With the arrival of spring, we look forward to the return of hundreds of species of migratory songbirds from their wintering grounds.
Sparrows, swallows, warblers and thrushes, among other songbirds, will be returning from their wintering sites anywhere between the southern United States and distant South America.
Some of these birds will return with a small “backpack” that has recorded their entire migration from their North American breeding grounds to their wintering grounds and back.
Birds provide important ecosystem services, such as preying on insects, dispersing seeds, scavenging carcasses and pollinating plants. Unfortunately, there have been dramatic declines in many migratory songbirds over the past few decades, with some of these populations dropping by more than 80 per cent.
If we are to find ways to slow or reverse these declines, we must first figure out what’s causing them. Climate change, habitat loss and predation by cats are among the leading causes of bird declines.
But with the vast distances these birds move over the course of the year, it can be difficult to pinpoint the main cause for a given species —and where it’s occurring.
To answer this question, we need to know where individual birds spend their time throughout the year.
We have a good idea of the range —or the total area —the birds occupy during the breeding and wintering periods. But ranges are composed of many populations, and we still have a very poor understanding of how individuals within each of these populations are connected between seasons.
Individuals from different breeding populations may remain segregated during the winter. For example, some ovenbirds winter in the Caribbean whereas others spend their winters in Mexico and Central America.
Or a bird may mix with individuals that originate from other breeding populations, such as bobolinks that mix in South America during the winter.
These patterns of migratory connectivity have critical implications for predicting how migratory songbirds will respond to environmental change.
Habitat loss —deforestation, for example —in one place can have different effects. If habitat loss occurs in a wintering area where breeding populations mix, it may have wide-ranging, yet diffuse, effects on the breeding populations. But if the habitat loss occurs in a wintering area that is occupied by a single breeding population, the effect may be more focused.
For example, habitat loss in South America will likely have range-wide effects on bobolinks, while habitat loss in the Caribbean may only influence a portion of the breeding populations of ovenbirds.
Backpacks for birds
We know that the breeding and wintering populations of most species mix to some extent, but…