Signs of Spring Update: March 13, 2000
Today's Report Includes:
Swallows On a Mission
Did You Know?
Some Birrrrrrds Like It Cooler
Most swallow species spend the northern winter in South America, and start heading north in February. Days are getting noticeably shorter in Argentina by then, and the birds grow restless. People have counted thousands of swallows passing over Panama as early as February 24. As they continue northward, they join up with Tree Swallows and Violet-green Swallows from the Gulf Coast, and move into the US in early March. The first swallows to continue on to colder areas of the central and northern states and the Canadian provinces are almost always Tree Swallows, which can survive colder temperatures than other swallows.
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Mighty Tiny, Mighty Welcome
Swallows are little birds. The Cliff Swallow weighs less than an ounce (about 24 grams). The Barn Swallow, who can migrate from the bottom of Argentina all the way up to Alaska, weighs even less (about 19 grams). Barn Swallows are found over most of the globe. The only swallow of New Zealand, which looks a lot like a Barn Swallow, is called the Welcome Swallow, but wherever they go, just about every swallow is a welcome one.
What Do Swallows Swallow?
Swallows eat mainly flying insects, including mosquitoes and other harmful species, so people benefit from swallows being around. And their graceful movements are among the loveliest of any bird.
Mates for Life: Discussion of Challenge Question #4
Last time we asked, "What do bird species that mate for life have in common? Give reasons that would explain why these species might mate for life when others don't."
Birds that mate for life include a variety of species, from ravens and eagles to swans and cranes, but they have a few things in common: relatively long life spans, large territories, and, for the most part, only a few young produced every year. If a Tundra Swan or Sandhill Crane arrived on its vast, empty territory in spring without a mate, it might take a while to find one. Both of these species find their mates during the winter, when large groups of their species gather, and the pair migrates together. Every year, the two of them become more familiar with their huge territory--all the little hiding places and good feeding spots, and predators that live there--and every year they become more and more familiar with each other. Year after year they become better at successfully raising babies together. Cranes only lay two eggs a year, and usually only successfully raise one chick each year, so it takes many years together to produce what Red-winged Blackbirds can produce in a single season.
Birds with shorter life expectancies would waste time learning all the tiny details about their mate if their mate gets killed before the eggs are even laid. They have to adapt quickly when their mate dies so they won't lose their chance to produce young before their own death. Red-winged Blackbird males put on enormous shows with plumage and song to attract females, and then produce five or six young per nesting. There are so many millions of blackbirds, and the males are so conspicuous, that females have an easy time finding males. And if either male or female dies, it's an easy matter to find a new mate during the breeding season.
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The Next "Signs of Spring" Update Will be Posted on March 27, 2000.
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