Biology Reference
In-Depth Information
are so well hidden that you can't find them through systematic searching or looking on the
ground in a known territory. You have to use parental behavior as your key.”
First, she had to spot a female carrying nest material or a male carrying food and follow the
bird back to the nest. The parents, though, saw her as a predator and didn't want her to find
their vulnerable eggs or young. “If you're not sneaky,” Sarah said, “they'll never go to the
nest; they'll just fly around in circles with the food.” She learned to hide behind jack pines
and crawl on her stomach to see under the branches and, eventually, watch parents go to their
nest.
As she grew more adept at keeping a low profile, she picked up other tricks. For one,
the males tend to sing while they deliver food, but the quality of the song is different, more
muffled and gurgly. The closer they get to the nest, the quieter and sweeter the song becomes.
“Then the male goes silent while feeding the incubating female or nestlings, and after a few
seconds he'll pop back up high in a jack pine, singing his regular, loud-and-proud song. You
can almost locate a nest by ear alone.” Of course, it takes experience. In her first year, Sarah
and her team found sixty-three nests. The next year, they almost doubled that total.
To learn about the effects of overwintering and summer survival, Sarah and her colleagues
net as many birds as they can, band and measure them, and collect blood samples. This
provides data on body condition, and she uses the blood for stable-isotope analysis to help
infer winter diet and habitat quality.
Stable-isotope analysis, an exciting new approach in ecology, involves measuring the ratio
of the heavy form of an element to its lighter form. For instance, carbon atoms with an atomic
mass of 12 are the most common in nature, but a small proportion have an atomic mass of
13 because of an extra neutron. “What's really interesting is that isotopes of some elements
vary in a regular way that is useful to ecologists,” Sarah explained. “For example, the ratio of
13
C [carbon 13] to
12
C [carbon 12] in plants is greater in dry habitats than in wetter environ-
ments—we call this being more enriched in δ
13
C” (a measure of the ratio of stable isostopes
carbon 13 to carbon 12). Habitat quality for many overwintering migrants varies along this
same wet-to-dry gradient that can be measured by δ
13
C. A habitat-specific stable-carbon ra-
tio is incorporated into birds' tissues through the fruits and insects they eat during the winter.
Nitrogen is another useful element; the ratio of
15
N (nitrogen 15) to
14
N (nitrogen 14) varies
by trophic level, with organisms further up the food chain becoming more enriched in δ
15
N
(the measure of the ratio between heavy and lighter nitrogen). So Kirtland's warblers that
consume a greater proportion of insects than fruit will have a higher δ
15
N signature in their
tissues.
When Sarah captures a bird soon after its arrival in Michigan and takes a blood sample, its
red blood cells retain the stable-carbon and stable-nitrogen signatures of its late winter habit-
at type and diet. Researchers have called isotopes in birds “flying fingerprints.” The real ad-
vantage of this technique is that Sarah doesn't need to know exactly where each of her birds
