We’re at the height of the nesting season for most of Maine’s breeding birds. Most of our perching birds (songbirds and flycatchers) are on their breeding territories now. The tremendous energetic cost of migration is now replaced with the energy costs associated with reproduction: nest-building, incubation of eggs and feeding of nestlings.
Bird movements haven’t stopped. To be sure, territorial birds will stay put until young are fledged. But other birds are always on the move. The best local examples are red crossbills and white-winged crossbills.
These birds rely on the cones of coniferous trees for food for themselves and their young. Their crossed bills and powerful tongues are prefect for forcing apart adjacent scales of 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. In some years, few cones at all are produced but in other years, called mast years, a superabundance of cones is produced.
Biologists believe the reason for this high variability is a response by the trees 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 can’t reproduce quickly enough to eat all the seeds. Some conifer seeds will give rise to a new generation. Then, a following year of low seed production spells curtains for many insects.
The crossbills wander broadly until a large cone crop is found and nesting behavior starts. White-winged crossbills have been found nesting in every month of the year. I love to tell my story of seeing white-winged crossbills feeding nestlings in January in the Northeast Kingdom of Vermont when the temperature was 35 below zero.
Behaviorists call this wandering behavior irruptive migration. Unlike standard migrations, where end points are fixed and timing is predictable, irruptions maintain the element of surprise. You never know when and where an irruptive species might appear.
Not all members of territorial species claim a territory. A portion of a population is nomadic on a small scale. Mostly 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 that might have died. These bachelors are called satellite males or floater males.
A rather gruesome experiment performed in Maine in 1936 provides valuable insight into satellite males. I always relate this particular study with trepidation because of the killing involved. This experiment is offensive in our 21st century ornithological sensibilities.
Performed in a 40-acre tract 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 researchers went out with shotguns and shot as many males as they could. They reduced the population of singing males to 21% of the original number by June 21 and kept the singing birds at that level until July 11. At the end of the study, the biologists had killed 528 birds, 3.5 times the original number of territorial males. Clearly, floater males were rapidly coming in to take the place of males that had been killed. There seems to be no lack of satellite males.
I think the chances of such a study being approved today is slim. Federal biologists enforce the protection of our native birds provided by the Migratory Bird Treaty. To kill birds for scientific purposes, a federal collecting permit is required from the Bird Banding Laboratory, an agency in the US Fish and Wildlife Services. A state collecting permit is also required. The application requires the applicant to describe the purpose of the study and justification of the collection.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at [email protected]