This article examines the four chief reasons why birds migrate.
Migration requires an enormous amount of expended energy and risk, including significant jeopardy of predation and the perils of extreme weather conditions en route. So why do so many bird species have a seemingly irresistible instinct to head for distant climes at certain times of the year when, in isolation, those dangers alone would seem to place them at an evolutionary disadvantage? In short, there are four reasons, all of which are closely interrelated. And a fifth, less scientific yet intriguing.
Availability of food, climactic conditions, the successful raising of young and the length of daylight hours are the four generally accepted reasons that play their part, in differing proportions depending on the type of bird.
Food is the most important motivation of the four reasons for most breeds. Consider, for example, the swallow – an insectivore that catches prey in flight, swooping low on balmy evenings as the temperature cools, a high mobility maximizing a harvest of midges and gnats as we marvel at its skill. Fast forward that scenario to the depths of winter and it’s immediately clear that the swallow has only two viable options. Find another source of food or move on to another warm climate in a new location. Being ill suited to alternative forms of nourishment, the hirundinidae are faced with little choice. A similar dilemma, of course, applies to many other migratory species.
A swallow in flight
Climactic conditions and temperature are the underlying reason why the swallow’s daily source of food dwindled and then disappeared - the reason it could not follow its normal dietary practice in winter. So why then do we regard climate alone as a distinct and separate cause of migration from that of food?
Temperature is, in and of itself, a reason for migration for some species, especially mountain and moorland varieties that perform smaller treks, often based on altitude, to avoid the cold. In addition, birds who have arctic breeding grounds such as the snow bunting, some gulls and the arctic tern – there are over 100 species that fall into this category in total - could not be expected to survive there in the depth of winter. Thirdly, it is believed that many smaller varieties such as the hummingbird also migrate for reasons of temperature due to their low body weight and inability to conserve internal warmth. The term for this is thermoregulation.
Studies have shown however that, if artificially fed, small birds may remain in situ but the evidence is inconclusive and more research is needed on this topic.
However, although it may be not be a mandatory cause of migration, maintenance of body heat requires energy and therefore calories, which may be enough motivation for some.
Lastly, water fowl plainly cannot function in geographies where the water is frozen. So we can see it is the case that weather and food are both interrelated and yet separate factors for migration dependent on the species.
A pair of Arctic Terns at their breeding grounds
We must first return to nourishment again before considering the case for breeding as an independent cause. To be sure, with all those new hungry mouths to feed, the need for food sources increases and many of those sources would likely be too scarce if all species remained together, but there is another precious resource that migrating birds which are keen to reproduce are in search of – suitable nesting sites.
Adequate shelter plays a part and for some it is the safety in numbers that breeding colonies provide that they require and these sites are commonly returned to year after year. For others, the areas that they inhabit for the rest of the year for reasons of diet are simply incompatible with the physical requirements of nesting – fish feeders such as sea birds, divers and some waders for example.
This factor applies to birds whose habitat before migration is in the northern and southernmost latitudes, as the length of the day varies less the closer to the equator. Conversely, contrast the permanent twelve hour days found there to the advantages of extreme latitudes where it is possible at certain times of the year for carnivorous birds to forage and hunt around the clock. Plant life and associated insects also thrive in these conditions to the benefit of the herbivores and insectivores. Where the midnight sun exists, so may midnight feeding – and midnight nest building. Whatever an individual species needs to thrive and to propagate, the longer the day the easier the task and, of course, the reverse applies in the shortened sunlit hours of winter.
Osprey in flight
Not all bird species migrate – in fact less than half of them do. Modern patterns of migration are also adapting due to climate change, with the timing of departure seemingly under flux as a result.
If we consider other historical epochs when the earth’s climate changed, during ice ages for example, then another perspective arises for our consideration. As the poles got colder, it is likely that birds moved closer to the equator. As the ice age subsequently receded, millennia later, it is equally likely that some ventured back further north or south again.
The ancestors of many of the birds now familiar to us may not have otherwise have had any cause to visit the areas they now inhabit for parts of the year and we assume to be their natural home. We also know from skeletal remains that those ancestors were established far earlier in history in what we now regard as far-flung places and that, in evolutionary terms, they are relatively recent visitors to the major areas of human population.
Given that nature has many examples of natal philopatry, that being the phenomena where adults eventually return to the place they were born to reproduce, such as salmon, eels, turtles and others, could it be that the swallow, the osprey, the goose and the sparrow are not, as we see it from our human perspective, leaving but returning - to their ancestral homes – until in the distant future the climate changes sufficiently once again and they can remain there all year? From an avian perspective, is their presence in our neighbourhoods merely an inconvenience to be abandoned once the next interglacial period begins, as it inevitably will?
The curious reader may also wish to investigate whether the presence of predators and the reduction of endemic disease due to dispersal may also play a part in bird migration, or indeed research the prevalence of migration in other animals such as insects and mammals, from butterflies to whales, and many others.