How Different Spring Migrants Decide When to Head North

(Kenn Kaufman;22 March 2017)

Will warmer weather bring the birds back early? It all depends on what type of migrators they are.

Spring officially started this week, but for many of us it had already begun. Back in February, unseasonably warm temperatures swept over much of North America, buds began opening on trees, and flowers began to bloom weeks early. Naturally, birders began to ask: Will our migratory birds come back earlier, too?

That question doesn’t have a simple yes or no answer because the timing of bird migration is . . . complicated. Every species is slightly different; short-term changes in weather do have an impact, but so do a variety of other innate and environmental factors. Here’s a quick primer on how North American avians schedule their spring journeys to aid in your own birding ventures.

Two Types of Migrants

To figure out how migrating birds could be affected by balmy weather, we should start by categorizing them into two groups: obligate and facultative (to use the fancy terms). These labels aren’t ironclad—many birds fall somewhere between these extremes—but the definitions are helpful in understanding what triggers a species’ migration.’

For obligate migrants, the timing of travel is dictated by hard-wired instinct. An unusually warm or cool season won’t make them suddenly decide to change their departure dates. They’ll move at about the same time each year, regardless of weather. Meanwhile, facultative migrants are more tuned in to the conditions of the moment. They have a standard timing for their migration, but they might tweak it by days, or even weeks, if the season is chillier or more temperate than usual. They’re flexible.

So how does this work during spring migration? When birds start moving north from their winter homes, the hard-wired, obligate migrants run like clockwork. That includes certain songbirds, raptors, shorebirds, and others that commute between the far north and the deep tropics or temperate southern zone. A Blackburnian Warbler spending the winter in South America or a Wood Thrush wintering in Costa Rica won’t have any way to judge what’s happening up the United States and Canada. Such species may wait for clear skies and favorable winds to launch each stage in their journey, but a major warm spell won’t cue an early arrival.

Meanwhile, most of the flexible, facultative migrants are birds that only move short distances, wintering right in the United States. This allows them to sense local conditions and seize opportunities to move closer to their breeding grounds. Overall weather patterns tend to apply to broad regions, so if the season continues to be warm, facultative birds may gradually move north ahead of schedule. For example, if the weather is mild in late February, a Fox Sparrow in Tennessee might guess that life won’t be too bad a few hundred miles farther up in Ohio. Red-winged Blackbirds, Common Grackles, American Woodcocks, Killdeers, Eastern Phoebes, and even Tree Swallows are other species that may turn up earlier than average during a warm spring.

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