Spring migration is peaking now. Many warblers and thrushes are now arriving. Some are passage migrants, on their way to more northerly breeding territories, while others will find places to nest here in Maine.
Diet strongly influences when spring migrants will arrive in Maine. Birds like Red-winged Blackbirds, Common Grackles and sparrows that rely on seeds are among the first birds to arrive. Vireos, tanagers and warblers are leaf-gleaning insectivores, meaning they gobble up caterpillars feeding on tender leaves of plants. There is great risk in arriving in Maine before the leaves appear because caterpillar abundance is low.
Many naturalists like to keep track of the unfolding of spring. They track the night when the first spring peepers are calling, the first appearance of blackflies, the first appearance of leaves of various plants, the first flowers of Trillium, blueberries and other plants. This study of the effect of climate on recurring natural events is called phenology.
We know that the earth’s climate is warming. The past five years were the warmest on record. These upward trend in temperature is associated with increased carbon dioxide in the atmosphere. Carbon dioxide emissions have been increasing steadily since the Industrial Revolution began about 1850..
Phenological records indicate that many natural events are occurring earlier now than they did in the past, consistent with warmer winters and earlier springs in modern times.
For birds, we have some intriguing long-term records of spring-time arrivals in the northeastern United States. Chris Butler analyzed first-arrival dates from the records of the Cayuga Bird Club in upstate New York and the Worcester County Ornithological Society in Massachusetts for the period 1903 to 1993. He showed that all 103 species of migratory species are arriving early in the period of 1951-1993 than they did in the period of 1903-1950. Butler found that short-distance migrants like Yellow-rumped Warblers and Common Grackles are arriving about 11 days earlier now. Long-distance migrants that winter in the Caribbean, Central America or South America are arriving only about four days earlier. He suggested that short-distance migrants are more responsive to local temperatures since they are already in North America. There is no way that a Bobolink wintering in Argentina can know whether a Maine spring is warmer or colder than normal.
Vitale and Schlesinger analyzed the spring-time arrival dates of 44 species in Dutchess County, New York over the period of 1885-2008. They found earlier arrival dates for 40 of those species, with the average change being 11.6 days per century.
What do we know about changing arrival dates for Maine birds? We do not have a continuous record of arrival dates as seen for the New York and Massachusetts regions. However, the short-lived Maine Ornithological Society did publish the Journal of the Maine Ornithological Society (JMOD) from 1899 through 1911. The journal included bird records, including first-arrival dates.
From 1994 through 2017, I coordinated a citizen-science project to track the first arrival of Maine migratory breeding birds. We therefore have a way to compare average arrival dates at the beginning of the 20th century with the late 20th/early 21st century.
My students harvested all of the spring-time arrival dates from the JMOS for comparison to our modern records. We had sufficient data to analyze 78 species of migratory breeding birds.
The results stand in stark contrast to the results from the Massachusetts and New York regions. In Maine, only nine of 78 species arrived earlier in the 1994-2017 period than in 1899-1911. Twenty-two species are arriving later now! For 47 species, we found no difference in average arrival dates.
Why do we see such different results in Maine? I suspect the observers who contributed sightings to the JMOS were out in the field more often than the contributors in my arrival date project so hence were likely to detect migrants earlier in the season.