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size. Kirtland's are quite abundant in the right habitat patches. I have found up to one male
per hectare in the busiest locations.”
Sarah was not the first to study the Kirtland's for her doctorate. In the early 1990s, Carol
Bocetti compared the breeding success of Kirtland's warblers nesting in managed (that is,
harvested) jack pine plantations that had been started a decade earlier with the breeding suc-
cess of Kirtland's nesting in jack pine groves that had naturally regenerated following wild-
fires. Remarkably, Carol found that the density of territorial males was similar in managed
and natural stands.
Carol's work and that of others illuminated the basics of the warbler's summer range and
breeding biology. Meanwhile, in the Caribbean, Joe Wunderle, a noted tropical ornithologist,
and his research team had started to put together a picture of the winter ecology of the spe-
cies. Sarah saw her opening when she heard of Joe's study. “Are there aspects of their winter
life,” she asked—the climate where they stay, their habitat use, their diet—“that affect repro-
ductive success and survival?”
Sarah had predicted that birds that arrive early on the breeding grounds would be the most
successful parents because they would occupy the best sites during the long winter stay in
the Bahamas. For a long time, biologists were unable to track individual birds throughout the
year or match wintering conditions to breeding success up north. Some who studied neotrop-
ical migratory birds simply assumed that these species led separate lives in the two locales
and that their winter behavior had no effect on their summer activity. Sarah's study questions,
supported by collaboration with Joe's Caribbean-based team, required her to follow the fate
of her birds through an entire annual cycle.
Many dangers confront these long-distance night travelers, about 60 percent of which die
in their first year, Sarah noted. Along their 2,500-kilometer route, they must avoid colli-
sions with radio towers, transmission lines, windows of homes and office buildings, and other
obstacles. En route and on the home breeding grounds, migratory songbirds must avoid being
eaten by predators. The fate of migratory Kirtland's was no different from that of many spe-
cies of migratory songbirds. But for extreme rarities with breeding numbers as low as theirs,
the loss of many adults could be devastating.
Joe had his own set of unanswered questions that meshed with Sarah's. The first addressed
what winter habitat qualities the birds preferred. Over several years, his team had identified
the conditions the warblers favored in late winter just prior to migration, which might have a
bearing on their rarity and their survival. On Eleuthera Island, where he found a small con-
centration of wintering Kirtland's, the birds eat a lot of fruit and prefer snowberries, black
torch berries, and the fruits of wild sage. That means the birds hang out in second-growth
areas where these plants occur. Habitats resulting from second growth eventually diminish in
size, however, unless they are preserved by some big disturbance event. Historically, disturb-
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