Nature Trail stream. Photo by D. Sillman
(Click on the following link to listen to an audio version of this blog … Winter Birds
This is another essay from the “Virtual Nature Trail” website. It is sad that the nature trail on the Penn State New Kensington Campus, functionally, no longer exists. You can, though, step out almost anywhere and see wonderful birds all winter! Do it!
What birds could I expect to see on the Nature Trail in winter?
The diversity and numbers of birds on the Nature Trail in winter are less than the rich summer and transition season populations. You should be able to see, however, a nice variety of bird species throughout the winter season. Some of the birds that you could expect to see include chickadees, titmice, downy woodpeckers, blue jays, and song sparrows. You may also find turkey tracks (although they are usually quite secretive and very hard to see), and you may even see an over-wintering red tailed hawk.
American robin. Photo by D. Sillman
Why do some birds migrate and others do not?
The main reason for bird migrations is the availability of food. You can think about migration in terms of food energy “economics”. It costs a bird a great deal of energy to fly “south for the winter”. This migration expense is “paid” by the bird because the winter food supplies in its summer range are not adequate to ensure its survival. Further, this migration cost must be “paid off” by a rich food supply in the area into which the bird is migrating. This balance of “survival energy” in the summer range and “revival energy” in the winter range is a dynamic one and is constantly being tested and restructured by variations in individual bird’s behaviors. Within any population of “migrating birds” there will be some individuals that do not respond to the environmental cues for migration. These individuals, then, remain in their summer range even as the winter season asserts itself. The usual fate of these birds is death from exposure or starvation and thus the migration behavior is reinforced within the successful behaviors of the species. Some non-migrating individuals, though, may in fact survive the winter possibly by exploiting a new food source or possibly as a result of a site’s changing climate. Over time, with the survival and subsequent reproduction of these non-migrating birds, a migrating species could either form a non-migrating sub-population or if the advantage for non-migration is
Wood thrush. Photo by D. Sillman
strong enough, may become an entirely non-migrating species. The American robin is an example of this process of selection and change. The robin is the classic example of migration and is often referred to as the natural harbinger of spring because of its predictable return to its northern, summer ranges. There are, however, increasing numbers of American robins that are either reducing their migration treks or eschewing migration altogether primarily as a result of human modification of their northern habitats. The reliable occurrence of food (berries and other fruits) and increased quality of shelter in suburban and urban areas make the northern regions of the United States an increasingly tolerable winter habitat for robins. These changes along with warmer conditions in urban zones (which prevent soils from freezing until later in the winter season thus allowing earthworms and other invertebrates to be increasingly available for food) alter the balance between migration costs and possible benefits to the robins. On- going changes in the migration behaviors of the American robin and its increasing winter presence on our Nature Trail are examples of the dynamic nature of adaptation and selection within the natural world.
Blue jay. Photo by D. Sillman
How do over-wintering birds survive?
As mentioned above, food is the primary concern for over-wintering birds. If food is present in sufficient quantities through the winter in a bird’s “summer” range, then the expense of migration is not biologically justified. Anything, then, that will increase total food supply or even increase the ease by which food is found will greatly influence the survival of over-wintering birds.
One very interesting behavior that can be observed in the many of the winter birds of the Nature Trail is the formation of mixed species flocks. The chickadees, titmice and downy woodpeckers in particular join together into large, complex flocks for the winter. At first glance this flocking behavior would seem to be disadvantageous to all of the species and to all of the individuals concerned. Food is in short supply in the winter Nature Trail ecosystem, how could the clustering of many individuals that all eat approximately the same prey items (primarily insect larvae) do anything but decrease the survival of respective species? The answer involves the facilitation of food finding and the reduction in the average individual’s energy devoted to searching for food. In the large mixed flock there is a very high probability that some flock member will find a food source (a cache of larvae under some tree bark for example). Exploitation of this food source by the entire flock unit with the subsequent high probability that another individual will find another food cache “smooths out” the expected boom and bust food cycle of the winter system, thus increasing the survival of a higher percentage of individuals in the mixed flock. Further, from the perspective of the downy woodpecker, flocking with the very alert and excitable chickadees and titmice also increases their awareness of incoming predators and thus adds to their chances of winter survival.
Another interesting behavior that was described by Aldo Leopold (author of the “Sand County Almanac”) is the positive response of chickadees to loud, explosive noises (like shot gun blasts or tree limb breaks). The chickadees swarm toward the sound very energetically (and in a mixed flock carry along the titmice and downy woodpeckers with them). The breaking of a tree limb or the falling of a tree opens up the woody encasement that may be densely inhabited by ants, ant larvae and or other insect larvae. Drawn to the loud noises, these birds can rapidly exploit a suddenly available food source.
Photo by D. Sillman
Why don’t the birds on the Nature Trail sing in the winter?
The calls of birds are one of the most beautiful aspects of nature. Theories have been proposed that the first music made by ancient humans was an attempt to recreate and control the haunting beauty of bird songs. The woods do seem empty in the winter without the whistles and melodies of the native songbirds. We do hear the buzzing of the chickadees and the rapping of the woodpeckers, but the lack of full throated singing makes the forest seem empty and barren.
But why don’t the birds sing in the winter? The answer really lies in the reason for bird songs in the first place. Although many birds do use song as a mechanism of individual recognition and contact, the primary reason for song especially in males is advertisement of themselves! The male bird sings to declare his individual territory and to attract a mate (or as many mates as possible depending on the species!). Mating is not one of the biological functions of the winter season, so songs are unsung until spring.
Photo by D. Sillman
Why don’t we have bird feeders and seed out on the Nature Trail in the winter?
The decision not to have bird feeders out on the Nature Trail was made after very careful deliberation. We decided that we wanted to manage the Nature Trail ecosystem to optimize natural habitats to provide food, water and shelter for the over-wintering bird populations. To this end raspberry, wild grape, sassafras, barberry, crabapple, and even poison ivy vines are allowed to flourish within the Nature Trail habitat. Further, many “weeds” are encouraged to grow on the edges of the wooded areas of the Trail. These weedy plants generate substantial amounts of natural seeds that are harvested by the birds throughout the winter. We are fortunate to have the spring fed stream down in the ravine that provides water nearly year round for the birds and many other members of the Trail’s rich fauna. Now, many of us associated with the Nature Trail do have bird feeders at home
Cardinals in late winter. Photo by D. Sillman
which we keep stocked with seeds throughout the winter. One of the reasons for this “home feeding” has to do with the impact of human activity and societal preferences upon the natural seed sources for many important birds. Typical landscaping around a home involves both the extreme simplification of the types of plants growing in the yard and also the energetic prevention or removal of “weeds” from the entire vegetative habitat. Without a complex base of plants and especially without these “weeds” and their incredibly abundant seeds, there is very little for a small songbird to eat. A mowed grass lawn to a bird is as attractive and as sustaining as a concrete parking lot. Bird feeders are a fairly easy compensation for the potentially drastic impacts of this over-simplification of the yard habitat. It is possible, though, to manage your yard habitat in a way to provide a more natural food and shelter base for songbirds. There are a number of books and organizational publications that discuss landscape plans and plants that can make your yard both aesthetically attractive and naturally productive especially for birds. An excellent contact for information on bird gardens and yards is the Cornell Laboratory of Ornithology.