The fall migration is on the decline now, with most of our flycatchers, swallows and warblers gone for the next seven months. All of these birds depend on insects for their sustenance, a resource in short supply now.
Sparrows and other seed-eaters have a more leisurely migration. They can find seeds, at least until the first snows arrive. Even so, by the end of the month most of our sparrows will be gone to more moderate southern areas.
As I discussed in the last column, we know that the majority of migratory bird species have an innate knowledge of where they should go to spend the winter. It boggles the mind to realize that many first-year birds find their way – unaided by adults – to a wintering habitat they have never seen. Travel instructions are encoded in their genes.
In considering how migratory birds find their way, we need to recognize two different abilities of birds. First, the birds have a well-developed sense of navigation. In other words, they can set a course and follow it, barring intervention from hurricanes or other weather phenomena.
Second, some birds have well-developed abilities of orientation. Most migratory birds can navigate well but fewer can orient.
A famous experiment done with European starlings in Eastern Europe nicely distinguishes navigation and orientation. Some starlings were captured and placed in a cage in the spring. This particular population of starlings is migratory. In the spring, the caged birds attempted to depart on a northwesterly vector to reach their breeding grounds.
Other birds were transported several hundred miles to the west. Again, the direction that the captive birds chose was recorded. The transplanted birds again tried to migrate to the northwest. They were unable to correct for the fact that they had been moved westward. The starlings showed a good sense of navigation but a poor sense of orientation.
Contrast that result with the abilities of white-crowned sparrows. A wintering population of birds in southern California migrates each spring to Alaskan breeding grounds. Wintering birds that were flown either to New Orleans or Maryland ultimately found their way to their Alaskan breeding grounds. These birds were able to compensate for their eastward displacement by biologists. These birds are great at both navigation and orientation.
The abilities to orient and navigate are not restricted to migratory birds. During the nesting season, birds need to be able to find their way to their nests. The need is particularly acute for birds like bald eagles that maintain huge territories, or ospreys or albatrosses that may fish miles away from their nests.
Domestic pigeons have been the subjects of the most illuminating studies on navigation and orientation. Pigeons can return to their roosts from distances as far as 1,100 miles.
They use multiple cues for navigation. An internal clock allows them to determine direction from the position of the sun in the sky. This so-called sun compass is the most important cue. They also can sense the earth’s magnetic field. On cloudy days, magnetic cues become important. We even have evidence that pigeons can smell their home over the last few yards.
Pigeons are able to fly steadily at 50 miles per hour. It’s not surprising that competitive homing pigeons beat their owners home from a release point.
Pigeons can be used for nefarious purposes as well. Recently, a pigeon whose roost is in an Argentinian prison was caught smuggling 8 grams of marijuana and a memory stick.
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