For the Birds Radio Program: Fall Migration
Fall migration is on an entirely different time scale than spring migration. Why are spring migrants rushing in potentially dangerous conditions? And why are fall migrants so much more leisurely?
In spring, birds return to us in a fairly short window of time. Almost all our backyard birds return to the Northland in April and May, the bulk of them coming back within just a couple of weeks in the middle of May. Very few early birds return in March and barely a handful of stragglers hold back until the start of June. Fall migration is far more leisurely. Swallows start gathering in July, and most are gone before August begins. The first warblers take off sometimes in late July, crowds of nighthawks flutter by in August, and the bulk of warblers move throughout August and September and well into October. Fox Sparrows and White-crowned and White-throated Sparrows turn up sometime in mid-September, and stragglers can remain until the Christmas Bird Count in December. And we usually don’t see our last Yellow-rumped Warbler of the year until November or even December.
In spring, the bulk of robins move with the 37-degree isotherm, coming back almost the very moment the ground thaws making earthworms first available. Loons often return to their territorial lakes the very day of ice-out. Red-winged Blackbirds and ducks return to marshes as little pockets of water open up, weeks before any new vegetation has greened up. Hummingbirds come back before many blossoms are available, taking advantage of the first oozing sapsucker drill holes and trees flowering for their first meals. Warblers return as trees are budding out. The tiny hatching caterpillars on the newly sprouting leaves provide them with food, but in the years birders love, when the warblers come before leaf-out, there is very little food to sustain these little avian gems. For most spring migrants, the abundance of food that will carry them through the breeding season is barely starting to be available until a week or more after they actually return. An April blizzard or ice storm can send robins retreating south, and the deaths of millions of songbirds have been recorded during bad spring weather. So why do they rush in? Those first migrants, if they survive, will be first to stake their claims to territories. And increasing day length revs up their hormones, giving them a more powerful reason than mere real estate to hurry back to their mating area. Spring migration is a gamble, but the winners are richly rewarded
In fall, birds are headed to their wintering grounds. Many will claim a new territory in the tropics, and the first to arrive do have some advantages, but no romantic urges propel them. And for most birds, their winter food resources are fairly constant, as long as their tropical area hasn’t been deforested, so the timing for their arrival isn’t critical.
Up here, most birds leave while their food is abundant. Swarms of dragonflies and other flying insects sustain swallows, nighthawks, kestrels, and flycatchers. Berries and other fruits provide huge meals for a wide variety of songbirds. By August, many flowers are starting to dry up, which makes hummingbirds take notice, but they still have a wide variety of food until the first frost. So they can mosey along, moving fast when weather is most favorable for flights, but otherwise enjoying the season’s abundance.
For birders, the two migrations are very different. After a long winter, we hunger for spring migration, and the returning birds fill our senses by wearing their most brilliant plumages and filling the air with song. Fall migration is much quieter, with a few Red-eyed Vireos still singing now and then but most birds silent except for quiet little call notes. By fall migration, many adult birds have molted out of their prettiest feathers, and adult birds are outnumbered by the young they’ve produced—baby birds still in their drabbest feathers. Fall birders take their pleasure in the identification challenges presented by these more subtle plumages, and in witnessing the sheer numbers of birds. While spring birders take hope in the end of a long winter, fall birders take hope in the promise of all this new life surging through the north woods, trusting that most of them will survive their long journey and return to us once again come spring.