North American crossbills

North America has a far greater variety of conifer species than Europe, and many seem to crop synchronously over wide areas. It also has a wide range of Red (Common) Crossbills, at least eight different types having been described, each occurring in mainly different regions from the others and adapted to the conifer species that grow there (Box 18.3). Conifer species differ in the dates their cones mature, and in the dates the cones fall or shed their seeds. Hence, while spruce-feeding, Common Crossbills in Europe move mainly in May-July, Red and White-winged Crossbills in North America make their main movements anytime from June to November, depending on the conifer, with more minor movements at other times in response to changes in seed availability (Benkman 1987, 1992). In eastern North America, White-winged Crossbills often arrive in areas of Tamarack Larix laricina and White Spruce Picea glauca in late May-July, as the new cones are forming, and breed on the strength of the new crop. They stay only until the seed has fallen, usually by November, switching then to Black Spruce P. mariana, whose seeds can remain available in the cones until the following summer. But if the Black Spruce crop is poor, White-winged Crossbills then irrupt into regions south of the usual range. The timing of this movement, in October-November, coincides with the timing of seed fall from Tamarack and White Spruce. In the same region, Red Crossbills (with their larger bills) can exploit all the conifers eaten by White-winged Crossbills, but can also tackle pines efficiently, so they have a greater range of conifers available to them. However, if pine cones are scarce, Red Crossbills also emigrate, mostly from November onwards (Benkman 1987). Thus, the southward irruptive movements of crossbills in North America occur from October-November onwards, compared with May-July onwards in Europe, but in both regions the timing of the movements matches the phenology of the food plants, as the birds respond to a dwindling food supply.

Some individual ringed Red Crossbills in North America have been recorded at the same site two years later, but others were recovered more than 2000 km away

(Adkisson 1996). Of particular interest are two birds, recorded in the same months of different years. One bird was recorded in May of 1991 and 1992 at localities 1409 km apart, and another was recorded in August 1990 and 1993 at localities 521 km apart. Without knowledge of local breeding seasons, it is hard to interpret these movements, but they would not be expected in a regular migrant moving annually between fixed breeding and wintering areas.

Was this article helpful?

0 0

Post a comment