Wednesday, June 8, 2011

Fall Migration is Underway


Male Common Grackle. Photo from the US Fish and Wildlife Service collection
Although the calendar says that we still have two more weeks of spring remaining until summer arrives, mother nature, at least in south Florida, has her eyes on fall migration already.

This evening while out bicycling (14 miles in the evening heat!) I encountered the first post-breeding flocks of Common Grackles I have seen this year. Despite some people having a bad taste in their mouth for this rather obnoxious bird (maybe that's why I can identify with it?) I have had a kinship with it for a long time. The research I did for my Master's degree was on the reproductive ecology of common grackles and mourning doves nesting near the Prairie Island Nuclear Generating Facility near Red Wing, Minnesota. For two nesting seasons I climbed up into jack pine trees in a small plantation about 1 kilometer from a nuclear reactor gathering baseline data on these two species. The thinking at the time was that if any environmental issues were being caused by the operation of the nuclear reactor they were likely to show up quickly in birds nesting within a stone's throw of the nuclear reactors.

As part of the research I made daily observations of the behavior of the birds and noticed that usually (at nearly 45 degrees north latitude) when nesting was completed and the young had fledged, the birds gathered in large post-breeding, pre-migrational flocks and flew around the countryside looking for food sources to sustain them through the beginning of winter. With few exceptions common grackle migration was well underway by the middle of August and virtually all of them had vacated the northern latitudes by mid-September.

I started thinking about that this evening when I encountered a post-breeding, pre-migrational flock of common grackles on the north side of Sarasota. The flock, consisting of about 100 birds, was made up of adult males and females and the ratty looking juvenals that hatched this spring.

At this latitude common grackles begin nesting in late February and early March when many other resident birds (northern cardinal, northern mockingbird, Carolina wren, to mention a few) also begin nesting. With a short incubation period of about 13 days and a nestling period of about 14 days before fledging, many young of the year common grackles are already out of the nest by mid-April when many neotropical migrant birds (Kentucky warbler, hooded warbler, black-throated blue warbler to mention a few) are just arriving.

The birds I saw tonight have had at least six weeks of post fledging time to be taught by their parents how to be common grackles. It just seemed strange that they were in pre-migrational flocks in early June!

Which brings up another question - where are they going to migrate to? The range map below, downloaded from the Cornell Laboratory of Ornithology site, clearly shows that a common grackle in south Florida isn't really going to be migrating that much further south. There are some records for the Bahamas and some for Cuba but that's about it in the West Indies. So where to they go when they leave here?

Nesting, Migrational and Winter Range of the Common Grackle
Lacking any band recovery data on Florida-banded common grackles or any satellite tracking data its difficult to guess. All I know is that the local cadre of common grackles is done nesting and setting its sights on some other place to be.


Purple Martin in Flight. Photo from the US Fish and Wildlife Service collection
The other evidence that fall migration is about to begin was the first of the season's post-breeding flocks of purple martins. The flock I saw this evening, made up of males, females, and recently-fledged young, numbered about 20 birds and was seen at the upper end of Cooper Creek Park on the north edge of Sarasota. Another group of maybe 15 birds was hawking insects over the south edge of the artificial wetland and a third flock of 15 birds was along the powerline right of way that passes over Honore Avenue.

Purple martins arrive in south Florida in mid-late January and are busily nesting by the first of March. Producing only one brood per season their nesting responsibilities are largely over by late April and they have nothing to do but get ready for fall migration after that. Most purple martins will have left this part of Florida by the middle of July.

Nesting, Migrational and Winter Range of the Purple Martin
Unlike the common grackle with its southern range limit here in Florida its another story with purple martins. These bird which almost everyone loves to love nest over an extensive area of the Midwest and eastern United States and Canada and spend their winters as far south as north-central Argentina. The bulk of them apparently winter in central Brazil.

Why they migrated that far south and why they leave here in the middle of summer to get there are two questions best left to mother nature to answer.

This little biological conundrum is another reason I enjoy being a biologist and trying to figure out the rhythms of nature. Although fall migration is on the verge of beginning here, in my natal Wisconsin both common grackles and purple martins are just now in the middle of the nesting season. And while that is happening, Arctic nesting shorebirds like red knot and semipalmated sandpipers each of whom were on Sarasota beaches and mudflats a month ago are just now arriving in Arctic Canada and Alaska and setting up territories and thinking about building nests and laying eggs. Most of this is related to 1) sunlight triggering their behaviors, and 2) available food supplies to support young birds at various latitudes. Still its fun to watch and try to figure out what the earth is trying to tell us and why.

2 comments: