Minnesota's buggy summers draw birds from thousands of miles

The golden-winged warbler left its nesting territory in Sherburne National Wildlife Refuge in mid-August, its destination an organic coffee farm in the mountains of Nicaragua.

It would take the bird 10 to 12 weeks to complete the trip. There is no breeding imperative in the fall. This trip would include the first of two crossings of the Gulf of Mexico, the round trip about 4,400 miles.

Spring migration takes four weeks or less, the goal to arrive on breeding grounds by an optimal date. The birds want the best chances for successful courtship, nesting, hatching, feeding, fledging. All of those vital activities have evolved to fit a tight schedule dictated by season.

But why not just stay in Nicaragua? It's always warm. There's food, no gulf, no rush. What's the attraction up here?

It's our explosive spring, according to biologists. When March warms to April, plants sprout, flowers appear, insects hatch. We offer a pantry so full that competition for food among birds is not a limiting factor.

If the warbler were to nest near that coffee farm she would likely raise only two chicks. There is no explosion of resource in Central America.

Resident nesting birds would compete with her for food. The warbler and her mate would work hard to feed their pair of babies.

Larger families do mean higher mortality after fledging. Nature maintains a population balance. Parent birds replacing themselves is the goal, and that can be a challenge.

If birds are born altricial — helpless — clutches tend to be small to accommodate the higher level of care the babies must receive. Game birds, precocial, have larger clutches because the young, feathered and out of the nest upon hatching, are more subject to predation.

A wood duck hen appeared on our pond one spring day with 14 ducklings. She led them into the channels lacing the adjoining swamp, home to mink and fox and raccoon. Two days later she had 11 ducklings, then seven, then three.

That fall we watched two immature drakes patrol the pond, all that was left of the family, and, perhaps coincidently the replacement number.

Birds lay more eggs inland as opposed to on the coasts, and more eggs at higher elevation. More eggs would be laid the farther north the bird was nesting, all because greater insect populations in those locations offer more food.

Adult birds that do not breed have higher survival rates than those that do, according to the Cornell Lab of Ornithology "Handbook of Bird Biology."

Early breeders have better chances for finding a fit mate, and their chicks have more time to mature before winter or the demands of migration. That early breeding benefit can be passed forward genetically because those chicks have that better chance of survival.

At the time eggs are laid, songbirds usually lack reliable cues to what chick-rearing conditions will be when that time comes. Will there be sufficient food? They rely on the experience of ancestors as recorded genetically.

I wonder if that will be good enough as a warming world changes.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at startribune.com/wingnut.