Migratory songbird breeding linked to wint...( or nestlings that succumb to predation...)

To test their hypothesis that events taking place on the birds' wintering grounds influence their condition and arrival time at the breeding grounds, Marra and colleagues studied American redstarts in two of their winter habitats in Jamaica - a moist, black mangrove forest and a drier, scrub habitat of small second-growth trees. The sex- and age-ratios of the redstarts in these habitats were quite different: older males, who are more aggressive, prevailed in the lush mangrove forest, forcing females and young males into the drier scrub habitat. As the season progressed, the researchers found that birds in the wet forest maintained body mass and showed other signs of being in robust physical condition, whereas those in the scrub habitat lost body mass and showed signs of deteriorating condition. These factors had little effect on the birds' winter survival, but birds in poorer condition - mostly females and young males in the scrub habitats - were slower to depart for the breeding grounds.

To link winter habitats with events on the breeding grounds, the investigators used a novel tool: carbon isotope signatures which varied among the winter habitats. Carbon, like other elements, occurs in nature in different atomic weights, called isotopes. Plants take up carbon from the atmosphere during photosynthesis and selectively incorporate different carbon isotopes into their tissues. The pattern, or signature, of carbon isotopes differs in a predictable way for plants in moist or wetter environments compared to those in drier environments. Marra and colleagues tested the insects feeding on plants in each Jamaican habitat and found that they incorporated the isotope signature of the plants in those habitats into their tissues. Redstarts feeding on these insects subsequently were found to have th
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Contact: Nancy Serrell
nancy.serrell@dartmouth.edu
603-646-2117
Dartmouth College
3-Dec-1998