NESTING MORTALITY OF CAROLINA CHICKADEES BREEDING IN NATURAL CAVITIES

Nest-holes are conventionally thought to enhance avian breeding success by the protection they offer against inclement weather and predation. Studies of birds nesting in natural cavities are rare, however, and much remains to be discerned about the reproductive costs and benefits of the hole-nesting habit. This study documents nesting mortality within a southern Illinois population of Carolina Chickadees (Parus carolinensis) breeding in natural cavities, and relates variation in mortality with variation in nest-hole structure and microclimate. Ambient-air vs. nest-hole temperature differentials were used as indices of nest-hole insulative capacity. Little variation in air-nest temperature differentials was found among nests and variation in nest-hole structure was not strongly correlated with variation in any of the indices of insulative variation. No total nest failures were attributable to ambient extremes. Predation was the greatest influence on nesting success, accounting for every case of total nest-loss (11/51 nests) and 78.7% of all egg and chick mortality. Nests excavated nearer the ground and in softer wood were preyed upon significantly more than higher or more solidly-housed nests. The availability of nest-holes appeared to decline at greater, thus safer, heights as the breeding season progressed. This apparent temporal decline in the availability of optimal nest-sites may reflect the subordinate status of chickadees among larger, more aggressive hole-nesting species. I suggest that chickadees may compensate for this low ranking status with their great nest-site selection plasticity as well as their ability to rapidly renest following nest destruction.