Nests and nest sites of the San Miguel Island Song Sparrow
Nests and nest sites of the San Miguel Island ( SMI ) Song Sparrow ( Melospiza melodia micronyx ) are described; nests are compared with those of 16 other races of Song Sparrows. Bush lupins ( Lupinus albifrons ), coyote brush ( Baccharis pilularis ) and golden bush ( Haplopappus venetus ) were the shrubs used most commonly as nest sites by Song Sparrows on SMI. As a result of its location, the nest was effectively concealed from gray foxes ( Urocyon littoralis ), the major predator of this sparrow. Nest and nest site also moderated the combined chilling effects of cool air temperatures and strong northwesterly winds on the eggs and nestlings. Even in the absence of these moderating effects of the nest site, the energetic cost of incubation, estimated at 41-53% of the sparrow's resting metabolic rate, was modest. Twenty-nine percent of the canopy above the nest was open and as much as 73% of the nest cup was in the sun at midday, a time when surface temperatures of foliage, nest and nestlings sometimes exceeded 40 C. Whereas this exposure did not apparently reduce fledging success, it may explain why the incidence of addled eggs was so high in this population of Song Sparrows compared to others. Significant differences existed among races of Song Sparrows in the size, porosity and insulation of the nest. In most cases, these differences were not related to the latitude of the races' nesting areas.