Weather is a fine-tuner of bird migration, determining the particular days a bird will move. The length of daylight (photoperiod) orchestrates the general timing of migration, dictating the months when a species is on the move. Get a copy of ?Birdwatching in Vermont? (Pfeiffer and Murin) to find out when species are generally expected to pass through, and where to find them.

Then, get cozy with meteorology. A thorough understanding of weather patterns is ideal for translating the weather forecast into a bird migration forecast. But a few tips will go a long way toward helping you to decide whether your time is better spent stacking wood.

Most of our weather comes from the west. When a low pressure system (rotates counterclockwise) is followed by a high pressure system (rotates clockwise), winds come out of the north. This is the classic ?cold front.? End of meteorology lesson, on to the birds.

In autumn, birds take advantage of northerly winds produced by the change in pressure systems and surf the tailwinds. The farther north the front extends, the more birds it will sweep along with it.

Songbirds are most easily seen in ?fallout? conditions: weather that makes the birds move followed by weather that forces them to stop. If a southbound cold front stops or reverses, and especially if it brings precipitation, birds will literally drop out of the sky to forage until conditions are favorable again. On a drizzly, calm, overcast morning following the arrival of a cold front, the trees are dripping with warblers.

When the north winds are too strong for warbler-watching, Lake Champlain offers a waterbird rush hour. Visit a shoreline offering good views with a spotting scope to see hundreds to thousands of loons, ducks, gulls, geese, cormorants, and terns pumping down the lake, especially in the early morning.

When the waterbirds peter out, move on to raptors. The best time to view raptor migration is late morning through early afternoon. As the earth warms, heat rises in columns or ?thermals.? Raptors use the rising air to soar upwards in a circular fashion (forming ?kettles?), then ?stream? directly south on the tailwind until they reach another thermal. A ceiling of clouds provides a helpful backdrop for seeing the ?dots? in the sky and keeps raptors from flying too high, but rain will ground them.

Now let?s put these tips into practice. A cold front blew in on Monday, bringing north winds and a cool, clear night. Predictably, birding was good on Tuesday for songbirds and raptors, less so for waterbirds because winds were light. Then a high pressure system moved in and winds shifted to the southeast; time to stack wood. At this writing, a weak cold front arriving Wednesday night with predicted north winds and some precipitation might make for decent birding on Thursday.

- Rosalind Renfrew
Vermont Center for EcoStudies
www.vtecostudies.org