Egg Laying: Evolutionary Biology and Behavior

Environmental & Science Education, STEM, Biodiversity, Nature, Wildlife, Biological Evolution, behavior

Ed Hessler

One (of many) reasons I find to read Jim Williams's Minneapolis Star Tribune columns on birds is because they often include comments on biological evolution and behavior.

A recent example is a column about tree swallows in which reported on nesting and reproductive behavior of a pair, which nested in an egg box Williams maintains, followed by comments on the timing and sequence of egg laying.

First he notes that "monogamy -- one paired mate -- is a fact of life for most bird species." Still, birds sometimes do "mate outside the pair in a nesting season." He noted one likely attempt when a male "flew close to the (nest) box" and was immediately "attacked by the resident male" in "brief but serious combat."

This behavior is all about genes and getting them into the next generation with each male vying for to do this and each female making a choice on the best mate based on features such as defense of territory. As Williams wrote about the incident he witnessed, the resident male "wanted maximum return on investment. The intruder was looking for return with minimum investment."

Songbirds, in general, are early morning egg layers, and "in the next hour "the next egg in sequence is fertilized." Here are some possible reasons for this sequence based on research hypotheses: "eggs are most vulnerable to harm just before they are to be laid" and night provides "the critical hours for completion of the egg," which is the time "the bird is less likely to be active." Tree swallows "feed on the wing" which frees the female from carrying a fully developed egg. Robins, on the other hand, lay eggs midmorning and because they are ground feeders finding sufficient food, rich in protein, eggs are not likely to suffer damage until laid by "holding an egg," while the egg is completing final development.

By the way, the male resident tree swallow, "given the chance...could well have been" a rival to another pair. The ideas discussed fall under the evolutionary concept of fitness --" reproductive success " and "reflects how well an organism is adapted to its environment." (see here)

Mr. Williams wrote the column titled "Birds pair off and get down to business" for the April 20, 2022 edition of the Minneapolis Star Tribune. The week following (April 27, 2022) another column was on genetics and evolutionary biology. Birds have an inactive gene have for teeth which is now used to make feathers. But some times a mutation will start the tooth forming process although the birds do not survive. Then he continues with a discussion of the formation of the bill, "the avian equivalent of the Swiss Army knife." Another terrific article, one that includes indirect observations on the need for suitable habitat. "Where can you find birds? Where their tools are best suited for use." And when you begin to spend some time observing birds in their habitats you will find that they further partition the habitat. Warblers provide an example, especially on their journey north. Some choose bushes near the ground, while others choose their tops; others distribute themselves in trees.

Both articles and the entire previous 229 he has written to date are found here.  Star Tribune articles are behind a subscription paywall as you would expect.  If you are interested in birds, in science, in nature and how the feathery side of the world works, Mr. Williams is a reliable, knowledgeable and friendly guide.