All of us have heard of bird migration which is the regular seasonal movement of some birds, often north and south along a flyway between breeding and wintering grounds. Roughly one in five bird species migrate.
Bird migration is one of the great wonders of the natural world. A huge variety of birds make the journey and avian migration is considered to be a natural miracle. Migratory birds fly hundreds and thousands of kilometres to find the best ecological conditions and habitats for feeding, breeding and raising their young. When conditions at breeding sites become unfavourable, it is time to fly to regions where conditions are better.
It is said that albatrosses circle the earth, flying over the southern oceans, while Manx shearwaters migrate 14,000 km (8,700 miles) between their northern breeding grounds and the southern ocean. Some bar-tailed godwits have the longest known non-stop flight of any migrant, flying 11,000 km from Alaska to their New Zealand non-breeding areas. The Red Knot has one of the longest total migration routes of any bird, travelling up to 16,000 kilometres twice a year. It breeds in Siberia and overwinters on the west coast of Africa, some even going down to the tip of South Africa. The tiny Rufous Hummingbird migrates up and down the North American continent, while the Arctic Tern migrates from pole to pole.
Migratory birds are said to have the perfect morphology and physiology to fly fast and across long distances. Often, their journey is an exhausting one, during which they go to their limits. Prior to migration, 55% of their bodyweight is stored as fat to fuel this uninterrupted journey. Birds need to alter their metabolism to meet the demands of migration. The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. In addition, the feathers of a bird suffer from wear-and-tear and require to be moulted. The timing of this moult – usually once a year but sometimes twice – varies with some species moulting prior to moving to their winter grounds and others moulting prior to returning to their breeding grounds. Apart from physiological adaptations, migration sometimes requires behavioural changes such as flying in flocks to reduce the energy used in migration or the risk of predation.
Most migrations begin with the birds starting off in a broad front. Often, this front narrows into one or more preferred routes termed flyways. These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water. The routes taken on forward and return migration are often different. Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Geese in a V-formation may conserve 12–20% of the energy they would need to fly alone.
Often, the migration route of a long-distance migratory bird doesn’t follow a straight line between breeding and wintering grounds. Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. For most land-birds, such barriers could consist of seas, large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns (important for broad-winged birds). The same considerations about barriers and detours that apply to long-distance land-bird migration apply to water birds, but in reverse: a large area of land without bodies of water that offer feeding sites may also be a barrier to a bird that feeds in coastal waters. Detours avoiding such barriers are observed: for example, brent geese migrating from the Taymyr Peninsula to the Wadden Sea travel via the White Sea coast and the Baltic sea rather than directly across the Arctic Ocean and Northern Scandinavia.
It is truly amazing how migratory birds can navigate with pin-point accuracy. Exactly how migrating birds find their flyways is not fully understood. Scientists believe that birds navigate through an innate biological sense resulting from evolution. Migratory birds may use two electromagnetic tools to find their destinations: one that is entirely innate and another that relies on experience. Our books claim that the timing of migration seems to be controlled primarily by changes in day length and that migrating birds navigate using celestial cues from the sun and stars, the earth’s magnetic field, and mental maps.
Scientists admit that the ability of birds to navigate during migrations cannot be fully explained by endogenous programming, even with the help of responses to environmental cues. The ability to successfully perform long-distance migrations can probably only be fully explained with an accounting for the cognitive ability of the birds to recognize habitats and form mental maps.
Points to think about
We should start at the beginning i.e. the first flight. As per our school books, some animals which had looked longingly at the skies did not just sit back on their haunches but actually grew wings and assorted paraphernalia required to fly and ….well started flying. The rat had become a robin.
The first thing we learn from the above mentioned facts is that migratory birds require some special adaption in their bodies, storage of body fats, moulting of feathers, etc. So, the first question is how did the birds know about what changes would be required even before their first flight? Remember, a flight without proper preparations would have resulted in certain death and dead birds would not have been able to pass on their wisdom to their young ones.
The second question is how did the first bird know its destination i.e. where to migrate? Birds had just evolved from land animals; they did not have the benefits of maps and aerial photographs; so, how did the first bird in Alaska know that the perfect place for its needs was in New Zealand which lay 11000 kms away?
The third question is how did the first bird navigate its way from Alaska to New Zealand? Scientists have surmised that they used cues from the sun and stars, electromagnetic field and mental maps. Really? The first bird would not have had the benefit of memories of ancestral experiences, so a pre-existent mental map is ruled out. So, how could the first bird which had just come into existence, know about using cues from the stellar bodies and the magnetic field? It is illogical. Logic demands that the bird would have to first create in itself the ability to read such cues. So, how did it equip itself? Who taught it to read the cues? How did it know the right flyway? Without a global map in the brain, how did it know about the geographical obstacles which lay ahead?
In view of the lack of credible answers to any of the above questions, it is rationally impossible to accept that a bird which had just come into existence miraculously knew about a place thousands of kilometres away which was perfect for its needs; proceeded to equip itself for such a hazardous flight and then –wonder of wonders- actually completed such a flight.
Sam's Page 