May 31, 2001 Amazon forest birds are particularly sensitive to habitat fragmentation -- even patches as big as 250 acres are missing many species -- but no one knows why. New research offers a clue: birds in fragments have slower-growing feathers. This suggests that they are more stressed, which could decrease survival and reproduction.
"There might be physiological consequences for birds that live in fragments," says Jeff Stratford of Auburn University in Auburn, Alabama, who did this work with Philip Stouffer of Southeastern Louisiana University in Hammond, Louisiana. This work is in the June issue of Conservation Biology.
This the first evidence that fragmentation may have direct physiological effects.
Stratford and Stouffer compared feathers from two common bird species (the white-crowned manakin and the wedge-billed woodcreeper) that were captured in either forest fragments or continuous forest near Manaus, Brazil. To determine how fast the feathers had grown, the researchers measured the daily growth bars. Healthier birds presumably have feathers with wider growth bars.
The researchers found that feathers from birds captured in forest fragments had grown slower: for instance, feathers from birds in 2.5-acre fragments grew 10% slower than those from birds in continuous forest.
Why do birds in fragments have slower growing feathers? Stratford and Stouffer ruled out the obvious possibility of insufficient food. The manakin's diet includes fruit and the woodcreeper eats insects living on tree trunks and branches, and fragmentation does not reduce either type of food.
In fact, fragmentation may not even affect feather growth directly. Rather, less robust birds may be more likely to end up in undesirable habitats like fragments. This is supported by the finding that manakins were rarely recaptured in fragments, implying that they had grown their feathers in continuous forest. "We suggest that these birds are social subordinates that are wandering about the landscape," says Stratford.
Birds in fragmented habitats elsewhere may be even more stressed because the fragmentation in this study was relatively mild. For instance, the forest fragments were separated by pasture and regenerating forest rather than by parking lots and houses. "Even though things look bad, this is a 'best case scenario'," says Stratford.
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