The Relation of Breeding Schedule and Clutch Size to Food Supply in the Rufous-Sided Towhee

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Abstract
The relationship between food supply and breeding schedule and clutch size in rufous-sided towhees Pipilo erythrophthalmus] in 2 habitats. One population of towhees inhabited a mesic oak-hickory forest (HMF) in New Jersey [USA] and 2 other populations were in xeric pine barrens in New Jersey (LSF) and on Long Island, New York (BNL) [USA]. Samples of invertebrates in the litter and on low vegetation in the New Jersey study areas showed that numbers and biomass of potential prey were several-fold higher in the mesic habitat than in the xeric one. Differences in biomass density were noticed as early as late April-early May when returning towhees were establishing territories and pairing. Towhees in HMF commenced egg-laying about 1 to 2 wk earlier on the average than those in the pine barrens. This difference was correlated with the earlier development of foliage and the concomitantly earlier appearance of additional invertebrates on and near the ground in HMF. The timing of 1st nests in LSF suggests that towhees there started breeding as early as possible and that poor feeding conditions at the time of egg-formation may have prevented them from initiating nests earlier than they did. Mean clutch size of early nests in HMF was significantly larger than the mean of those in pine barrens started at the same time. Early clutch size vareid directly with food supply between the 2 habitats. The importance of food as a possible proximate factor affecting breeding schedule and clutch size and the problem of plasticity in clutch size are discussed.

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