As March begins, thoughts turn to spring and the return of migrant birds that have spent their winter in warmer climes. The question is, do birds fly south just to escape cold weather? If this is so, then why do some birds stay here?

The answer is manifold. Escape from harsh weather is often not the overriding reason for a bird to migrate; many birds spend the entire winter in harsh climates. Scott Weidensaul, in his book “Living on the Wind: Across the Hemisphere with Migratory Birds,” recounts that in 1994 scientists found an enormous population of spectacled eider ducks wintering far within the pack ice of the Bering Sea.

As far as the researchers could tell, Weidensaul writes, “the openings [in the pack ice] were maintained in the 30-below cold solely by the body heat and movement of the eiders themselves, which were presumably feeding on the mollusks and invertebrates in the waters below.”

This scene gives me shivers just thinking about it, but it contains a partial answer to the above question: special adaptations that allow birds to withstand such severe cold, as well as enable them to exploit food resources, dictate which will stay and which will go.

But there is a twist to this scenario: for example, many migrant songbirds that feed on insects in the summer switch to a diet of fruit when they reach their wintering grounds. Robins, grosbeaks, crossbills, and even yellow-rumped warblers do the same, but they stay in more northerly climes. The yellow-rumped warblers, for instance, have been observed in Nova Scotia in the dead of winter. Why is this the case?

The availability of winter fruit is highly variable and of patchy distribution; these birds may need to travel widely to find it, and as soon as it runs out, there will no more until next season. If all of our birds stayed to rely on this ephemeral food source, they’d soon be in trouble. So they travel to Central or South America, where food sources are plentiful and more reliable.

Some birds have particular adaptations that allow them to eat food that others can’t. The yellow-rumped warbler has special digestive juices that enable it to consume bayberries, which remain on the plant well into winter; the wax of this common plant’s fruit renders it indigestible to any other bird.

So, this warbler makes a tradeoff: it avoids the enormous cost of migration to exploit local food sources, putting up with the attendant harsher climate. This can backfire on them, however, in years that see unusual weather extremes: ice storms coat their food with a layer of rime, preventing them from eating and causing them to starve to death or succumb to the cold from lack of fuel.

There are other factors that influence whether a bird will migrate long distances or stay in northern climates. I hope to address these in another column, perhaps next week.

Chris Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com