Adult American Redstart male (larger photo) with its striking plumage and American Redstart females with more conservative yellow-on-graey. Photos by Josh Engel.

This article was supposed to be about how small back yard birds – the juncos, cardinals, goldfinch, chickadees, nuthatches, house finches, house sparrows, mourning doves – survive the vicissitudes of our northern winter. Do they huddle together, do they have territories they defend?  They certainly squabble at the feeders and at my heated birdbath, and it seems as if I can discern a pecking order among and within species.

Looking for answers, I turned to a 2019 book called “Birds in Winter,” by Roger F. Pasquier, highly recommended by Bernd Heinrich, himself a renowned biologist and one of my favorite nature writers. The book is more than a compilation of studies of birds in winter throughout the world. It includes, much to my surprise, fascinating information on how “our” spring and summer migrating birds spend wintertime in in the south. 

Topic change.

Like snowflakes, each species is different.  For today, let’s look at just a few of the wintering warblers.

American Redstarts and Black-throated Blue warblers wintering in Jamaica have been the subject of multiple studies.  American Redstarts breed in our deciduous forests. They are dimorphic – adult males sport a striking orange-on-black black plumage, while young males and females are a more conservative yellow-on-gray. (see photo).  One study found when Redstarts reach their wintering grounds in Jamaica, they establish territories not only in forests but also in farm fields and urban areas, especially in the lowlands. They eat only insects, as they do in summer. Black-throated Blue warblers, on the other hand, can live more densely than they do in their breeding territories because they eat a variety of foods, including insects.

What scientists call “sexual habitat segregation,” where one sex predominates over another in a species’ winter habitat, has been demonstrated for 14 different songbird species. And it is not uncommon for the females to get the raw end of the deal.  For example, on a shade-grown coffee farm in Chiapas, Mexico, Inga trees provide the shade. Ingas are a tropical pioneer nitrogen-fixing species that produces copious white nectar-rich flowers and fruits. They are light-loving, grow quickly, and can thrive in poor soil.  Their leaves have small, nectar-producing glands that attract insects, many more than are attracted to coffee plants. Black-throated Blue warblers are attracted to those insects. A research study showed that males dominated in the Ingas. Females were relegated to the less productive coffee leaves below.

Inga trees also grow in isolated islands in cattle pastures in Chiapas.  These islands are the only areas that can support migrating birds.  A 1994 study showed that during the winter, these islands were dominated by male Yellow warblers who successfully fought off 29 other bird species, resident and migrant, including Magnolia warblers. The males controlled the canopies, where there are more arthropods. Females were relegated to the lower shrubby vegetation which supports less nutritious meals. In spring, however, when the Ingas flower, the story changes. Orchard Orioles arrive to take advantage of the flowers.  And, that is it for the male Yellows; the Orioles chase them off.

Studies indicate that birds’ winter habitat can affect how successful they are in breeding.  The nutritional health obtained during winter may affect how early they arrive on their breeding grounds, how early they nest, and how many young they fledge.  It is important to look at a bird’s entire year to understand how it is faring.

A case in point.  A study at the Font Hill Nature Preserve in Jamaica was published in January, 2001 in The Auk. During the non-breeding season, American Redstarts establish strong territories and are easy to observe and capture.  The study examined consequences for annual survival, longevity and changes in the body mass at two different habitat types. They found that the habitats differed in their proportions of males and females. Males predominated in the best habitat, the mangroves, where water and leaves are present throughout the winter. Females were predominant in second-growth scrub forest, which, during the winter dry season, is without leaves and never has water. The Redstarts of both sexes in the scrub lost body mass and muscle. They lived fewer years than those wintering in the mangroves. They were less fit to undertake migration and left their wintering grounds later, thus arriving at their breeding grounds late. 

Female Redstarts wintering in healthier breeding grounds will arrive earlier and in better condition and will produce more eggs.  Redstarts have only one brood a summer and stay only two-and-a-half months on their breeding grounds. Early arrival and physiological health make a difference in successful breeding. The birds, mostly females, wintering in low-value habitats are at a definite disadvantage.

While science continues to increase our understanding of the importance of wintering habitat, the amount and healthy condition of migration and breeding habitat continues to be critical for migrating birds. Here in the Prairie state, we are fortunate, in Evanston, to have a lot of trees that provide safe stopover areas for woodland migrants to refuel in spring and fall.  It is vital that we preserve and even increase our tree canopy and natural areas for the birds.

Libby Hill is the author of "The Chicago River: a Natural and Unnatural History. She has been writing about birds and trees and Evanston's natural history for the Roundtable since 2004.