The natural world is a mosaic of habitats. An aerial photo of Maine will show patches of forest, marshes, lakes, grasslands, and blueberry barrens along with human-altered patches of towns and farms. Each of these habitats offers a particular set of food and other resources available for animal use. All animals need to find and occupy the habitat that best meets their own needs.
The choice of habitats occurs by a process that ecologists and animal behaviorists call habitat selection. Research on habitat selection is important for crafting methods to help protect and conserve animal populations.
We know that habitat selection operates at different spatial scales. Ecologists use the term grain to describe the spectrum of habitat selection. For instance, we can speak about coarse-grained habitat selection to describe birds that nest in forests. Most of our wood warblers, Winter Wren and Scarlet Tanager are good examples.
However, not all forest tracts are the same. So, looking at a finer grain reveals that some birds prefer deciduous forests, others like old-growth coniferous forests and some like forests that are regenerating after a fire, strong winds or clear-cutting. So, to find a veery or wood thrush, head to a forest dominated by maple, beech and oak. Swainson’s Thrushes and Winter Wrens will be in old-growth coniferous forests. Mourning Warblers and Lincoln’s Sparrows will be in regenerating stands.
We can look at bird habitat at a yet finer grain. One of the first and most influential studies of fine-scale avian habitat selection was done on Mt. Desert Island and in Vermont by Robert MacArthur for his doctoral research at Yale University. He published his work in 1958.
MacArthur later joined the faculty at Princeton and became one of the leading ecologists of the 20th century, using his mathematical skills to advance theoretical ecology. But he was also a crackerjack naturalist. Sadly, he died in 1972 of renal cancer at the age of 42.
For his doctoral research, MacArthur studied habitat selection at the level of a single conifer by five species of wood warblers. Picture a conifer as a triangle on a stick. Five species of warblers tend to be found in particular parts of the tree. Cape May Warblers occur at the top of conifers. Yellow-rumped Warblers occupy the lowest level of our triangle, both on the periphery and in the center of tree. Blackburnian and Black-throated Green warblers like the periphery of the middle third of the tree, with the latter also found toward the middle. The central upper two-thirds of a tree is the bailiwick of the Bay-breasted Warbler.
You can confirm these patterns when you are birding in the boreal forest. The information is useful in finding that sneaky Bay-breasted Warbler skulking in the middle of a conifer.
Some ornithologists take issue with MacArthur’s explanation of this division of habitat by the five warbler species. He claimed the five species are reducing competition among themselves by occupying different parts of a conifer. He did not demonstrate aggressive behaviors of these birds, defending their part of the tree and driving off that Yellow-rumped Warbler that tried to feed in the top of a conifer.
We do have some evidence for competition between species in conifers. Doug Morse studied warblers in coastal Maine forests and on islands where species diversity was lower. On the mainland, Northern Parulas prefer to feed at the tops of conifers but are frequently attacked and displaced by Golden-crowned Kinglet (feisty birds half the weight of a parula!). On off-shore islands where kinglets are absent, the parulas forage happily in the tree tops.
In another study, Morse found patterns of habitat use similar to those described by MacArthur but further noted a difference between the sexes within a species. Male Magnolia Warblers, Black-throated Green Warblers, Yellow-rumped Warblers and Blackburnian Warblers foraged higher than females, usually above the nest location. Females foraged at the level of the nest.