Hey! What's going on with Dickcissels this year? Birders from Wisconsin and Minnesota are reporting extraordinarily high numbers of them compared with recent years. Some have suggested the present drought might be responsible, causing the birds search for more suitable habitat outside of their core breeding range. That's one possibility and here's a citation from Birds of North America to support that hypothesis:
Drought in core breeding range apparently forces many Dickcissels to move outward in search of more favorable conditions for nesting, as, for example, occurred during droughts in 1964, 1973, and 1988 (Emlen and Wiens 1965, Sealy 1976, Igl 1991).
Dickcissel Range Map
Here's an eBird graph showing the anomaly for Wisconsin (2008-2012):
And the same time period for Minnesota:
Incredible, isn't it?
Also note how much earlier they arrived this year. From this data I assumed I should to be able to find locations within their core breeding range where Dickcissel numbers are down (because they shifted north to escape the drought). I checked states like Kansas, Oklahoma, Nebraska, Missouri, and Iowa, but numbers there appeared to be pretty normal. Perhaps these birds came from some part of their breeding range where nobody is counting them? Could these higher numbers in WI and MN be on account of just one (or few) isolated high-density breeding location(s) they abandoned this spring? That seems unlikely to me, but it's possible. However, when I looked at a national graph, it appears Dickcissel numbers have been steadily increasing for the past several years:
Additionally, June maps from 2009 to 2012 seem to indicate a steady increase:
One explanation might be that Dickcissels had a highly productive breeding season in 2011 and experienced low mortality (from persecution) on their wintering grounds. The increase we're seeing in Wisconsin this spring and summer appears to be too great an anomaly for that to be the only explanation, so maybe it's a combination of drought, a general increase in numbers, recent breeding success, and/or something else we haven't discovered yet. I confess it remains something of a mystery to me, but I'm grateful there are more of them here for us to enjoy this summer!
Dickcissel image © 2012 Mike McDowell