Breeding biology of Xantus's Murrelet at the San Benito Islands, Baja California, México

We report the first quantitative information on hatching success and assortative mating of Xantus's Murrelets Synthliboramphus hypoleucus in Mexico, obtained at the San Benito Islands, Baja California, during 2003 and 2004. The San Benito Islands are one of the southernmost murrelet colonies, do not have native Deer Mouse Peromyscus maniculatus egg predators, and support both murrelet subspecies. We compare murrelet breeding biology at San Benito Islands with that at the well-studied colony of Santa Barbara Island, California, near the northern end of the breeding range where Deer Mice are present. In 2003 and 2004, murrelets began laying eggs 3-6 weeks earlier on the San Benito Islands than at Santa Barbara Island. The mean number of eggs hatched per nest at the San Benito Islands was 0.55 ± 0.83 in 2003 and 0.72 ± 0.79 in 2004, significantly lower than at Santa Barbara Island in both years (1.21 ± 0.78 and 1.11 ± 0.94, respectively). Causes of hatching failure differed between colonies. Nest abandonment was most common at the San Benito Islands in both years. On Santa Barbara, mouse depredation was the largest cause of failure in 2003 and mouse depredation coupled with abandonment in 2004. Most breeding individuals at the San Benito Islands (62.5%) were S. h. scrippsi, 22.5% were S. h. hypoleucus and 15% were intermediates (n = 40). Based on facial patterns of 20 breeding pairs, most pairs (70%) were monotypic (II S. h. scrippsi and 3 S. h. hypoleucus), but five pairs involving intermediates and one mixed pair indicated some interbreeding between subspecies. Long-term monitoring is needed at the San Benito Islands to further assess breeding success in the southern portion of the range and especially to investigate the effects of avian predators and variable prey availability.