Last spring, I wrote a column summarizing the citizen-science project I conducted over the past 24 years to monitor the spring arrivals of Maine migratory breeding birds. With the aid of several hundred birders who contributed first arrival dates, we have learned much about the regional and year-to-year patterns of spring bird migration in our state. Here is a webtool to explore the data: https://hobbes.colby.edu/arrival/
When I launched the project in 1994, I simultaneously launched a project to monitor the fall departures of those same Maine migratory birds. That project never took flight, mainly because of the time commitment involved. It’s easy for observers to record their first sighting of an Eastern Phoebe or Yellow Warbler in the spring. Determining the last data of sighting in the fall requires continuous record keeping.
With the advent of eBird in 2002, many data on the occurrence of bird species in Maine became available to researchers. Although most of the records in eBird date from 2002 onward, some birders and researchers have uploaded older data into the eBird database. I used this treasure trove of information to determine the fall departure dates of our migratory breeding birds and to examine differences in departure among regions in the state on the north-south axis.
To make this study comparable to the spring arrival project, I only used eBird fall occurrence data from 1994 through 2017.
You can download a copy of the paper at: https://bit.ly/2RyVCIs
For the analysis, I defined the beginning of fall as the Last Safe Date provided by the Maine Breeding Bird Atlas Project. The Last Safe Date is the latest date for a species when nesting is expected. For instance, the Last Safe Date for Barn Swallow is July 21 but the Last Safe Date for Eastern Bluebird is August 21. I defined the end of the fall migration as December 31 although the vast majority of fall migrants departed well before then.
I divided the state into three bands of equal latitude. The North Region extended south to 45.7 °N, the South Region extended from 42.6 °N south and the Central Region was in between. I analyzed the data for each region separately.
For the occurrence dates between the Last Safe Date and December 31 for a particular species, I examined the latest half of the records, the latest 10 percent of the records and latest 5 percent of the records. For each set, I calculated the mid-point (median) of the arrival dates. Using only the latest 5% of the records provides a better measure of the last departure date but at the expense of small sample sizes.
Space limitations prevent the listing of the departure dates here so download the paper to see those dates.
As an overview, one would expect that birds would depart from Aroostook County and other North Region areas first. Occurrence records in the South Region should be the latest.
These expectations were met for most of the species analyzed. Median departure dates from the North Region were a few days to several weeks earlier than departures from the South Region with departures in the Central Region being intermediate.
A few species (Cliff Swallows, Canada Warblers, White-throated Sparrows) unexpectedly showed latest departures from the Central Region rather than the Southern Region. I suspect the Central Region provides more favorable staging areas for these species, causing them to linger, and then pass quickly through the Southern Region where they are less likely to be detected.
Nat Wheelwright, recently retired Professor of Biology at Bowdoin College, has just completed his weekly video series, Nature Moments, on the natural history of the common plants and animals of eastern North America. You can visit at: https://research.bowdoin.edu/nature-moments/
The site is a great resource for naturalists and educators. Don’t miss the video called The Sound of Extinction.