, and both mate and site fidelity were significantly lower after a breeding season was skipped. In addition, mate fidelity was significantly lower when a site was switched and vice versa. 5. Lifetime reproductive output increased significantly with mate fidelity but showed no relationship with site fidelity. This suggests that fitness is optimized more through mate selection than site selection and that mate fidelity is not a by-product of site fidelity. 6. Annual reproductive success showed a significant linear increase with age in males but not females, and a strong parabolic relationship with breeding experience that was significant in both sexes and significantly greater in males than females. 7. These results suggest that (i) males may be more responsible for mate selection and females for site selection; (ii) improved foraging experience with age and a cost of reproduction may be more important factors in males than females; and (iii) reproductive success may be optimized by behaviour of the male rather than the female. 8. Controlling for the age and experience terms of both parents, experience with a mate had a significant positive linear effect on annual reproductive success. This suggests that mate fidelity is adaptive in Cassin’s auklets, and that studies examining the effects of age and experience on reproductive performance should separately consider the duration of the pair bond. 9. Controlling for all other variables, neither experience at a breeding site nor breeding density showed significant correlations with reproductive success. 10. We suggest that reductions in food supply, which correlate with reduced breeding densities, may prevent all but the highest quality breeders (those which have already established a pair bond) from reproducing, and that the increase in quality offsets the reduction in food availability.