Capital versus income breeding in a migratory passerine bird: evidence from stable-carbon isotopes

Birds meet the energetic demands of egg formation by using either endogenous reserves (capital breeding) or recently ingested nutrients (income breeding). Examining these strategies in migratory birds has been difficult because of the inability to assign the origin of egg nutrients. We used stable-carbon isotopes (delta C-13 values) to determine whether American Redstarts (Setophaga ruticilla (L., 1758)) form eggs using endogenous reserves acquired on tropical wintering areas or local dietary sources. Redstart diet tends to be enriched in C-13 on tropical wintering areas; therefore, we predicted that if endogenous reserves are used to form eggs, then C-13 would be enriched in first clutches relative to replacement clutches. We analyzed yolk (delta C-13(YK)) samples from successive first, second, and third clutches and blood plasma (delta C-13(PL)) sampled from females over the same time period. Values of delta C-13(YK) in first-clutch and second-clutch eggs were significantly more positive than those in third-clutch eggs. Although the isotopic shift in yolk was in the direction predicted for a mixed capital-income strategy, delta C-13(PL), which represents the locally derived diet, varied seasonally in accordance with the shift in delta C-13(YK). Our findings indicate female Redstarts are primarily income breeders, forming eggs from an isotopically variable diet during the breeding season.