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 (δ13CYK) samples from successive first, second, and third clutches and blood plasma (δ13CPL) sampled from females over the same time period. Values of δ13CYK 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, δ13CPL, which represents the locally derived diet, varied seasonally in accordance with the shift in δ13CYK. Our findings indicate female Redstarts are primarily income breeders, forming eggs from an isotopically variable diet during the breeding season.
Intense surveying of 5 study plots, banding, mist nets, monitoring, eggs collected