Flock of Barnacle Geese during autumn migration
Many species of birds undertake seasonal journeys of
various lengths, a phenomenon known as Bird migration.
The different strategies followed by bird groups are
detailed below.
Long-distance land bird migration
Many species of land
migratory birds migrate very long distances, the most common
pattern being for birds to breed in the temperate or arctic
northern hemisphere and winter in warmer regions, often
in the tropics or the temperate zones of the southern
hemisphere.
There is a strong
genetic component to migration in terms of timing and route,
but this may be modified by environmental influences. An
interesting example where a change of migration route has
occurred because of such a geographical barrier is the trend
for some Blackcaps in central Europe to migrate west and
winter in Britain rather than cross the Alps. Theoretical
analyses, summarised by Alerstam (2001), show that detours
that increase flight distance by up to 20% will often be
adaptive on aerodynamic grounds - a bird that loads itself
with food in order to cross a long barrier flies less
efficiently. However some species show circuitous migratory
routes that reflect historical range expansions and are far
from optimal in ecological terms. An example is the
migration of continental populations of Swainson's Thrush,
which fly far east across North America before turning south
via Florida to reach northern South America; this route is believed to be the
consequence of a range expansion that occurred about 10,000
years ago. Detours may also be caused by differential wind
conditions, predation risk, or other factors.
The advantage of the migration strategy is that, in the
long days of the northern summer, breeding birds have more
hours to feed their young on often abundant food supplies,
particularly insects. As the days shorten in autumn and food
supplies become scarce, the birds can return to warmer
regions where the length of the day varies less and there is
an all year round food supply. Most of the passerine
migrants fly by night in small flocks. During dusk prior to
migration, they show a restlessness which is termed
zugunruhe. They may also sing at night during this
period of pre-migration restlessness.
The downside of migration is the hazards of the journey,
especially when difficult habitats such as deserts and
oceans must be crossed, and weather conditions may be
adverse.
The risks of predation are also high. The
Eleonora's Falcon which breeds on Mediterranean islands has a very late breeding season,
timed so that autumn
passerine migrants can be hunted to feed its young.
Whether a particular species migrates depends on a number
of factors. The climate of the breeding area is important,
and few species can cope with the harsh winters of inland
Canada or northern Eurasia. Thus the Blackbird Turdus merula
is migratory in Scandinavia, but not in the milder climate of southern
Europe.
The nature of the staple food is also important. Most
specialist insect eaters are long-distance migrants, and
have little choice but to head south in winter.
Sometimes the factors are finely balanced. The
Whinchat Saxicola rubetra of Europe and the Siberian
Stonechat Saxicola maura of Asia are a long-distance
migrants wintering in the tropics, whereas their close
relative, the European Stonechat Saxicola rubicola is a
resident bird in most of its range, and moves only short
distances from the colder north and east.
Certain areas, because of their location, have become
famous as watchpoints for migrating birds. Examples are the
Point Pelee National Park in Canada, and Spurn in England.
Drift migration of birds blown off course by the wind
can result in "falls" of large numbers of migrants at
coastal sites.
Another cause of birds occurring outside their normal
ranges is the "spring overshoot" in which birds returning to
their breeding areas overshoot and end up further north than
intended.
A mechanism which can lead to great rarities turning up
as vagrants thousands of kilometres out of range is
reverse migration, where the genetic programming of
young birds fails to work properly.
Recent research suggests that long-distance passerine
migrants are of South American and African, rather than
northern hemisphere, evolutionary origins. They are effectively southern
species coming north to breed rather than northern species
going south to winter.
Broad-winged long distance migrants
Some large broad-winged birds rely on
thermal columns of rising hot air to enable them to
soar. These include many
birds of prey such as
vultures,
eagles and
buzzards, but also
storks.
Migratory species in these groups have great difficulty
crossing large bodies of water, since thermals can only form
over land, and these birds cannot maintain active flight for
long distances.
The Mediterranean and other seas therefore present a
major obstacle to soaring birds, which are forced to cross
at the narrowest points. This means that massive numbers of
large raptors and storks pass through areas such as
Gibraltar, Falsterbo and the Bosphorus at migration times.
Commoner species, such as the Honey Buzzard, can be counted in hundreds of thousands
in autumn.
Other barriers, such as mountain ranges, can also cause
funnelling, particularly of large diurnal migrants.
Short-distance land bird migration
The long-distance migrants in the previous section are
effectively genetically programmed to respond to changing
lengths of days. However many species move shorter
distances, but may do so only in response to harsh weather
conditions.
Thus mountain and moorland breeders, such as Wallcreeper
and White-throated Dipper, may move only altitudinally to
escape the cold higher ground. Other species such as Merlin
and Skylark will move further to the coast or to a more
southerly region.
Species like the
Chaffinch are not migratory in Britain, but will move south
or to Ireland in very cold weather. Interestingly, in
Scandinavia, the female of this species migrates, but
not the male, giving rise to the specific name coelebs,
a bachelor.
Short-distance passerine migrants have two evolutionary
origins. Those which have long-distance migrants in the same
family, such as the
Chiffchaff, are species of southern hemisphere origins
which have progressively shortened their return migration so
that they stay in the northern hemisphere.
Those species which have no long-distance migratory
relatives, such as the
waxwings, are effectively moving in response to winter
weather, rather than enhanced breeding opportunities.
Wildfowl and waders
The typical image of migration is of northern landbirds
such as
swallows and birds of prey making long flights to the
tropics. Many northern-breeding
ducks,
geese and
swans are also long-distance migrants, but need only to
move from their arctic breeding grounds far enough south to
escape frozen waters.
This means that most
wildfowl remain in the Northern hemisphere, but in milder
countries. For example, the Pink-footed Goose migrates from
Iceland to Britain and neighbouring countries. Usually wintering
grounds are traditional and learned by the young when they
migrate with their parents.
Some ducks, such as the
Garganey, do move completely or partially into the
tropics.
A similar situation occurs with
waders (called "shorebirds" in North America). Many
species, such as Dunlin and Western Sandpiper, undertake
long movements from their arctic breeding grounds to warmer
locations in the same hemisphere, but others such as
Semipalmated Sandpiper travel huge distances to the
tropics.
Most of the wildfowl are large and powerful, and even the
waders are strong fliers. This means that birds wintering in
temperate regions have the capacity to make further shorter
movements in the event of particularly inclement weather.
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 is a barrier to a water
bird. Open sea 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.
For some species of waders, migration success depends on
the availability of certain key food resources at stopover
points along the migration route. This gives the migrants an
opportunity to "refuel" for the next leg of the voyage. Some
examples of important stopover locations are the Bay of
Fundy and Delaware Bay.
Some Alaskan Bar-tailed Godwits have the longest non-stop
flight of any migrant, flying 11,000 km to their New Zealand wintering grounds (BTO News 258: 3,
2005). Prior to migration, 55% of their bodyweight is stored
fat to fuel this uninterrupted journey.
Seabirds
Arctic Terns
Much of what has been said in the previous section
applies to many
seabirds. Some, such as the
Black Guillemot and some
gulls, are quite sedentary; others, such as most of the
terns and
auks
breeding in the temperate northern hemisphere, move south
varying distances in winter. The
Arctic Tern has the longest-distance migration of any bird,
and sees more daylight than any other, moving from its
arctic breeding grounds to the antarctic wintering areas.
One Arctic Tern, ringed (banded) as a chick on the Farne
Islands off the British east coast, reached Melbourne, Australia in just three months from fledging, a sea
journey of over 22,000 km (14,000 miles). Seabirds, of
course, have the advantage that they can feed on migration.
The most pelagic species, mainly in the 'tubenose' order
Procellariiformes, are great wanderers, and the
albatrosses of the southern oceans may circle the globe
as they ride the "roaring forties" outside the breeding
season. The tubenoses in general spread thinly over large
areas of open ocean, but congregate when food becomes
available. Many of them are also among the longest-distance
migrants;
Sooty Shearwaters nesting on the Falkland Islands migrate
14,000 km (9,000 miles) between the breeding colony and the
North Atlantic Ocean off Norway, and some Manx Shearwaters do the same journey in reverse. As they
are long-lived birds, they may cover enormous distances
during their lives; one record-breaking Manx Shearwater is
calculated to have flown 8 million km (5 million miles)
during its over-50 year lifespan.
Pelagic
birding trips attract petrels and other procellarids by
tipping "chum", a mixture of fish oil and offal, into the
sea. Within minutes, a previously apparently empty ocean is
full of petrels, fulmars and shearwaters attracted by the food.
A few seabirds, such as
Wilson's Petrel and Great Shearwater, breed in the southern hemisphere and
migrate north in the southern winter.
The tropics
In the tropics there is little variation in the length of
day throughout the year, and it is always warm enough for an
adequate food supply. Apart from the seasonal movements of
northern hemisphere wintering species, most species are in
the broadest sense resident. However many species undergo
movements of varying distances depending on the rainfall.
Many tropical regions have wet and dry seasons, the
monsoons of India being perhaps the best known example. An
example of a bird whose distribution is rain associated is
the Woodland Kingfisher of west Africa.
There are a few species, notably
cuckoos, which are genuine long-distance migrants within the
tropics. An example is the Lesser Cuckoo, which breeds in India and winters in
Africa.
In the high mountains, such as the Himalayas and the
Andes, there are also seasonal altitudinal movements in
many species.
Australasia
Bird migration is primarily, but not entirely, a
Northern-Hemisphere phenomenon. In the Southern Hemisphere,
seasonal migration tends to be much less marked. There are
several reasons for this.
First, the largely uninterrupted expanses of land mass or
ocean tend not to funnel migrations into narrow and obvious
pathways, making them less obvious to the human observer.
Second, at least for terrestrial birds, climatic regions
tend to fade into one another over a long distance rather
than be entirely separate: this means that rather than make
long trips over unsuitable habitat to reach particular
destinations, migrant species can usually travel at a
relaxed pace, feeding as they go. Short of banding studies
it is often not obvious that the birds seen in any
particular locality as the seasons change are in fact
different members of the same species passing through,
gradually working their way north or south.
Relatively few
Australasian birds migrate in the way that so many
European and North American species do. This is largely a
matter of geography: the Australasian climate has seasonal
extremes no less compelling than those of Europe; however,
they are far less predictable and tend to take place over
periods both shorter and longer. A couple of weeks of heavy
rain in one part or another of the usually dry centre of
Australia, for example, produces dramatic plant and
invertebrate growth, attracting birds from all directions.
This can happen at any time of year, summer or winter and,
in any given area, may not happen again for a decade or
more.
Broader climatic extremes are highly unpredictable also:
expected seasonal heat or rain arrives or does not arrive,
depending on the vagaries of
El Niño. It is commonplace to have stretches of five or
ten years at a time when winter rains do not eventuate
during the El Niño cycle, and equally common to have La Niña
periods which turn arid zones into areas of lush grass and
shallow lakes. Long distance migration requires a heavy
investment in time and body mass—and, given the random
nature of El Niño, an investment with an uncertain return.
In broad terms, Australasian birds tend to be sedentary
or nomadic, moving on whenever conditions become
unfavourable to whichever area happens to be more suitable
at the time.
There are many exceptions, however. Some species make the
long haul to breed in far distant northern climes every
year, notably
swifts, and a great many wading birds that breed in the
Arctic Circle during the southern winter.
Many others arrive for the southern spring and summer to
breed, then fly to tropical northern Australia, New Guinea,
or the islands of South East Asia for the Southern winter.
Examples include cuckoos, the Satin Flycatcher, the
Dollarbird, and the Rainbow Bee-eater.
Others again are altitudinal migrants, moving to higher
country during summer, returning to warmer areas in winter
such as several robins, or travel north and south with the
seasons but within a relatively restricted range. The tiny
10 cm
Silvereye is an example: most of the southernmost Tasmanian
race crosses the 200 miles of Bass Strait after breeding to
disperse into Victoria, South Australia, New South Wales and
even southern Queensland, replacing the normal residents who
fly still further north, following the band of fertile
country along the coast, feeding through the day and
travelling mostly at night. The northernmost populations,
however, are nomadic rather than migratory, as are the
Silvereyes of southern Western Australia, which is bounded by thousands of
miles of desert to the north and east, and sea to the south
and west.
Study techniques
Bird migration has been studied by a variety of
techniques of which ringing is the oldest. Color marking,
use of radar, satellite tracking and stable hydrogen
isotopes are some of the other techniques being used to
study the migration of birds.
Migration conditioning
It has been possible to teach a new migration route to a
flock of birds, for example in re-introduction schmes. After
a trial with
Canada Geese, microlites were used in the US to teach
safe migration routes to reintroduced
Whooping Cranes
[1].
References
- Alerstam, T. (2001). Detours in bird migration.
Journal of Theoretical Biology, 209, 319-331.
- Berthold, Peter (2001) Bird Migration: A General
Survey. Second Edition. Oxford University Press.
ISBN 0-19-850787-9
- Weidensaul, Scott. Living On the Wind: Across the
Hemisphere With Migratory Birds. Douglas & McIntyre,
1999.
- Dingle, Hugh. Migration: The Biology of Life on The
Move. Oxford Univ. Press, 1996.
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