Walk Softly: Migration – When things go terribly wrong!

by Geoffrey Carpentier

Last time we talked about migration and how birds and some insects use a variety of tools to safely move from their summering to wintering grounds and back again. However, what if things don’t go as planned? There are many factors which can and frequently do occur which can offset the positive migratory adventure our birds face. Setting aside things like predators, wires, buildings, hunters, pesticides and disappearing habitats (which usually lead to the death of the migrant), many other factors must be considered when discussing why birds end up in the wrong place.

Perhaps the most obvious one is, a bird, when only born a few weeks before it starts its first migration, is very likely to make a mistake. Recall, as discussed in my last column “The Mechanics of Migration,” birds ‘see’ polarized light and innately can calculate the exact position of the sun in relation to the bands of polarized light. These appear during the key parts of the day, when birds are most likely to migrate. Yet, what if the bird doesn’t factor in exactly how it was supposed to orient itself, at the start of the journey, using the sun as a compass point? Even miscalculations of a fraction of a degree, from the intended line of travel, can take it hundreds of kilometres off course, as it journeys southward. Couple this with the fact, this ‘path’ will be modified repeatedly, along the migratory route, to respond to local conditions, and thus the original small error will be augmented exponentially. Once it starts on the wrong path, it can rarely correct the error and often just keeps going in the wrong direction. In some situations, it may find others of its kind and then use them to assist in getting back on course. Alternately, it may find suitable habitat and simply spend the next season there, hoping to eventually figure out how to get back to where it should be. A great example of this is, a Steller’s Sea-eagle, which belongs in eastern Russia in the summer and Japan in the winter, but somehow, in 2020, it ended up in North America and simply never left!

Another potentially catastrophic scenario, regarding getting off course, arises if a bird doesn’t stop migrating when it should. Let’s say, it inherently should winter in Panama but ends up in Colombia. This may not actually be much of an issue, as long as it can find food and shelter. Yet, if it kept going to southern Argentina, the results could be catastrophic! Two outcomes come to mind. Firstly, recall birds use the magnetic pull of the Earth as a tool to help in migration, so once they cross the Equator, the pull is towards to South Pole, not the North Pole, as in our hemisphere. This means the birds, once that far off course, could likely never actually figure out how to get home because it will always be drawn southward. The second outcome is, terrain and fauna of the new wintering area may be quite inhospitable and the bird simple cannot find food or shelter. So, it’s left with the option to ‘stick it out’ (likely a fatal choice) or move on. The latter may be successful but really it is likely a risky option, as there really is no plan, just a wandering hope, things will turn out okay.

Inexperience in individual birds can augment errors which occurred early in the migration. Again, recalling another tool birds use is visual features, such as coastlines, rivers, lakes, and olfactory cues to aid in migration. So, a young bird, of the year, which has never been on its migratory route and therefore doesn’t have the memory of sight and smell, loses that tool from its arsenal, at least on the first migration. Some birds migrate southbound, along one path, but return along an entirely different one, so this inexperienced bird may be negatively impacted in both directions of its journey.

There’s much more to this fascinating story – my next column will continue the exploration of this annual event and what environmental factors can cause difficulties.

Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line on LinkedIn and Facebook.