C4

BY MIKE LUSHINGTON

Stumble Upon

del.icio.us

Digg

Facebook

Print

Email

Speak Up

Most birds begin life as insect eaters. Insects provide the protein necessary for quick growth in more concentrated forms than do other potential food sources, particularly for smaller birds. However, as adults, many birds rely on other food sources for their nourishment. It is the availability of those food sources on a year round basis that determines whether or not birds migrate, and when.

Some birds — swallows, swifts, flycatchers and their kin — rely on flying insects throughout their life span. These birds are among the most spectacular fliers in the avian world. Some swallows and swifts hardly perch except to nest; they often eat and sleep on the wing. The flycatchers are somewhat more sedentary, but they, too, rely on flying insects to eat. Obviously, these species must time their arrivals and departures in accordance with reliable hatches of flying insects. (It is not a coincidence that many of these species are in decline in the more northerly regions of their ranges - insect hatches are happening at different times than had been the case, due, it would seem, to climate changes — and many of these birds have not yet adapted to these changes).

Other birds — the chickadees, woodpeckers, and nuthatches among them - are also insect eaters, but they find their insects under the bark of decaying trees, where they remain available year round. Most of these birds are opportunistic feeders too, which is why many of them are among the most common of the feeder birds, particularly during the winter months. Of all the woodpecker species that we see locally, it is not accidental that Northern Flickers and Yellow-bellied sapsuckers are the most migratory: flickers rely on ground inhabiting insects and worms, while the sapsuckers, as their names suggest, feed on sap (and the insects that are attracted to the sap) that they tap from trees — which is in short supply in winter.

Others are seed and wild fruit eaters. Often these are the species that display the most irregular migratory patterns. Finches will stay around all winter if spruce and other conifers produce heavy seed crops; robins, waxwings, and grosbeaks will, if there is a heavy wild fruit crop — principally mountain ash berries. Many sparrows rely on seeds that they find in tall grasses. They will often migrate only as far as they need to find areas where the grass seed heads will persist above winter snows, which is why they are rare around here, but relatively common in the southern part of the province and elsewhere nearby.

Much the same case may be made for those birds that we commonly associate with our waterways. Next week, I will consider some of their migratory strategies.