Birds pair off and get down to breeding business

A pair of tree swallows that survived the record-breaking mid-March blizzard south of us last spring got directly to work when they finally arrived.

They chose one of the nest boxes I maintain at a nearby golf course. There was courtship. There was mating. The pair of birds combined efforts to raise four chicks.

Monogamy — one paired mate — is a fact of life for most bird species. A nest will be more successful if both parents care for the young.

That doesn't mean the birds never mate outside the pair in a nesting season.

I watched a female swallow atop the nest box while a male swallow, presumably her mate, idled in the air above her. Another male approached, flew close to the box, almost certainly with the female in mind. It immediately was attacked by the resident male, brief but serious combat.

Members of monogamous pairs sometimes will seek or accept polygynous sex with others of their species.

The intruding swallow may well have had a mate at another box. By breeding with other females, though, the intruder could increase the probability that some of his offspring would survive to breed themselves.

Females will accept extra-pair mating as insurance that their offspring carry the genes of the most fit male(s) available. Fitness, in this case of our fighters, perhaps represented by success in combat, equates with improved chances for survival of the chicks.

The male defending his mate and territory wanted his partner's eggs to carry his genes alone. He had a lot invested in finding that mate, building a nest, and, in weeks to come, feeding young birds. He wanted maximum return on his investment.

The intruder was looking for return with minimum investment.

Timing the egg

Human babies can arrive at inconvenient times in inconvenient locations. If you can equate delivering a baby to laying an egg, birds have solved those problems. Most songbirds lay their eggs first thing in the morning, on schedule, no surprises.

One researcher has suggested that females lay eggs early in the day to free them for routine daily activities that might otherwise endanger the egg.

Eggs are most vulnerable to harm just before they are to be laid. If an egg is laid early in the morning, the critical hours for completion of the egg fall at night, when the bird is least likely to be active.

The weight of the egg as the bird forages for food could be another factor. Tree swallows, for example, tend to lay between 5 and 7 a.m., according to research.

This species feeds on the wing. Free of the weight of the egg, the female swallow is better able to maneuver in the air once the day warms and insects become active.

For robins, research timed an average egg delivery time of mid-morning. Robins are ground feeders.

"Holding an egg probably is of little consequence to foraging efficiency (on the ground)," the researchers wrote.

Production of eggs at a certain time of day might also be helpful to those male swallows I watched. In most songbirds, eggs are fertilized at 24-hour intervals. An egg is laid, and in the following hour the next egg in sequence is fertilized.

This hour, then, is the best time for males to attempt fertilization.

The male swallows using my boxes flew out to meet perceived rivals whenever necessary. And, given the chance, they could well have been rivals themselves.

The females quietly stayed put, letting the inherited fitness of competing males determine the fate of their genes.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com.