Thursday, August 30, 2012
Dispersal or Migration?
What am I?
August draws to a close and I'm grateful for another spectacular month of birding at Pheasant Branch Conservancy. Maybe you've noticed leaves are falling a bit early this month. No doubt it's due to stress brought on by the summer's severe drought. It's still one of my favorite times of the year to walk the creek corridor. I love the cool mornings and the sight of yellow leaves on the path. So few mosquitoes ... that's different! Singing pewees are omnipresent and the flight calls of frenetic warblers looking for a place to forage can be heard throughout the woods as the sun slowly rises in the east. That's what I look and listen for when I go birding this time of year. Ya just gotta love the fanastic wood warblers of North America.
In terms of species and numbers, warbler migration has been about average with nothing unusually early or uncommon. With the Baraboo Hills only a few dozen miles to the north—and nearly two-dozen warbler species that nest there—a question whether the birds I've observed at Pheasant Branch are true migrants or dispersed juveniles has been proposed to me. I wonder where dispersal ends and migration begins. Are the patterns of dispersal predictable by species? Perhaps dispersal is merely the first stage of migration. It seems that the Tennessee Warblers I've seen must certainly be veritable migrants because are at least from at least 275 miles away from the southern terminus their breeding range, but I suppose they could be considered long-distance natal dispersed birds. There is such a thing. Perhaps life is simpler for birders who have fewer nesting warblers in their state!
Maybe this is more a point of semantics and merely a contemplative exercise for non-professionals like me. As we know, warblers are not available for interviews to birders, but there are ways of determining origins by measuring stable-hydrogen isotopes in captured birds. I imagine this must be tedious work. My gut feeling is that natal dispersal, whether long or short, must be more random than migration considering the behavior of inexperienced birds. But maybe it isn't unpredictable. Nature loves rendering patterns even where there seems to be chaos. The order and number of warbler species I've observed this month at Pheasant Branch seems typical, on the mark, and fits a seasonal pattern by my records. On the other hand, if what I observe in August at Pheasant Branch Conservancy actually is dispersal, then there must be a lot more complexity for it to have the same species show up in the same order at the same time and place year after year. Why shouldn't dispersal be similar every August? I have more questions than answers. This is one of the many rewards of birding the same location as often as I do at Pheasant Branch. It keeps the mind busy!
© 2012 Mike McDowell