Wednesday, December 28, 2011
When (and Why) Waterfowl Don't Migrate South
Here in Maryland, the state DNR is reporting roughly 10% of the typical flock and "one of the worst seasons on record." News releases from other mid-latitude states show the same thing. What's most surprising is that hunters seem aghast and totally confused that this is happening. I'm here to tell you: it's pretty simple.
Many bird species are driven to migrate. It is part of their internal programming. There are two major parts of this programming - route and movement. Route, for many species, is basically pre-determined. For timing of movement, they often need prompting - a cue. Sometimes it's as simple as the first full moon after a hard frost. Or the first day that water temperatures rise above 60 degrees. Speaking in huge generalities here, the spring migration tends to be more dominated by these cues than the fall migration. And since hunting regulations are specifically structured to guard the safety of the spring migration, it's the fall migration with which hunters concern themselves.
So why do waterfowl migrate south in the fall? Again, sometimes a combination of internal programming and external cues cause a massive flight to occur. But in the case of several species, and notably the Atlantic Flyway, migration is largely driven by necessity. Specifically, the necessity of each living bird to maintain their own caloric balance. With the exception of mating behavior, waterfowl are largely energy conservative species. What does that mean?
To put it bluntly, it means that if 100,000 geese are doing the backstroke in 55 degree water in Lake Ontario, and eating corn off of the ground every night, why would they ever migrate south......even if it's December? Sound like an exaggeration?
That's right. It's almost January, and there is almost no ice on the Great Lakes. Which means there's little snow in the areas around the Lakes (we'll get back to that). Many duck and goose species travel through the Great Lakes on their way to wintering spots in the Mid-Atlantic. Does 50 degrees sound like really cold water? According to Cornell University, water temperatures of 50-70 degrees are ideal (thermally neutral) for ducks, and it's widely written that large birds (ducks, geese, etc) are "heat neutral" down to about 55 degrees (air temperature). So, sure, the birds are comfortably in this balmy early winter weather we seem to have most years anymore, but how does that impact migration?
You see, birds use 35-50% of their food intake solely for body temperature regulation. As it gets colder, water bodies freeze, and snow cover starts to eliminate local feeding habitat, each bird must still somehow find 35-50% of their "summer ration" of calories, just to keep warm enough to survive until the next day.......or very likely die if they fail to eat enough calories. What did I just say about snow? Snow cover. If there is a 3' layer of snow across all of New York state, very few birds will be spending the winter in New York state (and many who stay will die before spring). Birds are hard pressed to try and find food in more than 1-2" of snow. Let's look at a snow cover map - from a year (2010) when a hard freeze in mid-December forced a huge (though delayed) migration on the east coast, and from a year (2011) when no such freeze occurred, and the fall migration is still pending.
In a world without industrialized, large scale agriculture, birds could be forced south as they eat themselves out of their natural wetland habitat in a stopping point. But the eastern half of the United States is now filled with giant farms with waste grain laying about, farms with green, delicious winter cover growing on them, and natural food sources (such as they are) available in wetlands and open water. Until weather (ice, snow, cold air) changes the equation, there is no food shortage for North American waterfowl, except for those dependent on aquatic grasses (largely banged up by water pollution). This is not to say there's no habitat shortage.
The bottom line is this. Many (though not all) species of waterfowl tend to be energy-conservative. As long as the breeding grounds, or any flyway area between the breeding grounds and your hunting grounds, contain accessible waterfowl food, ice-free water, and relatively mild night temperatures, you should be pleasantly surprised by any birds that do arrive. Especially in the Atlantic Flyway. This frustrating and recurring phenomenon seems to be happening almost annually, and is very interesting to me as a biologist, and very frustrating to me as a hunter.
It leads me to three questions that I don't have the answers for yet:
1. This is clearly a climatologic pattern that is impacting our 100-year old knowledge of migration dates. Is it part of a short (5-10 years), medium (10-50 years), or long term climatic shift?
2. Assuming that any of those are true, what are wildlife managers to do with hunting season dates? Earlier dates are becoming fruitless for migratory birds (although local birds are often around in ample numbers to fill up a few straps). Yet, later season end dates (February and beyond) would be very likely to impact the sensitive spring migration. For many species in many states, bag limits are becoming ridiculously liberal...and few hunters in the Atlantic Flyway can fill a bag limit consistently throughout the season. 25 snow geese? 6, 7, 8 ducks per day? 16 doves? Many hunters have a spot they can go once or twice a season and achieve this kind of harvest - but not with any regularity.
3. While the bulk of Atlantic Flyway birds are produced in the Great Lakes region and eastern Canadian provinces, not all of them are. Pintails largely arrive from the prairies. Scaup largely hail from the Boreal Forest in western Canada. Weather that forces those species to migrate south and east may never even touch the Mississippi/Central flyways or the Atlantic Flyways. In short - will we start to see a different composition of wintering waterfowl in the AF? Are we already?
As the Atlantic Flyway harvest numbers come in for the 2011-2012 season, we'll be taking a more detailed look at all of this. Are land management practices, long thought to be strong drivers of waterfowl migration, being supplanted by climatic trends as the primary force in predicting seasonal bird movement? We'll also take a look at the stunning 2009 report by Audubon that says, at least for waterfowl, "yes."
Plan on joining us for it!
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