Saturday, May 9, 2009

Horned Grebes

Yesterday I took a trip further northeast in Montana, away from the side channel of the bird migration river at Fort Peck, to the main stream, where the migration is even more evident in the extreme corner of the state. More on that later.

For now I wanted to share some photos of a pair of Horned Grebes. This species seems to follow the water around, and this spring there is plenty of potholes and wetlands to choose from in the little bit of the prairie pothole region that reaches into Montana. This couple was in a small pond next to the road just north of Medicine Lake National Wildlife Refuge.

I was watching the pair of a while when they started what appeared to be some courtship displays including rushing through the water together, side by side.

They soon came over to a shallow mat of old vegetation and much to my surprise, the bird I had assumed to be the male based on other behaviors and plumage, proceeded to the platform first, assumed the passive position, and was mounted by the other bird. Hmm, that was different, but I thought I had learned something about plumage differences in Horned Grebes. Turns out I learned even more when I got home.

After the "copulation", which lasted only a few seconds, the bird on top moved to the front of the passive bird and did a quick water treading behavior where the bird sits up in the water and paddles vigorously while stretching its neck up (notice the bubbles and churning water in the photo above).

The passive bird remained on the platform and preened for a while.

and they preened even more.

When I got home I looked at the Birds of North America account for the Horned Grebe and found that according to the this account, copulation in Horned Grebes is a reverse copulation with the male in the passive role. A little more digging and I found out that it isn't even necessarily reverse copulation but more correctly reverse mounting. Apparently reverse mounting occurs more often early in the courtship period and isn't a breeding attempt as much as it "constitutes a regular and integral part of their courtship behavior" (Nuechterlein and Storer 1989). Also, these early courtship copulations often occur on a temporary courtship platform, which would explain the rather unsubstatial nature of the platform I had assumed was a rather poorly built nest.

It turned out I was correct in my initial assumption about which sex was which, and Nuechterlein and Storer warn that mounting behavior may be unreliable to determine sex in grebes. However, I was certainly incorrect in my assumption on the traditional nature of Horned Grebe sex.

Stedman, Stephen J. 2000. Horned Grebe (Podiceps auritus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online

Nuechterlein, G.L. and R.W. Storer. 1989. Reverse mounting in grebes. The Condor 91:341-346

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