Breeding efficiency: a metric for assessing habitat quality and individual performance?

Assessment of habitat quality and individual reproductive performance are vital facets of conservation, which often prompt management actions to increase population size. Here, we present a metric (breeding efficiency; BE) for gauging productivity (of sites or individuals), which uses the ratio of chicks fledged to eggs laid; BE can be further dissected into egg efficiency (EE; hatchlings:eggs) and chick efficiency (CE; fledglings:hatchlings). Our data come from intensive, long-term (i.e., 18 yr) monitoring of a marked population of the Pacific coast Snowy Plover Charadrius nivosus, which is listed as threatened under the U.S. Endangered Species Act. Overall, BE averaged 0.19 ± 0.25 across sites and years (n = 126). BE correlated negatively with the number of nests (–0.13, P = 0.07), which likely stemmed from frequent renesting following clutch failure at some sites. BE also differed among sites, which suggests that habitat quality varied with predation, the principal cause of reproductive failure in the population. A
strong positive correlation (0.92) between BE and per capita fledging success (across sites and years; n = 106, excluding sites with predator exclosures at nests) suggests that BE can be applied to populations that are unmarked and less intensively monitored. The latter case may be accomplished by locating nests or incubating adults (assuming a completed clutch) and tallying fledged chicks at intervals of several weeks.