The Kingdom of Rarities (14 page)

Among the birders in our makeshift Grayling caravan was Sarah Rockwell, a University of Maryland doctoral student who in 2006
had chosen this bird to be the center of her life for the next five years. When I first heard of Sarah and her work through a colleague, I wondered why in the world she would undertake such a study. Successful PhD dissertations in biology are all about large sample sizes. Piles of data are essential for the rigorous statistical analyses and testing of hypotheses that graduate committees demand from students. Indeed, professors typically divert their more idealistic or naive students from the futility of field studies on rare species.

A major reason she became interested in studying Kirtland's warblers, Sarah explained, is that she really liked the idea of contributing to conservation efforts in her home state. When working with an endangered species, any data one can gather on its ecology might be immediately useful to managers. “Besides,” she noted, “I actually have no problem with sample 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 success of Kirtland's nesting in jack pine groves that had naturally regenerated following wildfires. 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 species. 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 reproductive 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 neotropical 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 collisions 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 species 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 concentration 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, disturbance meant fires or hurricanes. Today, however, it's human activities. “So we hypothesized that disturbance by humans actually favors the spread of fruit plants used by the warbler.”

The test of their hypothesis led to one of the most surprising and counterintuitive discoveries in the history of ecology. “In the
Bahamas,” Joe told me, “at least for Kirtland's warblers, goats are kind of a blessing!” The ecologist's scourge of the earth, the common goat, was resurrected as a hero. Here's why. The goats ignore the leaves and stems of plants that supply the Kirtland's with fruit and instead consume the competing plants. Thus they delay plant succession, the process by which a natural community moves from a simple level of organization to a more complex assemblage. Succession is a natural process that typically occurs after some form of disturbance has simplified the system. “In fact,” Joe continued, “our work shows that goat farms make great Kirtland's habitat.” There are probably dozens of biologists who work in Hawaii, the Galápagos Islands, New Zealand, and other islands where goats were never part of the native mammalian fauna who would pay Joe handsomely to keep quiet. Goats are often the worst culprits in the extinction of rare native plants, and they typically outcompete native herbivores for forage. On Eleuthera, at least for Kirtland's warblers, they were a force for good, especially on parcels where farmers rotated their goat herds among fenced rangelands to prevent overbrowsing.

Many questions remain unanswered about the Bahamian winter getaway. For example, as in Grayling, natural fire regimes in the Bahamas may provide the disturbance essential to support the fruiting shrubs upon which Kirtland's warblers depend. But are today's managed fire regimes too different from wildfires of the evolutionary history of the Bahamas? Are goat farms and smart, “green” bulldozing practices enough to offset the suppression of fire? Finally, little is known about the habitat needs of the warblers during their critical three-week migration from Eleuthera to Grayling.

As we walked through the pine grove in Grayling, Sarah reflected on my first impressions of jack pine landscapes. “I had the same response as you did when I first arrived!” she exclaimed with a laugh. “It may not be very spectacular at first glance, but the more time you spend in the jack pine forest, the more you appreciate that it
has its own beauty. You can be on a little hill in the early morning, watching the mist that settles in the low places when the sun rises. I often see spruce grouse, upland sandpipers, porcupines, fawns, occasionally a coyote. There really is a lot to like about it!” Sarah was starting to think like her study bird.

Her biggest challenge had been learning to nest-search. The difficulty of finding nests of a study species, or the inability to do so, has probably derailed more potential PhD dissertations in ornithology than any other obstacle. Sarah stopped to look under a pine. “The nests 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 environments—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 ratio 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 habitat type and diet. Researchers have called isotopes in birds “flying fingerprints.” The real advantage of this technique is that Sarah doesn't need to know exactly where each of her birds 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: reproductive 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 number 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 occurred, 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 returned had been depredated by egg and nestling eaters.

A warbler flew up to its singing perch. I asked Sarah the question the dissertation committee 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.