Arrival times, laying dates, and reproductive success of Snowy Plovers in two habitats in coastal northern California

Habitat quality, as indexed by the reproductive success of individuals, can greatly influence population growth, especially for rare species near the limits of their range. Along the Pacific coast, the Snowy Plover (Charadrius alexandrinus nivosus) is a threatened species that, in recent years, has been breeding on both riverine gravel bars and ocean beaches in northern California. From 2001 to 2009, we compared the habitat characteristics, breeding phenology, reproductive success, and abundance of Western Snowy Plovers occupying these two habitats. Similar percentages of yearling and adult plovers returned to gravel bars and beaches, but plovers breeding on gravel bars arrived and initiated first clutches 2-3 weeks later than those breeding on beaches. Despite this delay, however, the mean annual fledging success of plovers on gravel bars (1.4 ± 0.4 (SD)) was double that on beaches (0.7 ± 0.3). Differences in cumulative reproductive success produced a stronger pattern. By their sixth year, males on gravel bars had fledged 14.5 ± 2.1 chicks, more than four times the number of young fledged by males on beaches (3.3 ± 3.1). Over 9 years, local population size decreased by about 75%, coincident with a shift in breeding distribution away from high-quality gravel bars to ocean beaches. This unexpected population decline and shift to poorer quality beaches may have been related to occasional low survival of plovers that over-winter exclusively on beaches in our study area. Consistently low productivity of plovers breeding on ocean beaches suggests the need for intensified management to ameliorate the negative impacts of predation and human activity on the recovery of this population.