Dec. 7, 1998 For several decades, scientists studying the population dynamics of migratory birds have debated the relative importance of events in summer breeding areas versus tropical wintering grounds. Now, in a Dartmouth study reported in the Dec. 4 issue of the journal Science, Peter Marra and colleagues provide evidence that what happens in the tropical forests where migrant warblers spend their winters and what happens in the Northern woods where they spend the next summer are inextricably linked.
Research on the population dynamics of migratory birds has been hampered by the difficulty of tracking individual birds throughout their annual cycle across thousands of miles. By using carbon isotopes as a habitat signature, the investigators in this study were able to link American redstarts at Hubbard Brook Experimental Forest in the White Mountains of New Hampshire with the habitats they occupied the preceding winter. "We've found that the quality of the winter habitat affects not only when birds fly north to the breeding grounds but what condition they're in upon arrival. Both are crucial factors in how well the birds do on their summer breeding grounds," says Marra, who is now Postdoctoral Fellow at the Smithsonian Migratory Bird Center in Washington, D.C. "Clearly, natural selection is acting on these birds year-round."
Marra conducted this research as a graduate student with Richard Holmes, Harris Professor of Environmental Biology at Dartmouth, and their colleague, Keith Hobson, Research Scientist at the Prairie and Northern Wildlife Research Center in Saskatoon, Canada.
Previous studies have found that males in many migratory species arrive at the North American breeding grounds before females and that males in the best condition arrive earlier. These earlier arriving individuals have a competitive edge in reproducing: first choice of prime nesting sites and mates and an opportunity to replace eggs or nestlings that succumb to predation. But the factors that give rise to early arrival have been poorly understood.
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 the same habitat-specific signature. Similar results were obtained from redstarts wintering in wet and dry habitats in Honduras, suggesting that these isotopic differences are characteristic of wet and dry tropical habitats over a wide geographic range.
By testing for isotopes in American redstarts as they arrived at Hubbard Brook in the spring and settled into breeding areas, the investigators were able to determine that early arrivers were, indeed, those birds that had spent the winter in wet, tropical environments. Late arrivers carried the isotope signature of drier, scrub environments. Thus, even though the investigators did not know the actual wintering location of the individual redstarts arriving at Hubbard Brook, they could determine which type of habitat they had occupied during the preceding winter.
"These findings suggest that migratory birds in winter are competing for good quality habitats that are in short supply," says Holmes. "The consequences may show up at other stages of their annual cycle. Conservation efforts need to take into account this interplay of factors that affect these species throughout the year, including not only the quantity but also the quality of the habitats they occupy."
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