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 (δ¹³C 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 ¹³C on tropical wintering areas; therefore, we predicted that if endogenous reserves are used to form eggs, then ¹³C would be enriched in first clutches relative to replacement clutches. We analyzed yolk (δ¹³C_YK) samples from successive first, second, and third clutches and blood plasma (δ¹³C_PL) sampled from females over the same time period. Values of δ¹³C_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, δ¹³C_PL, which represents the locally derived diet, varied seasonally in accordance with the shift in δ¹³C_YK. Our findings indicate female Redstarts are primarily income breeders, forming eggs from an isotopically variable diet during the breeding season.