Door to Nature: The Mystery of Migration

The total story of migration is complicated, fascinating and frequently quite unbelievable, and it includes many mysteries that scientists and students of ornithology are continually researching. Perhaps a third of all species migrate, amounting to tens of billions of birds.

Why would a bird that has spent the winter in the vicinity of the equator — in the midst of plentiful food and good nesting conditions — suddenly migrate several thousand miles northward to breed, nest and raise one or more broods?

Days and nights are equal at the equator. Hours of daylight during early summer increase as you travel northward, until you reach the Arctic Circle — and there birds experience 24 hours of daylight. Nesting adult birds require more hours of daylight in order to find food for themselves and their young. For example, a robin in northern Alaska was observed feeding its babies for as much as 21 hours during a day.

You could not infer that the birds realize the difference, but two entirely different habitats would obviously offer them more varied and nourishing diets. Fewer predators, parasites and infectious microorganisms exist in the far north because they’re killed or reduced in number during the extremely cold winter.

A survival-of-the-fittest concept results from the particularly demanding annual flights of migratory birds. Surely this has a highly favorable effect on the year-to-year maintenance of strong species of birds.

Yellow-rumped warblers nest in the county and are abundant migrants, some lingering until December. Photo by Roy Lukes.

Much more extensive land masses exist in the Northern Hemisphere, which offer breeding birds the increased amounts of space that are so vital to their well-defined territories. Fewer squabbles with their neighbors might lead to more time to find food, incubate eggs and protect nestlings.

Consider that a bird returning to its identical nesting site of the previous year must, during its long flight, know where specifically that is, be aware of the direction to get there and be capable of maintaining its course. This is quite a magnificent feat for a warbler not much bigger than your thumb.

Geographical, astronomical and physiological factors are considered to be important influences during the long, arduous flights immediately prior to breeding. It’s also a well-established fact that many migrants nest at the northern limits of their challenging flight, then quite mysteriously return to a more temperate climate and considerably shorter period of daylight as soon as nesting is completed.

There is excitement when the spring migrating birds fly north in their brilliant breeding plumage while singing territorial songs. The more leisurely, autumnal, southbound journey is not as dramatic. The Wisconsin Society for Ornithology considers the fall season to occur during a four-month period from August 1 until the end of November, rather than the typical three-month time frame during the spring.

Red-headed woodpeckers nest here but usually migrate to the southeastern quarter of the country. Some stay here all winter. Photo by Roy Lukes.

There are more birds flying south now, with many youngsters joining the flow. The haste and urgency to reach a destination are not as extreme as they are during the spring. The bright colors — primarily of the breeding male songbirds — are more subdued, so identification can be challenging. That applies especially to the warblers, which are often identified by their song during the spring but now are not as vocal.

Some of the earliest migrants are the shorebirds. Many nest in the Arctic region and have a long journey to their wintering grounds in Central or South America. Some of our flycatchers also leave before the end of summer. Members of this group are often identified by their song because many look very similar. When the nesting season is over, vocalizing is not as important, so flycatchers may be present but not heard.

Ducks go through a complete molt of their feathers once the breeding season is over, so they must remain in seclusion near their food source for several weeks. That means they hide in the marshes and waterways until later in the summer.

Some swallows gather in huge flocks of young and adults after the nesting is finished. They’re so adept at catching flying insects, swooping low over expanses of water to snatch their food. Places such as the Lake Michigan shorelines and some larger inland lakes may have hundreds of them skimming the water. Most seem to be gone by mid-September.

Game birds such as the wild turkey do not migrate, but the American woodcock — so evident during the spring with its extraordinary courtship displays — is usually gone by the end of October. The Wilson’s snipe sometimes remains here until mid- to late November. 

Some large birds are very noticeable as they glide and soar high above us. Turkey vultures are early to arrive in the spring, and their population during the last 30 years has greatly increased in our county. Most head south by mid- to late October, but a few may remain until early November. Bald eagles — which, when viewed in flight from a distance, can look a bit like turkey vultures — usually remain here throughout the winter as long as Lake Michigan remains open. 

Dark-eyed juncos often arrive from the north by mid- to late September. Photo by Roy Lukes.

Many people put out seeds and suet and enjoy seeing songbirds at their feeders during the winter. I feed birds all year long because of the joy of watching the young come into the yard, begging their parents for food or learning how to feed themselves.

Rose-breasted grosbeaks are the most abundant summer species in my woods. Adult males are white and black with a red breast patch, and females are medium-brown and white. All youngsters just off the nest look like the female, but if you can see their underwing feathers, you can tell which are male (pink feathers) and which are female (yellow).

Hummingbirds are active at the feeders all summer, but by early September, the adult males head south for the winter. Adult females are the next to leave, and the young, hatched this summer, will go last. My records from 2008 through 2019 show that the final date when a ruby-throated hummingbird was seen has varied from Sept. 23 to Oct. 5.

Thrushes are another group of songbirds that many people are familiar with, such as the American robin and the eastern bluebird, but a few that travel through our area on their way south merit close examination. The hermit thrush nests in Door County, but the gray-cheeked and Swainson’s nest farther north and merely stop here during their journey. 

Robins that nest far to the north in Canada may migrate only to Wisconsin to spend the winter here rather than flying farther south. Eastern bluebirds begin to head south by mid-October.

Tennessee warblers are among the earlier migrants, sometimes coming through by late August. Photo by Roy Lukes.

Winter visitors coming from the far north to spend the “off-season” begin to arrive after Labor Day. Most years, dark-eyed juncos are here by the end of September. Another ground-feeding bird, the American tree sparrow, arrives in October and can frequently be seen foraging along roadsides with juncos. 

Pine siskins are also early to arrive. Common redpolls don’t come here every winter and are usually first seen in early November. Crossbills are quite rare — observed here only in 2008 and 2012. The beautiful, rather tame pine grosbeak arrives in November but has not been present during five of the 11 years for which we have data. Evening grosbeaks have been seen during nine of the 11 years, but very locally and mostly in the far-northern parts of the county.

Cedar waxwings can be present all winter, but many do migrate. The more colorful Bohemian waxwings have been seen only in 2010, 2012, 2013 and 2015. Small birds of the open country fields are the northern shrike — usually solitary — and snow buntings, which are sometimes in large flocks. They arrive by mid-October or into November.

It’s helpful to maintain a daily phenology record of your sightings and keep these records for future reference. Birdwatching is a common citizen-scientist project that, if recorded with eBird, can help ornithologists in many parts of the country monitor avian populations and assess which factors are affecting their health.

Whether you feed birds all year long or just during the winter months, learn how to identify them and help them survive the rigors of life in the wild. You’ll find enjoyment and know that the migrating birds that are going south in the fall may return to your yard next spring and offer you more pleasures in hearing, feeding and watching them.