Several people have mentioned to me recently that they saw huge flocks of robins in the snow.

Huh? The popular image of a robin is that it shows up when the ground has thawed so it can find -- and eat -- tasty worms that emerge.

Not so, says Audubon Pennsylvania's Keith Russell. I asked him about the huge flocks, and here's what he wrote:

The flocks of robins are perfectly normal for this time of year. Robins normally begin migrating north by mid February. This year they have been stifled by the cold snowy weather. The flocks observed on lawns are possibly anxious migratory flocks waiting to move. By the end of this week when it warms significantly a lot of these will finally be able to fly north.

We have robins in Philadelphia all winter of course so its not easy to separate migrating birds from wintering birds in February. But there will be a gradual turnover of birds during March as many of the birds that winter here leave for breeding areas further north and are replaced by local breeders returning from wintering locations further south," he said. "Robin migration probably continues here through sometime in April. Unlike most songbirds Robins migrate during the day and on warm sunny days during February and March you often see migrating flocks passing by overhead in a northerly direction. Common Grackles and Red-winged Blackbirds also migrate diurnally at this time of year.

Despite the extremely cold weather robins survive during the winter by eating berries from trees and vines like American Holly and English ivy to name a few. In a pinch they also eat the seeds of Japanese pagoda trees. Really cold temperatures don't seem to be a real problem for them as long as they have food in the form of fruit. But on warm winter days they readily look for invertebrates on the ground in loose soil or under leaves. And they also get water by eating fresh snow. Pretty resilient birds.

Maybe folks are just noticing the big waves of birds this year because their read breasts are so vivid against the white snow.

Journey North, a citizen science site that tracks all sorts of migrations, including robins, has a map logging robin "waves."

On Feb. 1, for instance, Philip Wallick of Haverford noted on the site: "We had had ten days of very cold weather and deep snow -- from 4 to 12 inches, and nighttime temperature of 10 to 14 degrees, daytime temperatures of from 14 to 24; but 10 days without being above 32. The day before it began to warm up. Today a large grassy bank became completely bare and the temperature was in the 40's. A flock of 15 robins descended and began pecking for worms. They stayed off-and-on for about an hour. I think they were finding worms."