Finding the Right Spot

When the tiny black-throated blue warbler arrives at its summer home in northern North America, it has to make a crucial decision: where to set up its nesting territory. If the habitat is good, food will be plentiful and the warbler has a better chance of surviving the season and raising a family.

But how does it know, when it arrives from its winter quarters in the Caribbean, where food will be plentiful several weeks later when its young have hatched?

Scientists at the Hubbard Brook Research Center in New Hampshire have been studying this common bird since the 1960s . The study site centers around a stream valley surrounded by the steep slopes of the nearby mountains.

Hubbard Brook study site in New Hampshire

The birds typically arrive in late April to a chilly landscape with the leaves just beginning to emerge. They promptly set up territories throughout the valley and on the slopes. By mid-June, most of the birds have decided that the best place to raise a family is in a dense stand of hobblebush (Viburnum alnifolium) halfway up the mountain slope. This shrub offers plenty of insects for food and is its large leaves and intricate, twining stems provide excellent places to hide a nest.

Can you find the nest in the picture below? (Hint: you can zoom into the image by clicking on it.)

Black-throated blue warblers don't live very long—just a few years—so choosing the right spot to raise a family is a vital decision. By waiting to nest until they can tell where food and cover will be most abundant, the birds maximize their chance of survival and ability to reproduce.

This article summarizes the information in this publication:

Dynamic occupancy models reveal within-breeding season movement up a habitat quality gradient by a migratory songbird. 2008. Matthew G. Betts, Nicholas L. Rodenhouse, T. Scott Sillett, Patrick J. Doran, and Richard T. Holmes. Ecography 31(5) 592–600.

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The timing of settlement decisions likely influences the quality of breeding site choices.This is particularly the case in migratory birds, because the conditions that enhance breeding success are often not apparent upon arrival after migration. A strategy that addresses this problem is to adjust settlement decisions when reliable information becomes available. We used a new indirect method – dynamic site occupancy modeling – to estimate apparent movement of black-throated blue warblers Dendroica caerulescens among sites within a breeding season. Because individuals should disperse to sites that maximize their fitness, we hypothesized that warblers would move up a habitat quality gradient when opportunities arose. For our study species, that would involve moving into sites with greater shrub density and at higher elevation within northern hardwoods forest, as these two features are positively correlated with reproduction and apparent survival in this species. Although the probability of site occupancy in our study landscape remained consistent throughout the breeding season (range: 0.66–0.69), occupancy models revealed substantial support for apparent movement of individuals within the breeding season. The mean probability of emigration from a point count site was 0.21 (±0.03 SE), and the mean probability of immigration to a site not previously occupied was 0.51 (±0.05 SE). The spatial distribution of this movement was a function of habitat quality. A portion of the black-throated blue warbler population appears to arrive on the breeding grounds and settle initially in sub-optimal habitat, moving subsequently into high quality densely shrubbed habitat at higher elevations. This modeling approach provides a new means to test hypotheses about habitat selection and movement by using presence–non-detection data…

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