A Good Year for Eagles

Spent the weekend at Starved Rock State Park, manning a table at the EagleFest, so:

Bald Eagles (Haliaeetus leucocephalus), Starved Rock State Park,
LaSalle Co, IL 1/26/2014

These are a few of the Bald Eagles that can be found just below Starved Rock Dam in the winter. This has been a particularly good year for them, I counted 40 or so, and that number will probably increase over the next month.

What has made it a good year? Ice -- lots and lots of ice. When the Illinois River freezes, they congregate below dams where the water remains open. It's the only place you can still catch fish, and the only place you can count on finding geese. So, even though you face some competition:

It's still worth being there. In a mild winter, with lots of open water, the eagles disperse up and down the river, and you don't see the big concentrations. This same pattern happens on the Mississippi River.

So a good year for Eagles isn't really a good year for the eagles, just the eagle watchers.

The same thing happens with migratory songbirds -- places like Point Pelee and High Island are known for fantastic concentrations of migrants. On good days, you can have warblers feeding almost between your feet!

But those good days come when birds moving north over water meet headwinds that exhaust them. While it's difficult to say how many birds fail to make the crossings (Lake Erie to Point Pelee, the Gulf of Mexico to High Island), Sillett and Holmes estimated that Black-throated Blue Warbler (Setophaga caerulescens) mortality rates increase 15-fold during migration. (1) Even inland, we see the same effect -- major fallouts occur when birds moving north meet a cold front coming south, and you end up with birds trying desperately to find food on a cold, often wet spring morning.

For a warbler, a good spring for migration means no more stopovers than absolutely necessary -- think business traveler, not tourist -- and the more of them you see in your backyard, the tougher the migration likely was for them.

So when you hear that this was a "good spring", stop and think about who it was good for.

(1) Sillett, T. S., & Holmes, R. T. (2002). Variation in survivorship of a migratory songbird throughout its annual cycle. Journal of Animal Ecology, 71(2), 296-308.