ALWAYS IN SEASON: Some dates, some lines divide the natural world

The Fourth of July divides the year the way the 100th meridian divides the continent.

Of course, these dividing lines are arbitrary and imprecise.

Yet the divisions are real and important, even if they are subtle.

North Dakotans are aware of this.

Weather is fluid in these parts, and the heat of summer can occur early in June. Still, most years, it gains strength in July. At the same time, humidity declines, usually, and the wind grows less.

This combination produces the peculiar heat of the Great Plains summer.

Similarly, though the 100th meridian divides the state, the physical distinctions between east and west are nowhere so disciplined as to occur in a straight line.

The great spaces that characterize the west occur east of the meridian as well.

But the horizon changes, moving away, suddenly lurching upward in ways that are undeniably western.

So it is with bird life.

The 100th meridian has long been regarded as a kind of boundary line between eastern and western species of birds. These mix at the center of the continent, giving North Dakota a wide variety of bird species.

But the eastern species do give way across the state. Red-shafted flickers begin to appear, replacing the eastern yellow-shafted species. Spotted towhees replace eastern towhees. Western kingbirds at first supplement and then supplant eastern kingbirds.

And so on.

Just as the meridian divides the continent's bird life (again, inexactly and by no means completely), so does the Fourth of July divide the activity of birds, or so it appears.

Actually, this is a coincidence. The Fourth of July occurs just after the summer solstice. In nature, this event is of more importance than our human holiday.

But for us humans, it is easy to associate the changes that occur in nature with the Fourth of July.

By this time, most birds have completed nesting and many have fledged young. This is evident in any wetland, now alive with ducklings, goslings and juvenile grebes.

It's clear in any city park and suburb, too, where spotted robins have shown up on mowed grass. These are young of the year.

Nature has turned down the volume, too. Bird song diminishes as summer moves along, because birds sing to advertise territory and to attract mates.

That work is pretty much completed for the season.

There are exceptions. The seasonal divisions are arbitrary and imprecise, just as the continental divisions are.

Some birds raise two broods in a year. Robins may. Mourning doves often do.

American goldfinches nest late, sometimes at the end of July.

But this is a particular adaptation. The food source that goldfinches most value, thistle seed, comes into abundance in late summer.

Any doubt that the year is well advanced evaporates with the onset of migration -- which occurs soon after the days grow shorter.

This is not surprising. Most birds are sensitive to light, and many of their activities are related to the amount of light at a given time of year.

That's probably how birds recognize when the time has arrived to begin migration -- and it accounts for the consistency in the arrival of some species in the spring -- including the swallows at Capistrano.

For some species, even a small change in the amount of daylight triggers migration. This is true of shorebirds, which begin the southward migration by mid-July -- soon after the Fourth, in other words.

The start of shorebird migration might be considered the beginning of fall.

For most of us, the idea that summer is over, or even ending, is unwelcome, of course. We mark the change of seasons by our own activities, including another of our holidays, Labor Day.

But nature is ignorant of these.

Rather, we humans probably chose our holidays in the same way that birds choose their nesting and migrations schedules. We sought a late-summer holiday and created Labor Day, for example.

In any case, nature moves at breakneck speed. Most birds don't have the luxury of the long view. Instead, their lives are constrained by the urgency of reproduction and migration.

These occur with a regularity that happens to match our calendars, and that makes the Fourth of July a kind of dividing point.