There is no doubt about it: spring has finally sprung. Although there are still snow mounds in a few places, it is time to look and listen for migrating spring birds, a tradition that goes back centuries, no doubt. In fact, around this very time, on April 2, 1852, Henry David Thoreau wrote, “The air is full of the notes of birds: song-sparrows, redwings, robins (singing a strain), bluebirds, and I hear also a lark, as if all the earth had burst forth into song.”
Some of those birds Thoreau heard, such as male red-winged blackbirds, returned to our area in February after migrating to the southern United States for the winter. You may have seen them already, with their distinct red-and-yellow shoulder patches, and possibly even heard them call out conk-la-ree.
But blackbirds are only one of many types of birds returning after months away.
Migratory Restlessness
Most birds return to our region from March through June, with the greatest number arriving in mid-May. Hardy migrants like waterfowl and raptors such as hooded mergansers and ospreys begin arriving in March. Woodcocks, another early arrival, might migrate 1,500 coastal miles from Alabama to Massachusetts to perform their popular acrobatic mating dance, in which males spiral upward 200 to 300 feet, loudly calling a buzzy peent to females nearby.
By early April, we might see tree swallows. By late April through May, look for long-distance migrants like warblers, tanagers, and vireos, which travel here from tropical regions. Scarlet tanagers, for example, travel more than 4,000 miles, from the Amazon basin regions of Brazil, Colombia, Peru, and Bolivia.
It all starts when daylight increases, which causes hormonal changes in migratory birds that make them enter a state of what’s called migratory restlessness, or an urge to move. Even caged birds display this behavior, orienting themselves in the direction that they would have migrated, fluttering their wings, flitting from perch to perch, and putting on weight in anticipation of the long journey ahead. Some species of migrating birds almost double their weight before they migrate.
Scientists think that chicks’ need for protein-rich insects is a big reason birds started migrating in the first place. It seems simple enough. Fly to an area filled with soft, protein-rich insect larvae (caterpillars) to feed your chicks. Longer summer days also mean that there are more daylight hours for parents to collect food to feed their young. (Unlike human parents, baby bird parents do not feed baby birds at night.) But climate change is making it more and more difficult for birds to sync their migration with the arrival of the insect larvae they need.
Changes to Bird Migration
There is no doubt that bird migration has changed. Thanks to observations from the likes of Thoreau and Cambridge’s William Brewster, we have historical records that help us understand how climate change may be affecting bird migration. The National Audubon Society estimates that more than half of North American bird species are arriving earlier than they did 50 years ago.
It might be because, thanks to a warming planet, spring is arriving earlier and earlier: Tree leaves emerge earlier, roughly two weeks before than they did in the 1800s. Wildflowers are now blooming a week earlier. Caterpillars that birds eat are also appearing earlier. Since insects feed on plants, and birds feed on insects, any mismatch in the timing can be a problem. If migrant birds arrive too late, they miss the caterpillars they need to feed their nestlings. And if chicks miss peak insects too many years in a row, that bird population spirals downward.
Short-distance migrants, such as robins, can respond to an earlier spring by moving north whenever temperatures warm. However, long-distance migrants in South America, such as ruby-throated hummingbirds and scarlet tanagers, are unaware of what’s happening in North America. They arrive north at the same time they always have. When spring is early, their populations tend to decline, probably because they miss the peak abundance of caterpillars. The success of bird populations under these conditions depends on their flexibility. If they can consume different foods, for example, they may not have as many issues as a species that does not have a flexible diet. Robins and catbirds and warblers are good at adjusting their insect-heavy diet to fruits and berries as needed. Massachusetts is becoming too warm for black-capped chickadees, our state bird. By the year 2050, according to Mass Audubon, they may not breed here anymore.
Bird watching
With all this songbird activity, it’s a wonder we don’t see them. But that’s because these great swarms of migrating birds travel at night to avoid predators, reduce overheating, and to fly in calmer air. We might not spot night-traveling birds, but radar can detect them.
A secret report of the British Army Operational Research Group in April 1942 recognized that birds can reflect radio waves with enough strength to be detected by radar. This information went public in 1945, and thus began radar ornithology.
Identifying birds on radar can be difficult because swarms of birds look much like swarms of insects or bats. But scientists have some tricks to spot the differences. Insects, because of their small size, tend to travel with the wind. Birds, however, travel at higher speeds than insects and can travel across the wind. While bats can also travel against the wind, they don’t tend to emerge just before sunset and are most active during warm summer months, which is a teller against birds, which begin to migrate shortly after sunset in the spring and fall.
Today, anyone can follow bird migration on weather surveillance radar. And Mount Auburn Cemetery, a birders’ paradise, has early-bird hours in April and May, opening its main entrance at 6 A.M. To see migrating raptors on days with westerly winds from April to early May, head to parking lot #1 of the Parker River Wildlife Refuge on Plum Island. You can join hawk watchers there to see rough-legged hawks, Cooper’s hawks, red-shouldered hawks, turkey vultures, American kestrels, merlins, northern harriers, sharp-shinned hawks, and others.
If you are curious to see daily data about the birds migrating through Middlesex County, look no further than this migration tracker.
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