Thursday, December 3, 2009

Red-breasted Mergansers

Red-breasted Mergansers (mostly) on Lake Erie, east of Cleveland, November 2006. Lake Erie, especially the waters between the islands (South Bass, Kelleys, etc) and east to the Ashtabula area, is a major staging area for this species.

Our Ohio Birds listserv has hosted lots of discussion about this species, some of it from veteran lake-watchers who have a good grasp on waterbird movements on the lake. There is a general consensus that numbers have declined, but trying to determine by how much, or what the causes may be, is not easy.

I remember on many occasions seeing passing swarms of mergansers that were so massive and so dense that the flocks looked all the world like storm clouds scudding rapidly low over the water. Trying to estimate their numbers was practically an excercise in futility; certainly falling more into the "guesstimate" than estimate category. As many as 250,000 mergs have been claimed in one day in years past.

There are still a lot of birds on Lake Erie. I've been fortunate enough to have the opportunity to fly low over the lake in an airplane a number of times in recent weeks, specifically to estimate bird numbers and species. Aerial perspectives give observers a far more realistic picture of what's going on out on Lake Erie than onshore observers can hope for, in part because we can travel far beyond the reach of onshore optics, can cover much more ground, and can see many more birds simultaneously.

Estimating Red-breasted Merganser numbers on Lake Erie is still tough!

Red-breasted Mergansers are flighty critters; far more so than other waterfowl that we encounter on the lake. They are prone to frequent liftoffs and aerial sorties, and may do lots of circular wanderings, compounding the likelihood of double-counting. This is especially true when estimating from an onshore site, where it's possible many of the same birds will pass by in a relatively short window.

So, while merg numbers may be down from record highs of the 1980's, there are still lots of birds out there. Estimates from the last week or so from aerial surveys calculate well over 80,000, and these tallies are a lot more solid than prior estimates made from the ground. And it's almost certain that many birds are missed - it's impossible to find them all, even from an airplane.

It's premature to speculate as to exactly what may be causing the demise of Red-breasted Mergansers on Lake Erie, or even if they truly are declining.

There will be some interesting data on this issue emerging, and a good upcoming opportunity to learn more about the status of migratory waterbirds on Lake Erie. I'll report more on that later.

No comments: