One of the early conservation challenges in rebuilding the Kirtland's Warbler population was providing sufficient nesting habitat. These birds nest in jack pines that are five to fifteen feet tall – in other words, very young forests. Jack pines need fire for their seeds to germinate, so stands of young jack pines are relatively uncommon in a context where forest fires are carefully controlled. Fortunately, conservationists found ways around this problem, and the Kirtland's Warbler population has recovered steadily since its nadir.
|Kirtland's Warbler / Photo credit: Dave Currie|
But those extensive efforts only occurred at the Kirtland's summer home, so a team of researchers reviewed the conditions of many a warbler's winter home - the Bahamian island of Eleuthera. They did this by painstakingly putting together Landsat data to create cloud-free images of the isle's forest cover....
The researchers did this not just once, but ten times, obtaining a record that spans a 30-year time period. According to Helmer, this allows them to tell how long it had been since the forest was last disturbed by fire, crops or grazing.
What the scientists discovered was that, like in their summer homes, Kirtland's warblers are found in young forests. On Eleuthera, these forests only occur after a disturbance of some sort - like fire, clearing for agriculture, or grazing. And grazing turns out to be a disturbance the warbler can live with just fine. Old forest whose underbrush has been munched on by goats provides the most suitable habitat for warblers, said Helmer.
The results, published in this month's issue of Biotropica, suggest that goat grazing stunts the forest regrowth, so that the tree height doesn't exceed the height beyond which important fruit-bearing forage tree species are shaded out by taller woody species. Helmer said that understanding how and where the warbler's winter habitat occurs will help conservation efforts in the Bahamas.
Helmer said that a unique feature of warbler's winter habitat is that the age of this forest correlates very strongly with its height.