Friday, August 9, 2013
Morning flight: an overview
One fun aspect of migration in south Louisiana is morning flight, or movements of birds along their migratory journeys in the early morning. Once you are keyed in to it, you can often see or hear it happening. This time of year, the main species doing it is Yellow Warbler, our earliest fall songbird to pass through in numbers.
Yellows like to call a lot while engaging in morning flight, a slightly husky ssst. It is subtle, but if you are alert for it, not uncommon. Most mornings this time of year, listening at the sky for ten minutes or so will produce one, and sometimes multiples. This lasts throughout August. If you look up, you can usually see the little bird flitting overhead, naked eye. Most are going westward, or some variant thereof. They are presumed to be migrating around the Gulf of Mexico en route to their wintering grounds in the tropics. They are coming from a wide geographic range extending across the northern USA and Canada.
Yellows are known to migrate at night as well (like most songbirds do), and we really don't know how much morning flight is continuation of the evening's movement, or fresh movement starting after sunrise. We don't see it much, if at all, when they come through northbound in spring, at which time they come across the Gulf rather than around it.
There are some places where morning flight becomes concentrated. Concentrated morning flight can be a really fun spectacle to watch, and so sleuthing out where and when it occurs is a great game.
At this point, the greatest Yellow Warbler concentrations known anywhere are, lucky for us, on the south shore of Lake Pontchartrain. They happen where the lake shore juts northward to a point (where the bridges leave for Slidell)- called South Point. At the base of the Hwy 11 bridge, and even better at the base of the railroad bridge to the north, flights can reach 1000 birds in a few hours. And they probably get bigger than what we have encountered so far.
These birds fly the "wrong" way for fall, heading north across the lake. The primary theory as to what is going on is that the birds have been wind displaced onto the coastal marshes, and undertake a retreat into the wind to compensate for this displacement- trying to get back onto their intended flight path. These flights seem highly dependent on there being a north or northeast wind. Unfortunately, this wind direction is rare in August, so these big South Point flights are infrequent.
It could well be that, on any day, there are migrants in our area orienting in more than one direction, and which direction you see them moving in depends upon where you are. That is, which headings of movement tend to be concentrated at your particular vantage point. For instance, at South Point, you only see northbound birds coming from the south- because there is no land to the west, north, or east for them to come from.
The reason there are so many heading northbound at South Point is presumed to be that they are reluctant to fly over large expanses of water while struggling into the wind. So as they retreat from the coast, they run into Lake Pontchartrain, and follow its edge northeastward to South Point, avoiding crossing the water. At the point, they must cross if they are to continue north- there is no more shoreline to follow northward. From observations elsewhere, I think that open marsh interfaces (vs. open water) have a similar herding effect. And many other species of land birds do this- not just Yellows.
In the absence of such concentrating barriers, Yellows seem to just head west over New Orleans in a diffuse flow. I have twice encountered flights of 100/hour, once at Bucktown and once in Old Jefferson, heading west. Smaller flights of 10-20/hour are quite common, and again are usually heading approximately west.
Every season I ponder maps of the shoreline configurations in the area, imagining at what other places I might find concentrated flights. I have often wondered whether westward moving birds would be concentrated by the northeast coast of Lake Pontchartrain, which could squeeze approaching birds (birds passing west through the space stretching from Pearl River to the Rigolets), into a concentrated flow around the lake's northern apex. A few days ago I had a chance to test this, since I was in Fontainebleu State Park in the early morning. I took up position on the end of the pier and scanned with my binoculars landward. Sure enough, a half hour of scanning produced 128 warblers flying by, all headed west. At this date, and exhibiting this behavior, it is pretty clear they would have to be Yellows, at least the large majority of them. So it looks like the northern lakeshore may well have a concentrating effect- 128 in 30 min translates to about 250/hour, more than the typical flights south of the lake (excepting at South Point). And since it is not dependent on uncommon north winds, it may be more regular in occurrence than flights at South Point.
Another piece in a fascinating puzzle. Later in the fall more species come through that also fly substantial distances in the early morning and thus have the potential to produce big counts- Eastern Kingbirds, Indigo Buntings, Yellow-rumped Warblers, American Robins, and Cedar Waxwings. Some of these have produced flights of tens of thousands in a few hours at South Point. They are always accompanied by other species in smaller numbers, adding diversity to picture.
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