Karl Overman: Birds and More


The Tree Swallow shown in the photo from Pointe Mouillee, Michigan on December 27, 2008 was one of 34 Tree Swallows found on the Rockwood Christmas Count.  That is an astounding number for a Christmas Count in the interior of North America as I hope to demonstrate in the discussion that follows.  Adam Byrne amazingly trumped that total by finding 48 Tree Swallows at Pointe Mouillee two days later on December 29th.

The Christmas Counts under the auspices of The National Audubon Society are a good indication of the status of birds across North America in early winter.  The count periods run from around December 14th through around January 5th.  For fifty years or so there was only one record of Tree Swallow on Christmas Counts in the interior of  eastern North America.  Even a coastal state like Massachusetts only had one year with Tree Swallows on the Christmas counts prior to 1969 and that was four in 1943 ( The exact year may be off one. See footnote 1) per the National Audubon website.

Michigan has had Tree Swallows three times during the Christmas Count period, all in the last decade and all on the Rockwood Count which includes Pointe Mouillee in Monroe County and Wayne County and Lake Erie Metro Park approximately 3 miles to the north in Wayne County:

December 21, 2002---4  (see footnote 2)

December 28, 2005--15 (Adam Byrne had 30 at Pointe Mouillee on December 30th)

December 27, 2008---34 (Adam Byrne had 48 at Pointe Mouillee on December 29th)

These Michigan records are extraordinary for the interior of North America.  Consider the Christmas Count history taken from the National Audubon website of other states and the province of Ontario compared to these records:

Ontario:                Ohio           Indiana          Illinois        Kentucky    Tennessee   Missouri

    1984--2            1977--5        1984--1        1984--1        1984--1      1968--3        1947--6

    1986--1            1978--1        1990--1        2004--1        1990--2      1971--1

    1987--3            1984--5        1999--1        2005--1                          1986-4

    2003--5            1986--7(total on 3 counts) 2007--1                         1991-4

    2006--1            1997--1                                                                 2005--1                                         

                            2005--1                                                                 2007-1

Minnesota & Wisconsin                  Iowa

None                                           2008--9 (total on 2 counts)

Viewing this data it is apparent that Tree Swallows attempting to winter or at least lingering into early winter in the interior of North America is a recent phenomena. Also I believe this data gives an approximate picture of what is happening with this species even if one would need to scrutinize the underlying data for each of these entries to be completely confident of the data, something I have not attempted to do. For example, the 1997 Ohio record applies to the December 19, 1997 Christmas Count for the Lake Erie islands of Ohio (CBC American Birds p260 (1998).  The problem is the one Tree Swallow on that count was in Ontario (Pelee Island) not in Ohio (per John Pogasnik on 10/6/1998). so Ohio needs to lose an entry for winter Tree Swallow and Ontario needs to gain one. 

It’s striking that no Christmas Counts in the interior have the quantity of Tree Swallows that the Rockwood Count in Michigan has had even though it lies north of all or almost all of the interior counts that have had Tree Swallow (I don’t know about the last two Ontario count years listed).  Even the Rockwood count of 4 in 2002 is above the median number and the counts of 17 and 34 are in a league by themselves for counts in the interior.  I am not a big believer in coincidences so that has to be an explanation other than random dispersal why Pointe Mouillee, Michigan would host larger numbers of Tree Swallows than anywhere else in the interior of eastern North America.   There is no warm water discharge in the area.  There is some open water but that would be true probably of all locations that have had winter Tree Swallows in the interior.  I am wondering if the vast, dense stands of phragmites at Pointe Mouillee might provide more insulation (and insect food gaining the same benefit of insulation?)  for the swallows unlike other interior locations where the non-native phragmites does not occur in large stands.

I find the large group (34 on  the count/48 on December 29th) to be particularly fascinating. These swallows just didn’t fly in the day before the count--no such invasion was noted anywhere in the Great Lakes region any time in December.  Adam Byrne had a count of 109 at Pointe Mouillee on November 26, 2008, a month before the Christmas Count and counts in the single digits for the species were had by observers in early December in the area.  On December 27th, the day of the count, the high was 60, making it a good day if you were a swallow; but that cannot be said for the weather that preceded the Count.  These birds decided to stay through the fall and early winter despite adverse weather including considerable snow. For example they had to endure seven days in December (16th thru 22nd) where the temperature did not get above 28 F with lows of 1 and 5 on two of the days.  Only 7 of the  40 days preceding the December 27, 2008 Rockwood Count were above average in temperature (Detroit weather data).   So unusually warm weather cannot be the explanation for why these swallows decided to linger (unlike arguably the 15 on the Rockwood Count in 2005)  What were these swallows eating is a big unknown.  The literature does mention that Tree Swallows can eat vegetable matter but it seems very unlikely that such a vegetarian diet could sustain them for a week at a time or else the species would already be wintering regularly in the southern areas of the Great Lakes states much like Yellow-rumped Warbler which switches from a summer time insect diet to a winter diet of insects supplemented with fruit (especially poison ivy in southern Michigan and Ontario).

Footnote 1

It must be noted that National Audubon’s website does not directly list years of occurrence; it lists what Christmas Count number the birds occurred in.  So the nine Tree Swallows for Iowa listed above for 2008 are listed in the Audubon website as being in Count #109 which ran from  mid December, 2008 to  early January, 2009.  Similarly Count #85, covers  mid December of 1984 to  early January of 1985.  Since probably 95 % of all counts in the eastern interior take place in  the month of December  and not in the first few days of January in the new year of the count period, it is helpful to reference the Audubon Count number by the beginning year of the count period, not the end year. Obviously, without checking the underlying record, a record listed above could be off by a year but that is unlikely to be the case on the vast majority of records.   For example, the first three Christmas count records for Ontario for Tree Swallow all occurred on the Point Pelee Count and all, not surprisingly were in December, not January:   2 December 22,1984; 1 December 22, 1986; 3 December 21, 1987.  See CBC American Birds 39 4 p446 (1985); CBC American Birds 41 4 p625 (1987); CBC American Birds 42 4 p597 (1988).

Footnote 2.

If you check National Audubon’s website for the record of 4 Tree Swallows on the December 21, 2002 Rockwood Christmas Count, you won’t find it.  That is because the reviewers erroneously, in my view, rejected the record, relegating it to the meaningless category of “swallow sp.”  The four Tree Swallows were seen by four observers, all very familiar with the species:  Robert Epstein, Jerry Sniderman, Mike Mencotti and Dave Mendus.  They saw the birds at least three times during the course of the day at close range.  Christmas Count reviewers do make mistakes, especially when a wintering trend in a species is just beginning,  and in this case the rejection of this Christmas Count record is not convincing.

Tree Swallow (Tachycineta bicolor)

Modified May 29, 2020