Think of spring as a ballet, perhaps, a choreography of dancers each moving to the beat of a complex music, each counting his or her steps and rests by measure so as to arrive the right marks at the right time. One dancer leaps just as another moves into place to catch and lift her at the exact moment the music swells.
Plants break dormancy after the danger of frost passes, but not so late as to miss the first warm weather. Insects hatch out just in time to eat the young leaves. Birds fly in to catch the caterpillars for their young.
Timing is everything, because a bird cannot wait until the food is available before starting to breed; making bird babies is a multi-step process that begins with hormonal changes weeks or even months before the little ones actually hatch out. For a bird, breeding is an act of faith, like the dancer who launches herself into the air knowing that her partner will arrive in time to catch her. And with climate change, increasingly, the partner just isn’t there.
The problem is that the ballet of spring is accompanied by something less like an orchestra and more like a frog chorus, where each instrument is free to play its own song to its own rhythm for its own reasons. Different dancers in turn follow the beat of different drums, so when some drums change their speed and others do not the choreography as a whole fails.
For example, some birds migrate at the same time every year, regardless of the weather, using changing day length as a kind of calendar. The insects the birds eat, on the other hand, might time their development according to warming temperatures. In a stable climate, warming temperatures occur at about the same time most years, so even though bird and insect are keeping time differently, they arrive at center stage together. But as climate change pulls the weather and the climate out of sync, species that time their spring by different methods fall out of sync with each other. A related problem is that organisms lower down on the food chain tend to be able to adjust their schedules faster than those higher up, again resulting in birds getting hungry when their food isn’t available.
These kinds of mismatches are a major worry for those who study the effects of global warming. It is difficult to predict when and where scheduling problems will develop, because ecology is complicated and sometimes counter-intuitive. We do know some scheduling problems are already happening.
Birds who migrate long distances face special scheduling challenges, because conditions on their wintering grounds bear no relation to those on their breeding territories. They can’t tell spring has come early until after they get there.
Some species cannot adjust migration at all. Purple martins, for example, which winter in Brazil and breed in the United States, migrate strictly according to the date, regardless of weather. Once they leave, they travel very fast, covering several hundred miles a day towards the end of their trip. If they discover warm weather on their way north, they can’t speed up in order to make up time. Purple martins are part of a group of birds that hunt insects in flight, migrate long distances, and are currently in decline. Scheduling problems as a result of climate change could be why.
Some birds can adjust their schedules. Pied flycatchers, for example, who winter in Africa and breed in Europe, can travel faster in response to warm weather, but that only saves them a few days at most. They can’t leave any earlier than they do because from so far away they can’t tell they need to.
Even birds who arrive late do have some options. Many birds normally lay one egg per day but only start to incubate when the clutch is complete. That way, all the chicks hatch at once. In an early spring, some mother birds start incubating right away so the oldest egg can hatch a few days earlier when there is still food around.
But these adjustments are probably only stop-gaps, not long-term solutions.
It is true that sometimes a timing mismatch is less of a problem it seems. If a bird population is limited, not by how many chicks fledge but by how many can survive the winter, then it might not matter if more chicks starve to death. The survivors will have less competition and a larger proportion of them will make it to spring, for no net population change. But eventually, if breeding success drops too low, it will start to shrink the adult population even in these species.
It is true that the climate has changed before and birds adjusted their nesting schedules and survived. But those changes were much slower than this one and schedules take a while to adjust. The problem is that at least some aspects of bird breeding behavior are instinctive. A bird cannot change its instinct by an act of will any more than it can change the color of its feathers. Only through evolution do instincts change and that can take hundreds or thousands of years.
Purple martins don’t have that kind of time.
We’re not necessarily looking at losing all migratory songbirds, because ecology is complex, as noted. But even if only a few bird species are lost from a few areas, these birds’ behavior could an important part of some other organism’s timing; the loss of one bird could then cause scheduling problems for other species.
The choreography of the dance of spring would then continue to unravel, progressively, possibly in ways we cannot yet even predict.
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