Irruptive bird species are those that respond to irregular changes in the food supply. The availability of seeds, berries, and small rodents, for example, can fluctuate wildly from year to year and season to season. Flocks of grosbeaks, pine siskins, purple finches, crossbills, and certain northern owls, among others, are commonly associated with irruptions, usually in the winter. Other bird species will often shift their wintering grounds in response to food availability. Irruptive migrations are not predictable and may occur one year and then not again for many years and may involve short or long distances. Large numbers of birds may be involved in irruptive migrations, or just a few.
Each winter or early spring, depending on the abundance of seeds (keys) on the box elder (Acer negundo) trees in our yard, an irruption of evening grosbeaks (Coccothraustes vespertinus) occurs for one or several days, lasting as long as it takes the grosbeaks to strip the trees of all their seeds. A usual irruption finds up to a hundred grosbeaks feeding in around the house.
I mentioned how the recent severe change in the weather brought several unexpected visitors to our yard. One, the merlin, Leonard and I never saw on our property before. A single evening grosbeak was feeding on the ground outside my kitchen window. This surprised us for two reasons: 1) We do not recall ever seeing a single evening grosbeak before, they always come in irruptive migrations. 2) Although the box elders are groaning with a bumper crop of seeds, the grosbeak never went into the box elder trees. It only picked among the fallen hybrid poplar leaves, for what I could not determine. After a long afternoon of foraging amid the fall leaves, the grosbeak disappeared and has not been seen since.
Leonard and I remain puzzled as to why this one, and only one, evening grosbeak was here. Was it a straggler separated from the irruptively migrating flock? And why did it not eat the box elder seeds? Perhaps they were not ripe. What was it eating in the grass?
This picture was taken through my kitchen window (Modoc County CA) on an overcast day so is not the best quality. It was interesting to see the single evening grosbeak, but its presence poses some unanswered questions.