The weekend of May 12, 2002 was a big one in Minneapolis for migrating birds. Practically overnight, it seemed there were migrating birds everywhere, both in residential neighborhoods and in natural areas such as the Mississippi River Parkway. For example, during the course of an hour-long walk along the West River road about 1.5 miles south of Augsburg College on the evening of the 12th, my family and I spotted multiple individuals of at least 5 different species of warblers, not to mention an eagle (appeared to be a juvenile golden eagle) and a black crowned night heron. The warblers were particularly conspicuous, and appeared to be highly distracted by the urgent need to find food. Normally wary of people, as they searched the grass, shrubs, and trees for insects many of these warblers seemed almost completely unaware of the people walking within feet of them. One of the most conspicuous was a male bay-breasted warbler searching for insects in a narrow strip of grass between the street and the bicycle path (see the small bird near the center of the photo below).

Above: Male bay-breasted warbler foraging for insects along the West River Road, roughly 1.5 miles south of Augsburg College in Minneapolis.

Below: Closer view of same bird

 

Bay breasted warblers are not common in this area, being seen only when migrating through to or from their breeding grounds to the north. Due to its apparent lack of concern about our presence, this bird attracted considerable attention from several sets of birdwatchers. We sat in the grass and watched and photographed this bird for about 15 minutes, during which time it came within a few feet of us on multiple occasions. Then, a bicycle came past and startled the bird, causing it to fly out in front of a passing car. Impact with the car killed the bird instantly.

 

Above: male bay breasted warbler moments after being killed by a car.

 

After observing this bird for so long, seeing it killed abruptly was disturbing and disappointing. More importantly though, this event illustrates several more important points. Namely, migration is a difficult time for birds, and human activities can have a big impact on wild populations.

Many of our North American birds migrate. If asked why birds migrate, most people would probably say that they migrate south in the Fall to avoid the cold of northern winters, but that is mostly incorrect. The main reason for these migrations is due to food shortages during northern winters. Bird species that can find food all winter in the North tend to stay in the North all year round. Many species of seed eaters stay all year round, as do birds such as some of the woodpeckers that forage for dormant insects in tree bark. Even very small birds such as chickadees can survive harsh northern winters, despite their small size (which gives them a large surface area to volume ratio, meaning that they will tend to lose body heat extremely rapidly). The 50 or so species of warblers that nest in North America, on the other hand, are specialists at gleaning insects from vegetation, constantly flitting from twig to twig or branch to branch in their search for insects, and occasionally even grabbing an insect or two out of the air. The sorts of insects warblers eat are not available in the winter in the North, so they fly south in the Fall to spend the winter in more suitable habitats in the southern states, the islands of the Caribbean, Central America, and even South America. Birds such as swallows, swifts, and nighthawks are specialists at capturing flying insects out of the air, and hummingbirds feed on flower nectar and insects captured from flowers. These birds, along with many other species, migrate as well. Often migrating birds travel enormous distances, even crossing large bodies of water such as the Gulf of Mexico without stopping until they reach the distant shore - even tiny ruby-throated hummingbirds cross the Gulf of Mexico. There are some species of songbirds and shorebirds that even travel from Canada or Alaska to south Pacific Islands! It is important to remember that these birds are not seabirds....the ocean is an inhospitable place for these birds so they cannot stop to feed or rest until they reach land (in contrast, migrating arctic terns, which migrate from the arctic to the Antarctic and back every year are seabirds that have the potential of feeding along the way).

Such migrations are absolutely phenomenal feats, but they are undertaken because these migrations allow the birds to utilize the rich food resources (e.g. abundant insects) of the North during the breeding season even though these areas are not habitable for these particular bird species in the winter. These migrations are costly, however. Long migrations require phenomenal amounts of energy, which can leave the birds in an emaciated state. Not all individuals survive the rigors of migration, and those that do need to feed fast and furiously when they get the chance to regain lost body weight. In the Fall, there is a certain urgency because conditions are deteriorating in the North, so the birds cannot linger too late into the season. On the return trip to their breeding grounds in the Spring there is another sort of urgency, for there can be intense competition for breeding territories in the North... a bird that arrives too late might have difficulty establishing a breeding territory. Thus, these birds cannot dally any longer than necessary, but the danger of pushing North too soon is possible starvation should there be a late Spring cold snap. And if they live long enough, and some do, the birds have to face these problems year after year for a number of years.

And you thought YOUR life is difficult sometimes!

In the case of the bay-breasted warbler whose life was cut short by a car, this individual had possibly already made several round trips. This species nests throughout much of Canada and in a few slivers of the United States near the Canadian border, but spends its winters in northern South America, so it travels a significant amount each year. My guess is that the bird in these photos and many of the other warblers we saw that evening, were distracted by great hunger, resulting in riskier feeding behavior than they might normally exhibit. Furthermore, given that they were not local residents, they were less familiar with their surroundings than resident birds would be, and they were feeding in habitats that might not be their normal habitats (bay-breasted warblers normally nest in coniferous forests, for example).

Cars of course kill a lot of wildlife, not just migrating songbirds, but there were other hazards that evening for the migrating warblers. On our short walk we saw an inordinate amount of hunting behavior on the part of the local cats. Within a three block stretch within the local residential area, we observed two different cats stalking and nearly catching yellow-rumped warblers, and earlier in the day we observed a neighbor's cat carrying a dead warbler in its mouth. Thus, not only do we humans reduce wildlife populations with our cars, but high predation from pet cats allowed to run free also reduces native bird populations.

Many birds, both migrants and otherwise, also die due to collisions with various man-made structures. Not the least of these hazards are large windows that birds attempt to fly right through, not noticing that there is glass blocking their path. To reduce such impacts images of hawks or falcons or models of owls are sometimes hung in or near large windows to scare off smaller flying birds. The photo below shows the profile of a falcon placed in the large windows of the Lake Harriet bandshell in Minneapolis.

Though the above factors do indeed kill many birds, the effects of cars, cats, and man-made structures are probably small compared to a much more insidious effect of human activity....habitat destruction. Populations of many North American songbirds (and other wildlife) are declining, in part due to loss of breeding habitat in the United States, but also due to alteration of habitats in the bird's tropical overwintering grounds due to human activities such as farming and deforestation. Indeed, habitat destruction is the most significant cause of the major declines in many animal and plant species that are occuring worldwide.

Fragmentation of forests into small patches also exposes forest dwelling birds to increased levels of nest parasitism by brown-headed cowbirds, which are birds of open country and forest edges. Cowbirds lay their eggs in the nests of other songbirds (removing a host species egg from the nest each time they lay an egg), who are tricked into raising the cowbird young. Often the young cowbirds are larger than the young of their adoptive parents, and the cowbird young compete for food and space and sometimes kick their nestmates out of the nest prematurely. This all can reduce the reproductive success by the host species.

 

Links:

 

For more information on cowbirds and the controvesy over their effects on songbirds, go to the following web sites:

http://audubon.org/bird/research/#introduction

http://cowbird.lscf.ucsb.edu/

http://www.suttoncenter.org/bhco.html

http://www.prbo.org/Cowbirdlinks.html

 

For information on a very interesting book on bird migration, go to:

http://www.fsbassociates.com/fsg/livingwind.htm