DENSITY AND REPRODUCTIVE SUCCESS OF CALIFORNIA TOWHEES

Models of habitat selection commonly assume that higher quality source habitats will be occupied at higher densities than sink habitats. We examined an apparent sink habitat for California Towhees (Pipilo criss alis) in which densities are greater than in nearby source habitats. W e estimated territory density using spot-mapping and monitored nests o f towhees in grazed and ungrazed oak pine woodland habitat. Breeding d ensity of California Towhees was higher in ungrazed oak pine woodlands than in grazed areas, yet birds in the ungrazed site experienced lowe r reproductive success. Predation during the nestling period was prima rily responsible for the lower nest success. Clutch size and the numbe r of young fledged were also lower in the understory. Grazed sites had greater cover of live oak than ungrazed sites, and successful nests w ere more often built in live oak than in other plant species. This pat tern could not be explained by dominant birds settling in grazed sites and excluding subdominant individuals because a large proportion of a dults continued to settle in the ungrazed area. Towhees may have perce ived the dense foliage of the ungrazed area as suitable due to abundan t nest sites, cover, and food, resulting in an ''ecological trap''. Th e resulting high density of birds ther may have contributed to density -dependent predation. Alternatively, towhees are not ideally adapted t o their habitats because of their sedentary habits and site tenacity t hat is not affected by persistent nest loss. We stress the need to exa mine the reproductive success and productivity of individual species w ithin specific habitat types.