We are at the height of nesting season for most Maine breeding birds. Most of our perching birds (songbirds and flycatchers) are now on their breeding grounds. The enormous energy cost of migration is now replaced by the energy costs associated with reproduction: building the nest, incubating the eggs and feeding the chicks.
The bird movements did not stop. True, territorial birds will stay put until the young have fledged. But other birds are always on the move. The best local examples are the red crossbill and the white-winged crossbill.
These birds depend on the cones of conifers to feed themselves and their young. Their crossed beaks and powerful tongues are great for separating adjacent scales from a cone and extracting the nutritious seed at the base of each scale.
We know that cone production is highly variable from year to year. In most years a modest crop of cones is produced. Some years few cones are produced, but other years, called mast years, produce an overabundance of cones.
Biologists believe that the reason for this high variability is a tree response to seed predation by insects. By producing few seeds in most years, insect populations are kept low. A mast year can then overwhelm the insects. The insect populations will increase but they cannot reproduce fast enough to eat all the seeds. Some conifer seeds will give birth to a new generation. Then a subsequent year of low seed production means curtains for many insects.
Crossbills roam widely until a large cone culture is found and nesting behavior begins. White-winged crossbills nest every month of the year. I love telling my story of seeing white-winged crossbills feeding chicks in January in northeastern Vermont when the temperature was 35 below zero.
Behaviorists call this wandering behavior irruptive migration. Unlike standard migrations, where the endpoints are fixed and the timing is predictable, inrushes retain the element of surprise. You never know when or where an irruptive species might appear.
Not all members of territorial species claim territory. Part of a population is nomadic on a small scale. Primarily males, these birds wander around trying to displace a territorial male (good luck with that), find an unmated female, or seek to take the place of a territorial male who may have died. These singles are called satellite males or floating males.
A rather gruesome experiment performed in Maine in 1936 provides valuable information about satellite males. I always recount this particular study with concern because of the murder involved. This experience is offensive in our 21st century ornithological sensibilities.
Carried out in a 40-acre stretch of forest in early June, the number of singing males was mapped. A total of 124 territorial males of many species were found. Then the searchers came out with shotguns and shot as many men as they could. They reduced the songbird population to 21% of the original number by June 21 and kept the songbirds at that level until July 11. By the end of the study, biologists had killed 528 birds, or 3.5 times the initial number of territorial birds. males. Obviously, the floating males were arriving quickly to replace the males that had been killed. Satellite males do not seem to be missing.
I think the chances of such a study being approved today are slim. Federal biologists enforce the protection of our native birds under the Migratory Bird Treaty. To kill birds for scientific purposes, a federal collecting permit is required from the Bird Banding Laboratory, an agency of the US Fish and Wildlife Services. A state collection permit is also required. The application requires the applicant to describe the purpose of the study and the rationale for the collection.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes comments and questions from readers to [email protected]
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