the real “snow-birds”

The Fall bird migration is in full swing here in Minnesota, and large numbers of some of the smallest migrants have come and gone already on their long journey from their breeding grounds in Canada and Alaska to their over-wintering areas in Central and South America. These are the species that are obligate insect- or fruit-eaters that simply cannot find enough to eat during the cold winters of northern North America to survive here. So they leave well before the snow flies.

Ruby-crowned Kinglets are some of the smallest birds (besides hummingbirds) that pass through this area in the spring and fall. They weigh just half as much as a Chickadee (0.2 ounces or about 6 grams), — tiny bundles of energy that never stop moving for an instant as they continuously search for food.
There are fewer insect blooms in the fall for these birds to fatten up on. Look at the size of the miniscule prey item this bird is after on the underside of the the leaf — it would have to eat hundreds of them to get enough energy to sustain it for one day, let alone try to put on enough fat to provide energy for migration.
Magnolia Warblers, and most other warbler species, are only slightly bigger than the kinglets, weighing about 7-11 grams (0.2-0.4 ounces) depending on whether they have just fattened up to leave an area, or have just arrived desperate to find their next meal.
Eastern Wood-Pewees are tiny little flycatchers (weighing just slightly more than a fat warbler at 0.5 ouces or about 14 grams) that perch on bare branches and fly out to attack insects flying by.
Red-eyed Vireos are about 50% bigger than a Chickadee, weighing in at about 0.6 ounces or 17 grams.

Kinglets, Warblers, Vireos and Flycatchers (like the Eastern Wood-Pewee) eat an insect-rich diet most of the year, but because there are fewer insects around in fall (compared to spring), they often utilize berries, suet, and even seeds as energy sources to store fat for the next leg of their migration. Because insects and fruit are mostly water (70-80%), these tiny birds need to eat about 1.5 times their body weight each day in order to put on just 0.5 grams of fat per day. But that’s not enough to fuel a 500 mile flight to the next stop on migration, so it takes 3-4 days of constant eating and putting on fat to get enough fuel on board.

Philadelphia Vireo with a mouthful of dogwood berry. Fruit-eating birds are really good at separating the nutritious part of the berry from the indigestible parts, and pass the waste through their digestive tract quickly to make room for more berries. (Note: don’t stand under a fruit-eating bird while it’s moving food through its gut!)

Blackpoll Warblers are the kings of metabolic physiology when it comes to putting on fat for long-distance migration. These half-ounce (14 gram) birds double their body weight before flying non-stop for 4-5 days on one leg of their journey across part of North America and the Atlantic Ocean to reach their wintering grounds in the Caribbean Islands and northern South America. That means their little bodies are 50% fat when they take off — literally, butter-balls of bird!

Figure from SciNews, March 2019. A fascinating summary of a study by DeLuca et al. published in the journal Ecology in 2019.
Of course, the bird that defies predictions of how far it can fly on its fat supply is the Ruby-throated Hummingbird — a 3-gram (0.1 ounce) bird that routinely flies 500 miles across the open water of the Gulf of Mexico for 18-22 hours to the coast of South America, rather than follow the longer land route through Central America. Now that’s a physiological marvel!

12 thoughts on “the real “snow-birds”

  1. Wonderful photos and information as always! Thank you! We wondered why were were seeing so many Kinglets or Vireos (we thought they were golden finches) that loved our sunflowers then disappeared and are now back catching bugs and look to be eating the Mimosa pods? We also just saw some Peewees this week for the first time – they are quite the bug catchers!

    • Thanks very much for your comment, David. I’m glad to know that you appreciate all the numbers! Telling Phoebes from Pewees is always a problem, and I relearn it every year, it seems. But here’s what I try to remember: Phoebes have a darker head, an all dark bill (Pewees have an orange lower mandible), a whiter breast (Pewees have a grayish vest), and no distinct white on their wing bars. And of course. In the spring, Pewees are easy to tell when they are singing, peeeee-wee!

  2. I know that we will be seeing some of these soon in Florida. Hard to imagine how they consume so much in such a short time in order to sustain themselves over a long flight. Stunning photos of these tiny birds, Sue!

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