This morning was a gorgeous morning to be in the park. A bit of mist in the air before the sun came up made for a very quiet, scenic run. As I turned down the greenway, a lot of birds were moving around and getting out of my way. There was a great deal of foraging going on. In distance, my ‘sweet Canada’ birds were back singing away, but it was a bird that flew in front of me that caught my ear. It was dark enough on the greenway that I had a hard time seeing it, but I could hear its single call note and based on its shape in the light, realized that there was a Gray-cheeked Thrush moving through the bushes. The one that I saw was calling out repeatedly, so there must have been some others around, but it was too dark to track them down. Either way, this thrush is just one of the birds passing through and loading up on food for the trip south.
A lot of the birds that are flying down to Mexico and beyond to spend their holidays do their flying at night and load up on food during the day. A lot of that morning activity could have been those little birds powering down some food after a long night. Many of the migrants lose a fair bit of weight during the night while they fly and these stop-over points become an important resource that they use to finish off their trip.
Not all thrushes have the same needs for food, though. Like with people, some of them are just in better shape and so have an easier time migrating than others. With people, well it’s about body weight, muscle tone and the distribution of weight. With birds, that ease boils down a ratio that we call wing-loading. Wing-loading is a way of thinking about how much weight their wings have to carry in flight: weight (in g) per unit area of wings (in cm).
One study compared the wing-loading of different species of thrushes and also looked at variation within a species.
They recognized that birds with lower wing-loading had an easier time flying and had to spend less time in stopovers loading up on food. In the end, this simple physical relationship can affect all sorts of behavioral choices (i.e. when to start migrating; what route to take; how much time to fly each night; how long to spend foraging each day).
Since these birds are just passing through, it’s a good time to get out and watch them. According to the banding data used by these researchers, most Gray-cheeked Thrushes only spend 2-3 days in any one stopover before they move on. Since they worked near the Gulf of Mexico, the last big stop before flying over open water, they birds might also have spent longer at that stopover than at others en route. So keep your eyes (and ears) out for the birds, but don’t expect them to stick around for the weekend.