Walk Softly – When nature gets in the way

by Geoffrey Carpentier

In my last two columns, I spoke about the mechanics of migration – how do birds migrate? What tools do they have to make these perilous journeys? What can go wrong when they lose those tools or use them improperly? Today let’s finish the story and see how the environment itself can cause catastrophic challenges.

I mentioned last time that small errors in the chosen migration route will be magnified each time the bird deviates from course. Gradually, the bird will drift east or west of the intended line of travel. Not a big thing if the destination is still within the range of where the bird should end up. But what if the bird is travelling to Central America, for example, where most of the suitable wintering habitat is restricted to the ‘Five Great Forests of Central America’? These forests are vibrant and can support vast numbers of birds, but the lands surrounding them are not nearly as hospitable. So, drifting too far west will leave the birds in marginal habitats. Drifting too far east will lead them to the Caribbean Sea, hoping to find a small island to live on, bad choice!

Speaking of heading out over the ocean, on purpose or by chance, can lead to interesting outcomes as well. Most migrants don’t do well crossing large bodies of water unless they can swim, land on floating debris or ships, and somehow find food and avoid predators. The odds are against them. In the Western Hemisphere, winds tend to move from west to east, so many birds flying into a crosswind drift eastward, and if they cannot correct course, they can end up in Europe or Africa. This is called wind drift and often results in many birds perishing at sea. Another factor which can make these scenarios even more devastating is that the magnetic pull across the Earth is not uniform; as such, birds which are already off course can’t necessarily rely on their innate knowledge of the magnetic fields across the globe and instead rely more and more on chance to correct their errors.

Mirror-image misorientation is another phenomenon which can affect migrants. In this case, birds that wanted to migrate SSE from their breeding grounds somehow get mixed up and go SSW, for example. This means a bird which intended to end up in the southeastern USA would end up in California. From there they ‘might’ figure out how to complete their migration, assuming this latitude was not their destination, or they might have to winter there and hope to survive and reconfigure next spring. This is a very common phenomenon. Even more frightening is when birds confuse north with south and head entirely in the wrong direction at the wrong time of year. Many tropical and sub-tropical flycatchers and Cave Swallows undertake this perilous track in the autumn, usually with catastrophic results.

Social interaction between different but similar bird species can fool an individual into going to the wrong place. Many cranes migrate in large flocks which may include several species. If a misplaced migrant encounters one of these flocks and gets separated from its own kind, it may choose to follow them wherever they go. Usually, this is not a bad thing, as they likely feed on the same food. However, the chances of it ever getting back to its own kind are quite slim.

Finally, the altitude at which birds migrate can radically influence what happens to them as they migrate. The winds throughout the various heights above the ground change in speed and direction. So, a bird that has done everything correctly and chosen the right path and the right day may still choose to migrate at a different altitude for one of many reasons. This may mean it is suddenly forced into an environment where winds relentlessly drive it across the ocean or to the wrong hemisphere!

Now you understand better how tough migration can be for our summer visitors. And you think vacation planning is tough!

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