Offspring size as an index of habitat degradation

Abstract

Disturbances that shift a community away from its potential natural state may also degrade the quality of that community for some species. Having an index to measure changes in habitat quality resulting from such disturbances would be useful in assessing the impact of human activities on native fauna. We propose that average egg mass per clutch and offspring size for a population in a particular habitat may be a useful index of habitat quality, and perhaps degradation, for that population relative to the status of populations occupying other similar habitats in that region. We studied American Robins (Turdus migratorius) breeding along streams in three canyons on the western side of the Toiyabe Mountains of central Nevada, USA. The level of habi- tat degradation associated with cattle grazing and other human activities was deter- mined a priori based on soil and understory vegetation characteristics. The density of adult birds and their body condition did not differ among canyons with differing habi- tat quality, nor did clutch size or brood size at day 8. However, nests containing larger eggs and chicks were associated with canyons assessed as having a higher quality, or lower level of degradation.