Buckthorn! > On Dec 6, 2018, at 4:32 PM, Val Landwehr > <0000012f44857088-dmarc-requ...@lists.umn.edu> wrote: > > I was intrigued by the discussion of whether in recent years > more robins are being seen during winter in the Twin Cities(Charles Neil, > Mary White and J. Baumann). I think that I > have seen more robins, particularly in large flocks, in > recent years. To put this hypothesis to the test I looked at the > results from four Christmas Bird Counts over the years that are > online at the Minnesota Ornithologists' Union website. The > Christmas Bird Counts I checked were Bloomington CBC, St. Paul > (North) CBC, Excelsior CBC and the Minneapolis (North) CBC. > I found a distinct trend of more robins being counted during > the counts since the winter of 1996 than were in prior years. > The numbers vary widely each year, but there is a distinct > inflection point around 1996 for a larger number of robinssince that date > than prior to it. Since 1996, the number ofrobins counted varies greatly from > year to year, but I don't > see any clear evidence that the numbers are continuing to > increase. > Average number of robins counted at Twin Cities CBCs: > Bloomington St. Paul (North) Minneapolis (North) Excelsior > Pre 21.2 17.1 2.5 > 5.3 > 1996 > 1996- 367.4 420.5 114.6 181.2 > 2017 > > Median number of robins counted at Twin Cities CBCs: > Bloomington St. Paul (North) Minneapolis (North) Excelsior > Pre 5 6 0 > 1 > 1996 > 1996- 278 255 26 > 132 > 2017 > I considered that the increased number of robins might reflect > more birders participating in these Christmas counts as well > as an increase in birding skills. If there are more skillful > birders involved in these counts I would expect a similar > increase in the counts of other species. So I looked at the > counts reported by the Bloomington CBC for the black-capped > chickadee, blue jay and dark-eyed junco. I didn't find any > trends or long term changes in the number of these species. > Therefore, I'm convinced that the increased number of winter > robins is real. > I didn't attempt to see if there is a link between the increasein the number > of robins counted and weather/climate. > That leaves a question in my mind. Are the larger counts just > a reflection that the Twin Cities population of robins has > increased but the same proportion of that population is > overwintering as in the past or have the overwintering habits > of the robins changed? > The CBC data speaks only to the presence of robins in December. > Maybe the only change is when the robins migrate to warmer > regions. Maybe if we had many years of counts for January and > early February we would find that the number of robins in > those months hasn't changed much over the years. As Manley > Olson pointed out, once the robins exhaust the availability of > crab apples, perhaps they head south and are now just as > uncommon in January and February as they were back in the > 1950s through 1980s. > Val Landwehr > Minneapolis > > ---- > Join or Leave mou-net: http://lists.umn.edu/cgi-bin/wa?SUBED1=mou-net > Archives: http://lists.umn.edu/archives/mou-net.html
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