Early April mornings have a certain quality that gives the impression that something is about to happen: cool air, pale light, and the faint sound of a warbler working through a still-bare canopy. Indeed, it is. For millions of years, that moment signaled the rough start of a meticulously planned series of events: plants are triggered by warmth, insects are triggered by plants, and birds are fed by insects. However, that chain is beginning to come loose. Where the links used to land, they no longer do.
Phenological mismatch is the technical term used by scientists to describe a timing issue that is surprisingly intuitive. The study of seasonal biological events, such as when a tree leaves, a caterpillar hatches, or a warbler lands after crossing the Gulf of Mexico, is known as phenology. Ecosystems work when those events coincide with one another. When they don’t, populations start to disintegrate in silence.
Spring is currently arriving earlier in many parts of eastern North America. Consistently, but not dramatically—not in a way that most people notice without paying close attention. Because they are ectotherms, insects react to temperature almost instantly. Because heat accumulation is essentially what drives their bodies, they emerge earlier.
In response to warmer spring temperatures, butterfly and plant phenology change at about seven times the rate of bird arrival and breeding, according to a recent synthesis of more than 15 years of data across three trophic levels published in the Journal of Animal Ecology. The plants and insects are moving in unison. The birds can’t keep up.
The issue is that gap. Caterpillars, which are soft, high in protein, and momentarily plentiful, are the primary food source for songbirds raising young in the spring. In the few weeks between hatching and fledging, a nest full of Wood Thrush hatchlings can devour an incredible number of them. There is just less food, and breeding productivity decreases if the caterpillar peak occurs before the chicks require feeding.
Wood Thrushes are now arriving approximately five days earlier in spring than they did in the 1960s, according to research from Pennsylvania’s Powdermill Nature Reserve, but the timing still lags behind the insects of the season. In an effort to stay near the food window, some species have nearly completely eliminated the once-slow post-migration recovery period by arriving and breeding almost immediately after one another.
Here, long-distance migrants are most severely disadvantaged. The unusually warm March in Pennsylvania is invisible to a Blackpoll Warbler spending the winter in South America. In order to initiate migration, it depends on internal clocks and day length, which have remained constant despite changes in the environment it is moving toward. As they migrate north, short-distance migrants—birds that spend the winter in the American South—can at least detect some indication of an early spring. Even so, the gap is not entirely being closed.
This is exacerbated at northern latitudes by another geographic factor. Higher-latitude breeding grounds, which are the destinations of many long-distance migrants, seem to have the biggest discrepancy between how quickly birds adapt to warming and how quickly plants and insects do. For example, in Canada’s boreal forests, where the brief late spring insect explosion is both intense and compressed, even a week’s mistiming could mean the difference between a successful season and one that yields virtually no young.
Birds shift their breeding by less than a third of a day for every day that spring arrives, according to a study using 18 years of MAPS bird-banding data. That ongoing lag adds up to something quantifiable and concerning when it is multiplied over decades of warming and dozens of species. According to estimates, North American songbird breeding productivity could drop by about 12% by the end of the century under current warming trajectories. Many of these species are already suffering from severe population declines over the past 50 years.
Certain species might be able to adapt. In order to compensate for temperature changes, birds have previously surprised researchers by compressing migration schedules, modifying stopover durations, and nesting at slightly different elevations. However, behavioral flexibility has its limitations, particularly when the underlying cues that control migration are ingrained in genetics that have evolved over thousands of years. Evolution is a slow process. Springs move quickly.
A reasonable question without a straightforward response is what can be done. Because more food options give birds a little more leeway in a tightening system, protecting and enlarging native habitat along migration flyways is beneficial. The relationship between certain caterpillar species and the plants they consume means that invasive shrubs and exotic ornamentals just don’t provide the insect base that migratory birds require, so maintaining native plant communities is also important. These interventions are not dramatic. However, they are real ones, and real margins are important in a problem this complicated.
The birds are still coming in. You can still hear them moving through the treetops on a calm April morning. However, before they have a chance to sit down, the breakfast they came for—the short, dense explosion of caterpillars that spring used to consistently deliver—is increasingly clearing the table.
