gather in groups called kettles, to make the longest
migration of any hawk, up to 17,000 miles round trip from Canada to
the Pampas of Argentina.
They have an efficient migration method, soaring up on warm thermal
currents, gliding gradually down to the next thermal, gaining
and then beginning the process again. As
soaring and gliding take very little energy, it is truly the lazy
bird’s key to
migration. Since the migrating Swainson's tend to group up, head
towards, and follow the land bridge of Panama on their way south, these
birds can be seen in flocks numbering in the thousands. Radio telemetry
and banding have greatly enhanced our
knowledge of the Swainson's migration specifically, and their movements
The Swainson's hawk eats more insects than other buteos do, often grabbing them on the wing, principally grasshoppers, crickets, dragonflies and locusts, and is sometimes called the grasshopper hawk. Insects make up almost of their diet in their Winter habitats. The Swainson's diet requires more nutritious foods while nesting and feeding their young, so they add rodents, rabbits and reptiles while on the American and Canadian deserts, grasslands and praries, and have been known to stand in groups at the mouths of ground squirrel burrows and grab squirrels as they emerge. Swainsons defend Spring and Summer territories of about two square miles, not only against other Swainsons, but sometimes against Red-Tailed Hawks and Ferruginous Hawks.
Swainson's nest in trees, shrubs and on cliff edges, so the continuing conversion of the Plains habitat to grazing land, alfalfa and wheat (which affords cover to their prey), combined with the destruction of the few sparse trees by cattle continually rubbing against their trunks, and converting the areas around the tree's base to muddy wallows, have made locating suitable nesting sites more challenging. Swainsons arrive at the nesting grounds in March and April, and spend a week or so building their nests. Nests are constructed of twigs and sticks, and from the one to four eggs laid a few days apart, chicks emerge with a similar time gap, with the result that older chicks are larger and more likely to survive by appropriating more of the rodents adults bring to the nest and shred for the chicks.
The Swainson's hawk is vulnerable to pesticide poisoning in its wintering grounds on the Pampas in Argentina, where it feeds on grasshoppers, and where pesticides, such as DDT, long since banned in the United States are still used. As the birds roost and hunt together in huge flocks in Winter, they are sometimes inadvertently blanketed by the aerial pesticide spraying, causing mass die-offs, including two severe seasons in 1995 and 1996 when thousands of Swainsons died. In an encouraging development, Argentine farmers and Ciba Geigy, a manufacturer of the relatively inexpensive but lethal pesticide monocrotophos, which has been directly linked to Swainson's mortality, have cooperated with raptor biologists from Canada, America and Argentina in an effort to use less lethal pesticides (which still have the effect of depriving the Swainson's of their natural prey), and restrict where monocrotophos is used.
Gary and Steve
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Steve & Wendy Hall
PO Box 555, 977 Springfield Road, Wilmington, NY 12997
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