Biology Reference
In-Depth Information
has spent the winter and what they have eaten. She can obtain the critical information from
the stable-isotope fingerprints that they've carried with them to Michigan.
Ultimately, the critical data Sarah needed were the standard metrics of animal biology: re-
productive success. In this case, it was how many young each pair of Kirtland's warblers
raised to fledge. Sarah and her team spent June and July searching for nests. Each time they
found one, they recorded a GPS location; later, they returned to the site to count the num-
ber of nestlings present. Sometimes this scrutiny didn't go over well with Mrs. Kirtland.
If flushed off their nests, the angry mother birds would perch right next to the researchers
and practically peck their hands while they counted eggs. After fledging should have oc-
curred, a researcher returned to the nesting territory to confirm it by looking and listening for
fledglings, angry chirping parents, or parents carrying food after the nest is empty. Not all is
Disneyesque. Sarah reported that about 25 percent of the nests to which the researchers re-
turned had been depredated by egg and nestling eaters.
A warbler flew up to its singing perch. I asked Sarah the question the dissertation commit-
tee at the University of Maryland would likely put to her: “So what have you found so far,
and how will it help managers?” Sarah's isotope analyses gave her the answers. After five
field seasons, her preliminary results showed that Kirtland's warblers originating in wetter
winter habitats, as indicated by more negative carbon-isotope ratios, arrive first, although the
pattern varies by year. The early birds have first pick of the best territories and mates and
have more time to breed. This additional time on the breeding ground offers them a chance
to renest if predation occurs or even to attempt a second nest if time permits. Also, hatching
failure and predation tend to be less earlier in the season. “The bottom line, I've found, is
that the arrival date of males is significantly correlated to the greater number of fledglings
they raise that year,” Sarah said. She also found that following wetter March weather in the
Bahamas—the month prior to spring departure—Kirtland's warblers tend to arrive sooner in
Michigan, be in better body condition, initiate nests sooner, and fledge more offspring.
Considerable research has linked winter rainfall, food supply, and overwinter condition of
other migratory birds. Now Sarah was finding that these factors carry over to affect birds
in the subsequent breeding season as well. She hoped her findings would help managers
choose the best-quality habitats in the Bahamas to conserve—those that retain moisture better
through the winter dry season. This factor would be especially important if long-term drying
trends in the Caribbean due to climate change occur as predicted. For the first time, biologists
realized that to understand how the population size of Kirtland's warblers is regulated and
what the species' prospects are, they need to understand what the limiting factors are—such
as winter climate and habitat—at each stage in the annual cycle.
