Thursday, March 7, 2013

Winter Bird Population Survey 2012-2013

The 21st year of RRBO's Winter Bird Population Survey has been completed. Over the late-December to late-February survey period, 39 species were tallied. The previous annual average number of species is 38. We added yet another new species this year, a Merlin recorded on 19 December*. This brings the total cumulative species list for 21 years to 73.

Comparing this season's results to the previous 20 years, a few things stand out. First, the number of American Robins was the lowest since 1995. Only previous two years have had lower numbers than this year. While many people still think of robins as harbingers of spring, a lot of robins will winter in southern Michigan so long as they have a food supply -- temperature itself is not the limiting factor. I looked at historical climate data (average temperatures and precipitation for December and January) for the past 20 years and found no correlation between any combination of these weather data and the number of robins counted each year.

For robins, food supply primarily means fruit. Due to the long drought over the summer of 2012, the fruit crop this season in our area was quite poor. Additionally, fruit set was diminished because of the unusual warm period in March, when some fruiting plants set fruit very early, or bloomed but failed to set fruit because pollinators were not yet on the wing.

This robin has staked out some wild grapes.
Photo by alsteele under a Creative Commons license.

For the past 20 years, the mean number of robins seen over the winter survey season was 348, with a mean of 28 robins per visit (each season the area is surveyed/visited an average of 14 times). This season a total of only 47 robins and 3 birds per visit were recorded. The highest counts (although still fewer than 20 individuals) came the last two visits of the season, when robins began to coalesce into larger flocks and move back into the area.

Pine Siskins provided another highlight, this one more positive. This species has only been recorded in three previous years, and this year they were present on all but one visit and in numbers greater than 20 birds each visit prior to mid-January. This winter was excellent for many "winter finches" although strong numbers of siskins were not predicted for this region.

The lack of recovery of American Crows in Dearborn from West Nile Virus (WNV), which first showed up here in 2002, continues. A group of three American Crows flying over campus on January 1, 2013 were the only ones recorded all season. Nine out of the last ten years have recorded fewer than ten crows all survey season. My 2011 post provides more background on the crow decline.


On the RRBO web site, you can find the full results of all 21 years of surveys, along with information on the protocol.

*Technically, this survey is supposed to take place from 20 December to 20 February.  Due to weather and scheduling difficulties, we ran it from 19 December to 21 February this season.

2 comments:

FAV said...

Julie In NJ we are discussing a 2012 appearance of Common Chaffinch. There is a mention here in NJ (and by you in the past, below) of breeding in the UP or elsewhere. Can you get me closer to the refenrnce or answer about breeding and ESCAPES?

Fred
avtrader AT comcast.net



Rod, certainly European Goldfinch are becoming established as breeding species. Great Tit and Blue Tit have nested, but I don't have recent reports. Chaffinch is or has likely nested in the UP of Michigan, but a nest has not yet been confirmed. If you visit, you know where to go to feel like you're at home!

Julie Craves said...

Fred -- you can go to the RRBO web site and see my work on European birds in the Midwest; there is a link there to my paper in North American Birds (www.rrbo.org/eugo). The UP bird info I have mostly heard from birder's reports.

However, in the case of New Jersey, there are so many importers and breeders in the greater NY/NJ region that I believe the vast majority of the reports from that area are of escaped or released birds. I have never heard any hint of an established population or evidence of chaffinches nesting there.