I think I know why birds go south for the winter. This revelation came to me after two weeks of hostile, frigid weather, not to mention a bomb cyclone nor’easter. But where is south?
South is a big place. For purposes of birds leaving north, even southern Maine is south. Song sparrows vacate eastern Maine in winter, but some can be found wintering in southern Maine and along the midcoast, wherever there is enough grass seed poking above the snowpack.
Birds that don’t rely on insects don’t need to migrate as far. Yellow-rumped warblers are bug eaters, but they can subsist on berries. Even in the coldest of winters, this warbler species is plentiful along the mid-Atlantic coast where it loves to eat bayberries.
Some birds just need to get far enough south that the water remains open. Belted kingfishers barely go south. There are winter records in Maine, and your odds of finding a mid-January kingfisher in Juneau, Alaska, are excellent.
The summer range map of the great blue heron stretches all over North America, and even to the northern tip of South America. In winter, the entire range map barely shifts southward. But individual birds can go anywhere they please, wherever ice doesn’t choke off fishing. Some go no farther than Massachusetts. More go to the Chesapeake. The rest head for the southern U.S. or the tropics, and there’s no rhyme nor reason for determining which bird goes where.
Great blue herons are declining in the state, and the Maine Department of Inland Fisheries and Wildlife wants to know why. So they’ve radio-collared a handful of herons. A bird named Nokomis was collared near the high school in Newport two years ago. For the second year in a row, she’s made her way to Haiti for the winter. With Maine temperatures at 10 below zero, I’m thinking she’s a smart bird.
Bobolinks and upland sandpipers go all the way to the grasslands of Paraguay and Argentina. Arctic terns go farther, to the Antarctic. Last weekend, I started to ask myself: “Where do some of our commonest summer birds go in winter?” I could either research that topic, or go outside and shovel snow in temperatures equivalent to Mars. I chose the former.
Black-throated green warblers are among the most common warblers in Maine. Black-throated blue warblers are closely related, and are only slightly less common. Do they winter in the same place?
No. There is overlap, but their winter ranges are not identical. Black-throated green warblers winter mostly in Central America, particularly along the Yucatan Peninsula of Mexico. Many greens do winter on the Caribbean islands, but blues are much more likely to be there.
Blackburnian warblers mostly fly past the Caribbean, concentrating along the coasts of Venezuela, Colombia and Ecuador, and also in the elevations of Costa Rica. Blackpolls share part of the same winter range, but are spread much farther eastward and into Amazon Basin. Bay-breasted warblers are about as closely related to blackpolls as you can get, but their wintering grounds are concentrated almost entirely around Bogota, Colombia, and along the rim of the jungle.
Such winter range distinctions matter. Until 1998, the Bicknell’s thrush was thought to be just a subspecies of gray-cheeked thrush. They are virtually identical, and even sound similar. The Bicknell’s thrush nests on Maine mountaintops, and all the way up to Cape Breton, Nova Scotia. Across the channel to Newfoundland and up to the arctic, the gray-cheeked thrush takes over. Among the most compelling pieces of evidence that these two birds were not the same, Bicknell’s thrush winters almost exclusively in the Dominican Republic. The gray-cheeked thrush skips completely over the Caribbean and winters in South America. There is zero overlap in winter.
The difference among other thrushes is equally astounding. Each has a completely different wintering destination. Most hermit thrushes don’t even leave the U.S. Swainson’s thrushes go all the way to South America. The wood thrush settles in between, spanning most of Central America. They may bump into each other in Maine all summer, but they won’t see each other again until spring.
Maine has two large summering flycatchers. The olive-sided flycatcher nests farther north. The great-crested flycatcher is spread from Bangor south. In migration, the first leapfrogs the second. The winter range of the olive-sided flycatcher extends from Guatemala to Bolivia, but the great-crested flycatcher barely makes it past southern Florida, and the longest migrants winter mostly in Central America. Maybe they just like the spicy food.
Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at firstname.lastname@example.org.