Bobolink

Doug Leier posted on April 22, 2010 09:21 :: 1736 Views

In the name of positive thinking, I've always operated under the assumption that spring arrives with March and winter isn't officially on the table until January. That makes for a short eight weeks of winter in my mind.

However, when the first snow flies in October and shelterbelts are still packed with white when spring turkey season opens in April, it presents a visual reminder of how off in left field my assessment usually is.

Bobolink Actually, in some years my philosophy isn’t as far-fetched as it has appeared the last couple of years. I remember how mild it was in Decmeber of 2007, and in some years the snow is gone in much of the state by early March.

In fact, those who document bird sightings with diaries can probably tell us which years were mild and which weren’t just from their records of earliest arrivals.

Weather conditions and seasonal temperatures influence migration timing by changing availability of food. We’re much more likely to see migratory birds show up earlier in years with little snow and warmer February and March temperatures. Think of snow geese, for example. While they begin their migration from Texas and other southern states toward their nesting grounds in Canada in early to mid-February, their northward push is limited by the southern reaches of the snowline. While a few scout flocks push beyond, the majority of the birds move forward as the snowline melts north.

That still leaves unanswered, however, the question of why some species might show up in North Dakota well before environmental conditions would indicate. An often overlooked indicator is what biologists refer to as photoperiodism, or the amount of light in any given day.

As winter wears on, day-by-day the sun shines a little longer, to the point we
acknowledge, “Well, at least it’s nice to see the sun shine past 6 p.m.,” even though the temperature is still below zero.

Dec. 21 has the least amount of sunlight, and after that until mid-June, the amount of sunlight increases daily. After that, daylight decreases gradually until Dec. 21 again.

In basic terms, day length triggers increased feeding activity so birds can lay on fat reserves needed in preparation for migration. This is an amazing adaptation, considering how critical migration timing is for certain species. For some, if migration did not begin until conditions at their destination were ideal, such as no snow, valuable weeks would be lost.

Think back to snow geese. During the past couple winters, snow cover in March has stretched down into Nebraska and even Kansas. Even without snowpack, the birds will advance each spring and the length of stay in North Dakota varies, but the ever-lengthening day still drives some internal urge to press on.

I still scratch my head in amazement at the migration of a bobolink (Dolichonyx oryzivorus). This small grassland bird begins its journey from wintering accommodations in South America’s Brazil and Argentina. They begin migratory preparation under decreasing day lengths, and then encounter increasing day length as they move north until finally arriving at North Dakota’s grasslands.

This is a remarkable accomplishment considering I have trouble remembering that Wednesday is garbage day. But it’s not necessarily surprising that birds and other animals are linked to photoperiods to stimulate migration and other activity. Of all the variables in the environment, only seasonal day length variation is constant. Habitat is subject to human devices and weather changes from year to year.

Some people use dirt that is dry and firm enough for the planting of spuds in the garden as their official first sign of spring, while others acknowledge the first Canada goose brood as a sign that winter has finally released its all-too-long grip.

While I haven’t seen any goose broods yet, it should be soon.