Migration

Fly away home…

Bird migration has fascinated humanity since historical times, although in earlier times it was believed that birds such as swallows, which disappeared in the summer, were hibernating at the bottom of ponds until the next spring. In northern Europe, the return of then Cuckoo still heralds spring. It became clear to science that birds were making the most of their opportunities by moving away from the cold and wet season when food would be hard to find. It was also part of the strategy to return to cooler climates in order to make use of the breeding possibilities when they were experiencing better summer weather. Some birds stay in the tropics. Other species stay resident, the cooler parts of the world, while some migrate to get the best of both worlds.

Because cooler climates can sustain fewer birds, it makes sense as a strategy to fill those gaps as those areas warm, and that warmth brings forth more food options, such as flying insects. Once advantage is taken of the food source allowing the birds to breed and raise young, the cooler air signals the time to once more migrate to the tropics. Some species only partially migrate, moving away from the most severe weather, but not going the whole hog and moving to the tropics. Once more, this makes sense as a strategy in that they are closer to the breeding grounds for their return when the weather improves than those who migrate further.

It is not unusual for some individuals to take a chance either on staying where they are rather than migrating to the tropics or staying in the tropics rather than moving north again or south in the case of the Southern Hemisphere.

These mass movements usually occur along distinct flyways. Sometimes there is safety in numbers and at other times that is actually a disadvantage. But geography and climate work together to make certain routes most viable. Moving across oceans or desert is clearly a risky strategy as there is nowhere to rest or to feed up. Nevertheless, hundreds of millions of birds do this every year. Science has not only researched this phenomenon en masse, but also looking at how individual birds manage to navigate, a process that has revealed some quite remarkable senses that birds possess that we as humans do not, such as an awareness of gravity. Initial studies were pure observation, but latterly, programs of ringing or banding birds was undertaken so that start and end points could be seen, and even routes taken by virtue of birds found that are ringed that did not survive the full journey.

Common Cranes Grus grus congregating in Germany before flying south – ©Andreas Weith, CC BY-SA 4.0 via Wikimedia Commons

Many migration studies now use modern techniques combining radio transmitters with satellite tracking. The studies involve both very scare and quite common species and have recently uncovered a great deal of information regarding both routes and en route feeding stations and hidden wintering grounds etc.

Birders gather to watch “viz mig” – visible migration. It really is a joy to see and can be a treat in an unlikely circumstance. For example, I was once waiting in a town centre in SE England while my wife shopped. Looking up (this was late September) I saw a few swallows passing south over the high street. 20 minutes later I was up to 800 – that afternoon others reported many thousands of swallows on the move. A joy to marvel at. Migration can be truly spectacular or more mundane. Watching in the spring may bring in early migrants and unusual birds blown off course, and viewing in the autumn can see mass congregations as birds ready themselves to fly, such as swallows occupying miles of telegraph wires. and rarity oriented. In the UK, many bird observatories are sighted along these migration flyways so that staff and volunteers can monitor movements.

Snow Geese Anser caerulescens Migration – Pacific Southwest Region USFWS Public domain, via Wikimedia Commons

For most birders, although spring migration can be exciting, it is during the autumn that we are most likely to be able to catch up with unusual migrants. Birds blown off course by bad weather or who have somehow become disoriented or have, by virtue of the way their brains are wired, migrated in the wrong direction, tend to have to make landfall en route in order to feed up before, for example, crossing water. In spring, many migrants will fly direct to their breeding grounds, high overhead and often at night. Bad weather may bring falls of birds, but in autumn, birds tend to begin their migration more slowly and therefore hang around, particularly at coastal sites where they spend time feeding to ensure they have enough fuel on board to make crossings of water, such as the English Channel or either end of the Mediterranean or across the vast expanse of the Sahara Desert.

Such spectacles of birds en masse are of course one of the reasons that people become attracted to bird watching. Migration is a wonder whichever way you look at it.