Nest predation, clutch size, and physiological costs of egg production in the song sparrow (Melospiza melodia)

We examined the effects of nest predation on both clutch size and the physiological cost of egg production using a clutch removal experiment in free-living song sparrows (Melospiza melodia), inducing “high nest predation” (HNP) females to produce many replacement clutches compared to “low nest predation” (LNP) females. In a preliminary analysis we investigated the utility of multiple measures to assess “physiological condition”, including inter-correlations between physiological traits, sex differences, and the relationship between physiological traits and reproductive performance (laying date). In our main study, experimental nest predation resulted in HNP females laying 11% fewer eggs per replacement clutch. As a result of frequent re-nesting, HNP birds produced 57% more clutches (3.0 vs 4.7) and laid 41% more eggs in total. Physiological condition of HNP females’ declined during the experiment associated with the increase in egg production and we suggest these results are consistent with physiological costs of egg production.