Abundant evidence exists to show that the earth has been warming over the past century.  This evidence includes the melting of the polar ice caps with the consequent rise in sea level, earlier leaf-out dates for trees and bushes, earlier ice-out dates of lakes and the northern range expansion of various species.

The spring arrivals of migratory breeding birds are also sensitive to a warming world. With leaves emerging earlier, caterpillars become active earlier, providing earlier food for warblers and vireos. Nectar for hummingbirds should be available earlier.

A number of studies have documented earlier arrivals of migratory breeding birds. Bird clubs in the Worcester, Massachusetts area and in the Ithaca, New York area have been compiling first arrival-dates of migratory birds for over 100 years.  The data clearly show earlier arrivals in recent years for nearly all species of migratory birds frequenting those areas.

In 1994, I began a citizen-science project to monitor the arrival dates of over 100 species of Maine migratory breeding birds. Each volunteer is asked to note the first arrival of each species along with the location of the sighting.  This on-going project has taught us much about the nature of spring bird migration across the state.  We know have over 55,000 arrival dates in the dataset.  The collective contributions of so many volunteers have made this project possible.

We know over the period 1994 to 2014 that some bird species arrive a bit later in cold springs and a bit earlier in warm springs.  However, Maine does not have a continuous record of arrival dates to rival those of the Ithaca and Worcester bird clubs.

Maine however did have a bird organization, the Maine Ornithological Society, active around the turn of the 20th century.  This organization published the quarterly Journal of the Maine Ornithological Society (JMOS) from 1899-1911.  The journal regularly published arrival dates of Maine birds along with censuses of birds. Danny Kipervaser, Scott Lilley and I collated the arrival date data from the JMOS to compare to contemporary arrival dates.  Our expectation was that birds should be arriving earlier now compared to then.  Our predictions missed by a mile.

We only had sufficient data from the JMOS for 80 of the species I track in the current arrival date project.  Of those 80 species, only nine are now arriving earlier in modern years.  These included inland Common Loons, Great Blue Herons and Red-winged Blackbirds, all of which need open water in their lake or marsh habitats.  The other six species were American Woodcock, Ruby-throated Hummingbird,  Tennessee Warbler, White-throated Sparrow, Rose-breasted Grosbeak and Indigo Bunting. Most of these species arrive between four and eight days earlier now.

The most common result was to see no significant change in arrival dates across the time span.  Fifty-one species are arriving at the same schedule now as they did a century ago.  Quite a contrast with the Worcester and Ithaca records.

Remarkably, 20 species are now arriving significantly later than they did around the turn of the 20th century.  These species include seven aerial insectivores (swallows, nighthawks), three warblers, four sparrows and Bobolink.

What’s going on here?   I see two possible biases.  First, the amount of forests we have in Maine now is far greater than in 1900 when farmland was more extensive.  As a result, habitat for the sparrows and Bobolinks that seem to be arriving later were almost certainly more abundant then than they are now.  Thus, the chances of seeing an early arriving bird were higher a century ago.

Second, the JMOS birders were likely more in tune with nature than we are.  Outside on foot or on horse-drawn carriages, these observers would be less likely to miss a first arrival.  Despite our fine optics, the JMOS ornithologists might well have been keener observers than we are.

[Originally published on May 16, 2015]