grounds in Asia and wintering sites in Africa, although some other
species undergo less conventional migrations to or from the peninsula.
Up to three-quarters of the species recorded in some states are
known only as visiting migrants, furthermore, many of Arabia’s
breeding species are absent for more months than they are present.
Simon Aspinall describes the various ways of studying migration
which have helped unravel some incredible journeys and highlighted
the particular importance of Arabia for migrant birds.
Wherever you are in the world you can see birds, but whilst some
are clearly resident around us others are most definitely not,
their appearance may be intermittent or unpredictable, or as with
the true migrants, something you can virtually set your watch
by. In the northern hemisphere the annual migration of birds,
northwards in spring to Eurasia and southwards in autumn to Africa
and southern Asia is a rather simplified standpoint but a good
start on which to build. Centering ourselves on Arabia what do
we know of the movements of birds to, from and through the peninsula
?
Let us first clear the way with some definitions about bird, or
indeed any animal, migration or movements. Migration is the easiest
to define, being the regular and predictable movement from one
area to another and back again at more or less fixed times. Generally
this involves movement to and from high latitude to low latitude
or away from landlocked ‘continental’ situations, for example
the central Asian steppes or Tibetan plateau. The distance actually
moved is unimportant and although for birds we often think of
movements of perhaps several thousand kilometres, for a mollusc,
for example, a regular seasonal ‘slide’ of just a few metres can
equally well be considered a migration. Altitudinal migration
works on the same principle and is generally facultative (only
taking place as necessary); a species found nesting on a treeless
alpine meadow in summer may simply descend to the lower elevation
of a valley floor in winter or, more rarely, further afield. It
is obvious to see why insectivorous species have to move, just
as it is for waterfowl and other groups. What is less easy to
explain is how and why migration evolved in the first place, although
what has to be realised is that migration is a recent phenomena.
In the 12,000 years since the last Ice Age receded, colonisation
and recolonisation will have followed the retreating ice as summer
isotherms moved north. Winters remain too harsh for many species
to reside in northern breeding sites hence the need for migration.
Migratory routes look to reflect (re-) colonisation history.
Nomadism is an alternative strategy whereby species roam, sometimes,
but not necessarily in flocks, to exploit richer food supplies
elsewhere or find a water supply. Examples would include Short-eared
Owls which seek out vole or mouse plagues, and as more obvious
examples in Arabia, Brown-necked Raven, Purple Sunbird and perhaps
Houbara Bustard. In other arid parts of the world, for example
Australia, some ducks and waders amongst many other species including
passerines (songbirds) exhibit a drought-related nomadism. Trumpeter
Finches in Arabia possibly behave in this way. Nomadism is more
or less predictable in occurrence, often being cyclical, whereas
less so is irruptive behaviour which is the term used when species
leave their normal domain en masse because of whole food failure
or because a bumper breeding season means the population has exhausted
the food supply. Irruptive species may then turn up in very large
numbers outside their typical range. Fruit and seed-eaters are
generally those species most affected. Examples in this region
are scarce but include Siskin, Sandgrouse species and to some
extent perhaps Grey Hypocolius. There is, however, not always
a particularly clear division between nomadic and irruptive species
and a species could arguably be said to adopt either strategy
on differing occasions. Excessive temperatures, hot or cold, are
often the underlying proximal cause and it is easy to see why,
for example, Black-crowned Finch-Lark and Cream-coloured Courser
vacate inland nesting sites within the peninsula for more coastal
locations post-breeding. Irruptive and nomadic species often remain
to breed in new areas (and often at rather atypical times of year),
sometimes for several years running before disappearing once again.
Some other species are opportunist, nesting when suitable conditions
exist; Pale Rock Sparrow appears to nest most commonly in the
peninsula after good winter rains.
In addition are local movements. These may involve post-breeding
displacement or dispersal of young from the adults’ territories,
in which case the former move off to find their own territories:
in Arabia the Namaqua Dove has undergone a rapid range expansion
and colonised many new areas as have Collared Dove and Desert
Finch, aided by a proliferation of irrigated agricultural schemes.
Alternatively, and once again facultatively, there are cold-weather
movements (displacements) of, for example, and for obvious reasons,
wildfowl, Kingfishers and Water Rails at the onset of freezing
weather.