Ecology is the study of the complex interactions between organisms and their nonliving environment. In the case of a bird, that means climate, habitat, other birds that compete with them, animals and plants that they eat, predators that pursue them and anything that modifies their habitat.
An example of the complexity of these relationships comes from a study in the high mountains of Arizona. There, climate change has led to drought and a reduction in snowfall. All but one of the area’s common nesting songbirds are declining. Why?
Less snow actually might seem like a benefit to these birds, which can’t start nesting until snow melts. But the absence of snow has allowed elk to remain in the mountains all winter. Formerly, elk migrated down into the valleys when it became difficult to traverse thick snow at higher elevations. These hungry elk now devastate the shrubs and small trees year-round at upper elevations once accessible to them only in summer. When spring arrives, there are no protective shrubs in which the songbirds may hide their nests and fewer leaves from which to pluck caterpillars for their babies.
It’s one thing to speculate that the elk, freed by climate warming from the constraints of snow, are the cause of the songbirds’ demise. But scientists must move beyond speculation about correlation to demonstrate causation, so researchers erected fences in six areas that kept out the elk, but didn’t affect other animals or the snowpack. Within five years, the vegetation had recovered and the birds were back. In six nearby areas without anti-elk fences, the plants, bugs and birds did not recover. There was one exception that proved the rule. A small sparrow sub-species called the gray-headed junco thrives in open, grassy savannahs created by browsing elk, and had not declined during the recent reduced snowfalls.
When the fences excluded elk, juncos declined as their grassy habitat was shaded under shrubs and small trees — the opposite pattern from all of the other songbirds.
Other scientists suggested that the birds recovered not because of elk exclusion, but because the fences kept out their predators. In fact, the special fences only kept out the largest animal in the area, the elk, while the regrowth of vegetation attracted more avian nest predators into the fenced areas. Because the thick recovering vegetation hid their nests so well, birds in elk-free areas actually lost fewer nests, despite the higher number of predators.
Once humans are added to ecological relationships, things get even more complicated. Our local deer overabundance is due entirely to a combination of human alterations of their habitat. Whether because of the reduction in free-running dogs, elimination of wolves and cougars, planting of delicious deer food in hunter-free suburban enclaves or the milder winters, deer populations have sky-rocketed. Scientists are still trying to untangle what this new ecological reality means for birds.
A study from William & Mary found strong negative relationships between deer populations and some forest songbirds among the fragmented forest patches on the coast. However, inland, where hunting pressure is higher and forests are less fragmented, deer are not yet depressing those bird populations. Restoring our missing songbirds in the face of an onslaught of deer, human development and climate change will require ecologists to further untangle the many interrelated pressures faced by birds.
Dan Cristol teaches in the biology department at William & Mary and can be contacted at dacris@wm.edu. To discover local birding opportunities, visit williamsburgbirdclub.org.
