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Connecting the Migratory Dots

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Blackpoll Warbler / © Jeff Nadler

Blackpoll Warbler / © Jeff Nadler

Life would be so much easier for VCE biologists if only birds could talk. Where did this Bicknell’s Thrush come from? Where is it going? In what kind of habitat did this Blackpoll Warbler spend last winter? Answers to questions like these don’t come easily. Banding allows us to track the origins and whereabouts of many songbirds, but the low probability of recapturing them allows for precious few results.

WHY STUDY POPULATION CONNECTIVITY?

Natural selection acts on individual animals throughout the annual cycle, and events during each phase of the annual cycle likely influence subsequent events. For migratory animals, understanding these selection processes has been impossible because of our inability to follow individuals year-round and determine where breeding populations winter, where winter populations breed as well as their routes during migration. An understanding of these factors, which could operate in breeding and/or during non-breeding periods, that limit and ultimately determine bird abundance, is of urgent conservation concern. The most pressing need, and to date the most seemingly intractable problem, has been to determine the movement patterns and population connectivity of individuals between the breeding and wintering grounds. This is critical for understanding how limiting factors (e.g. habitat destruction, climate change, etc.) operate in different parts of the birds’ annual cycle and for determining population size and local abundance.

Migratory connectivity is defined as the amount of population mixing between summer-breeding, winter-non-breeding as well as the stop-over and migratory pathways between them. Although understanding space-use strategies and habitat preferences of Nearctic-Neotropical migratory songbirds has grown substantially over the last 20 years, knowledge of migratory connectivity remains poor. Information from bird banding has been limited by the scattered and irregular nature of banding returns. Despite banding millions of individual birds, there remains a relatively poor understanding of migratory connectivity for most species because of extremely low recapture rates. The number of other Nearctic-Neotropical songbird species in which an individual marked in one portion of its range has been recaptured in another is exceedingly small.

Understanding the timing and extent of avian population limitation and regulation is complicated in the case of migratory populations that spend different periods of their annual cycle in ecologically disparate regions. The “seasonal interaction hypothesis” was first put forth by Fretwell (1972), who argued that breeding density is determined by winter survival, which in turn is related to events that occur during the breeding cycle. Recent studies of American Redstarts (Setophaga ruticilla) support the seasonal interaction hypothesis. In this species, winter habitat quality determined physical condition and timing of spring migration departure, which influenced arrival date and physical condition on the breeding grounds. The quality of each individual’s winter habitat was determined by measuring stable carbon isotope levels shortly after arrival on breeding territories. Subsequent monitoring of redstart breeding demographics revealed a profound interaction between seasons. Robust tests of the seasonal interaction hypothesis such as this require detailed knowledge of migratory connectivity.

VCE has turned to innovative techniques and technology that offer greater insights into songbird ecology and conservation: the chemical analysis of bird tissue and miniaturized data loggers. It is the next best thing to a talking bird.

STABLE ISOTOPES

Some basic chemistry explains how feathers can reveal their secrets. Take the element hydrogen, for example. Recall from high-school chemistry that a hydrogen nucleus has a single proton. Yet there are trace amounts of another form called “heavy hydrogen,” or deuterium. Its nucleus has one proton and one neutron. It is a stable (non-radioactive) version of hydrogen with similar properties. We call these and other naturally occurring elements stable isotopes. They are relatively easy to measure in the environment—in rainwater, oceans, soil, plants, humans, and, by extension, birds and other wildlife.

So what can deuterium tell us about where a Bicknell’s Thrush might have hatched? Consider water. A small portion of the hydrogen atoms in water across the planet will naturally be deuterium. And it turns out that the ratio of deuterium to hydrogen in water is predictable in certain regions. In North America this ratio fits a pattern that generally tracks with latitude (see map below). Rainwater falling in Vermont, for example, will contain different deuterium ratios than rainwa- ter falling in Virginia.

As rainwater passes up the food chain from plant to insect and eventually birds, this ratio of deuterium in rainfall is retained. As a result, a bird’s feathers carry deuterium ratios that correspond to the rain falling where those feathers were grown. In other words, you are what you eat. And a bird’s diet, which incorporates deuterium, can help tell us where it’s been.

Stable isotopes also offer VCE new insights into the ecology of our signature mountain bird species—Bicknell’s Thrush— here in North America and on its wintering sites in Hispaniola. As with American Pipits, VCE biologists use a clipped tail feather to examine how Bicknell’s Thrushes distribute themselves across various breeding sites—what’s called natal dispersal. Over the course of our 15 years working on this songbird, we have collected roughly 2,000 feather samples from across its breeding range—from the Gaspé Peninsula in Quebec to the Catskills. Do thrushes that hatch from breeding sites in Quebec disperse to subsequently breed in the Adirondacks?

We have preliminary evidence that there is indeed some beneficial dispersal of Bicknell’s Thrush genes across the breeding range. But stable isotope analyses, using deuterium, will provide more details and allow us to discover whether any populations are genetically isolated. Knowing that will help us design better conservation strategies for Bicknell’s Thrush—either across its entire breeding and wintering ranges or specifically targeting isolated populations.

Feathers aren’t the only tissues that speak volumes on a bird’s behalf. So do toenails. VCE has isotopic evidence that wintering Bicknell’s Thrushes may segregate by habitat—in cloud forests, where they can find ample insects for feeding, or lower-quality second-growth habitat, where fruit may comprise a larger part of the winter diet. Our findings also suggest that males may occupy the preferred montane winter habitat and females the less desirable regenerating forest, which may explain the skewed 3:1 male-to-female sex ratio observed on the breeding grounds.

Thrushes arriving on the breeding grounds each spring carry evidence, in their toenails, of the winter habitat they occupied. Deuterium levels will help us determine whether a given thrush was feeding in the montane cloud forests or in lowland habitats. Additionally, an isotope of nitrogen offers insights into whether a bird’s diet was largely fruit or insect. Our intent is to investigate any links between breeding success here in North America and where Bicknell’s Thrushes spend the winter and what they eat. The analysis of stable isotopes in bird tissue is a new and powerful tool that can help us direct conservation efforts.

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