The northern parula is a delightful spring treat in Wisconsin, as it skips along the canopy, headed to northern boreal and coniferous forests. This blue-gray bird, with a yellow belly and throat and a bejeweled necklace of blue gray and copper, looks to have arisen from the tropical forests of Central America, and does return to those forests to overwinter for over almost 8 months of the year.
Recent research indicates that northern parulas may not be keeping up with a changing climate. On their spring flights from Central America and the West Indies, the birds are falling behind leaf-out dates in North America. This asynchrony can have cascading impacts on the birds and their overall populations. A key to the migratory ecology of neotropical migrants is the correct timing of migration, yet as these birds follow the day length, northern climates are rapidly greening, and this leafing out of trees and shrubs brings a pulse of insects, including many caterpillar species. Missing this important pulse of resources could have significant implications for the conservation of the northern parulas.
Range maps of northern parulas show a distinctive gap roughly through the Midwest and portions of the Northeast. There are a number of possible explanations for this, though none very satisfying.
One possibility is habitat loss in these areas; the northern parula needs rather extensive blocks of forest, and remaining forest tracts in southern Wisconsin are relatively small. Yet, the parula makes it into areas of northern Illinois and Indiana, which are intensively agricultural and only have small forest fragments.
Another possibility is that parulas have some ecological requirement that is unfulfilled in the upper Midwest. In many observed cases of nest building, northern parulas use Spanish moss in the south (actually a bromeliad) and old man’s beard lichen in northern Minnesota, Wisconsin, and Michigan. These wispy materials are ideal for constructing nests. It is possible that a decline of the old man’s beard lichen (an Usnea species) due to air pollution has also caused a decline in northern parulas.
Lichens offer astounding insights into the overall quality of an ecosystem. Research in the Pacific Northwest has shown lichens to be reliable bioindicators, or representative of ecosystem health. Lichens accrue pollutants throughout the year, showing no seasonality, and they are long-lived. Lacking roots, lichens depend on atmospheric and water sources for nutrients, which also contain contaminants. Dynamic cycles of moisture and drying can concentrate pollutants over the lichen tissues, cell walls, and organelles. These features make lichens sensitive to pollution. In addition, widespread ranges offer clues as to where more severe pollution may have reduced or eliminated populations of lichens.
All of this is to say, the northern parula has an interesting range map; a curious gap in the central United States likely has no clear explanation, but a number of theories take aim at reasons why.
While northern parulas do not breed in most of southern Wisconsin, you can find them right now and in the next few weeks as they move through woodlands where the oaks are just starting to leaf out. Just the other day I was watching a group of parulas flitting through a stand of aspen. I got an excellent look at one of the birds as it dropped down onto a shrub and swallowed a massive caterpillar. The bird sat still for a few seconds—an eternity for a parula—and then flew off again, likely on its way to a spruce or tamarack forest in northern Wisconsin.
Written by Drew Harry, Faville Grove Sanctuary land steward
Header photo by Dan Pancamo