Can migration evolve on an as-needed basis? It has been adjusting for quite some time, as conditions tend to influence many things, even with humanity.
Having grown up in the extreme winters of the northeast, writer realized that things have changed a lot since then. There will always be a seasonal shift in necessary resources, and it used to be a lot more predictable than it is now.
Resident birds live with pros and cons: food available all year means seasonal weather changes, yet when the urge to procreate strikes, they are already on the breeding grounds. These are hardier species that can usually survive all types of weather and as such, can live longer, as they expend less energy without a migratory need. However, it can sometimes take a toll on these birds because very severe winters can resonate negatively on populations.
That’s when the breeding pool can be increased with new stock from moderate climates, like the Carolina and Black-capped Chickadees. Control of population increases by nesting success when needed is an adjustable entity.
For those insect eaters or obligate migrators, some of which are boreal breeders, we know that these birds head south for the winter. When food becomes unpredictable or variable, those currently irruptive species wander south, like the Snowy Owl, Pine Grosbeak, Pine Siskin, and Rough-legged Hawk.
We also experience partial migratory movement, which relates to birds that migrate and those that remain full time residents like the American Robin, Mallard, or Canada Goose. This phenomenon is based on genetics and can be variable.
Under this realm, we can also see how the Red Crossbill subspecies has increased over the years and is still doing so. They vary in bill types, and each variation will prefer a few species of trees. Cone crops vary every year, so the crossbill will wander like nomads from their core range when necessary.
Behavior evolves with the need for natural selection like any other living entity. Individuals show differences in migratory requirements, wing or bill length, or bill type. So for evolution, it must be hereditary or genetic, and it is being proven that it no longer takes as much as a century as was once preconceived.
With a warming climate, the annual cycles of birds must be synchronized and controlled, as our own circadian rhythms are adjusted by light and twice yearly time changes. Might this mean that nature will readjust itself on an as needed basis to coincide with migration and breeding? We have never lived through a new era, but we could notice minute differences eventually.
With environmental changes we must agree that natural selection is molded for changing conditions, like with the needs of the crossbills. However, it may take some time for birds to adjust to differing breeding times in the boreal forests. Hotter summers have already been noted in the South, and species like the Green Heron are attempting to change their own breeding needs by starting and ending sooner.
We shall see.
Deb Hirt is a wild bird rehabilitator and professional photographer living in Stillwater.