Migrating birds don’t need to be told twice to get going.

The snow and cold looming in late October pushed straggling migrants on their way, even without benefit of a weather service. Most of them left weeks ago, some of them months before that.

Migration depends on sufficient fuel as the journeys begin. One gram of fat will fuel about 125 miles of flight for one of the warblers that nest here, according to research.

A magnolia warbler, for example, might winter near San Jose in Costa Rica. It’s a flight of 3,800 miles from northeastern Minnesota, where it nests.

That warbler weighs about 8 grams, three-tenths of an ounce — feathers, bones, beak and all. Migrants can fatten for the trip, some of them doubling their weight before takeoff. That new weight will be fuel.

Add that to the bird’s usual weight, and a full tank will carry the bird about 1,400 miles, give or take. You see the problem.

This bird must stop to eat, to refuel, more than once before the journey from Minnesota ends.

The route map is imprinted in the warbler’s brain. Refueling points are not chosen as migration progresses. The bird is relying on a genetic map.

Not all birds do that. There are birds — geese, for example — that make what biologists call cultural migration decisions. These birds have an advantage.

A recent research paper discussed the pink-footed goose, a European species that makes a cultural migration. Other goose species do this, as well. The article in the Journal of Animal Ecology explored what can happen when genetically programmed migration stops are affected by climate.

Cultural migration schedules are mostly determined by the seasonal changes in plants and insect life at stopover sites, the authors of the study wrote. Decisions are made to fit the moment.

Geese explore options

The geese follow the development of spring vegetation along the route. If site A is not suitable, they’ll move to site B. This gives the geese some assurance of suitable migration conditions.

Biologists found that geese arriving at nesting grounds in less than optimal condition one year were likely to change their migration route the next year.

Our songbirds and shorebirds follow routes genetically imprinted. They meet the influence of climate — both short- and long-term — along that route.

Timing of spring migration for many species is based on the increase of daylight as the season progresses. This is particularly important when a time-critical nesting season awaits. Reliance on advancing daylight moves the birds north.

Length of day, however, does not dictate favorable conditions of habitat. Birds timing migration this way have no clue about conditions ahead of them. They don’t enjoy the advantage given to those geese.

Adapting to change

Changing weather conditions — wetter, drier, warmer — could provide migration difficulties for genetically programmed species, like our magnolia warbler.

The bird could arrive before or after expected food supplies are available — fruiting plants or hatching insects. This would create a mismatch between the peak availability of food and peak requirement, the research article said.

The birds could be unable to breed successfully if the migration route offered inadequate food. Negative conditions on the nesting site could make it difficult to adequately feed nestlings.

The birds expect to find the same migration and nesting conditions under which genetic programming evolved.

Birds can adjust. They did so many times over hundreds of thousands of years as glaciers advanced and retreated in North America. The key word here is retreat, weather conditions then returning to what they once were.

That guarantee looks like it has an expiration date.

Read Jim Williams’ birding blog at startribune.com/wingnut.