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Peregrine Falcon (Falco peregrinus)
The peregrine falcon is a very adaptable bird, found on all
continents, in rainforests and semi-desert, from 15,000 feet
to sea level. It is a highly adapted predator, killing most
of its prey on the wing. Before the 1950s, the peregrine falcon
was a hugely successful bird, but between 1950 and 1965 the
population crashed in most of Europe, Britain and North America.
After some research it was recognised that organochloride
chemicals used in pesticides were to blame. Other factors
did contribute as well, such as persecution from gamekeepers,
egg collectors and falconers, who would take the chicks for
training, but all these activities existed before the 1950s,
and seemed to have little effect on the overall population.
Numbers were sufficiently high that when an adult was killed,
there was another nearby that would take over its territory.
They were also persecuted during the Second World War, when
it was feared they would catch carrier pigeons with important
messages for the army. Their destruction was almost complete
in the south of England, parts of Wales and Northern Ireland,
but the effect was far less in the north of England and southern
Scotland, and almost non-existent in Ireland and the north
of Scotland. At the end of the war, the bird was afforded
a legal status which made it illegal to kill it, and by around
1955 the population had more or less recovered from the effects
of the war. Soon after, however, the population started to
decline again, with the worst reduction in England and Wales,
where the falcon was most dependent on birds of agricultural
land for their food. The cause of this decline was that pesticides
used on agricultural land were becoming concentrated as they
moved up the food chain, and caused the shells of the peregrines’
eggs to be thinner, so that they would break when sat on by
the adult.
The peregrine falcon lives on birds which
it catches on the wing. They are killed either in the
air as the peregrine dives on them from a great height
and hits them with its claws, or they are taken from
the air and carried off to a perch.
Mammals and even fish are occasionally taken, but it
seems that less than one in a thousand of the peregrine’s
prey is taken on the ground.
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Usually, the peregrine goes after whichever bird is most
abundant in the area it is occupying. Thus, in the highlands
of Scotland, the grouse, wheatear and blackbird are important
in the diet, but on Skomer it is more likely to take puffins,
guilleamots and Manx shearwaters. Peregrine falcons have a
distinct preference for prey items between 100 and 500g, around
one eighth of their own body weight. Having said that, they
are known to occasionally go for larger prey such as little
and tawny owls, buzzards, kestrels, and even other peregrine
falcons. It never kills unless hungry, and wastes very little.
It pulls the feathers off the body, but eats the head, the
intestines, the feet and most of the bones. Any indigestible
parts are regurgitated in a pellet.
There is some disagreement over how fast the peregrine falcon
can fly, but estimates of around 150mph when falling seem
to be reasonable. A peregrine falcon in the middle of a dive
is teardrop shaped, with a heavy, solid front, and a narrow,
light tail, with the wings folded inwards. This shape reduces
resistance and allows the bird to reach incredible speeds.
It is this speed that means the peregrine falcon has to attack
from the air, as hitting its prey on the ground would endanger
the bird and cause injury.
The amount of available food is probably the main factor
that limits the number of peregrine falcons that can live
in an area. On Skomer there are huge numbers of seabirds in
summer, but in winter these all migrate, leaving a few small
birds behind. In 2004 there were three pairs of peregrine
falcons with chicks on Sgomer, but after the chicks became
independent, the parents all returned to the mainland.
The peregrine falcon likes to nest on cliffs, often by the
sea, or in mountainous areas. They prefer high cliffs, but
will nest on lower ones if necessary. In the nesting territory,
the pair may use the same ledge year after year, or they may
have a number of favourite sites, up to four miles apart.
The area around the nesting ledge is defended fiercely, from
other peregrine falcons or any other animal that may pass.
Pairs often stay near their nesting site throughout winter,
and early in spring they spend the nights near the ledge where
the eggs will be laid. If both survive the winter, they perform
spectacular aerial displays, rising high up in the air on
the thermals, or rushing after each other, plummeting towards
the sea and then up again. The male brings the female food
during this time, sometimes passing it to her in the air,
sometimes sitting on a shelf in the rock. On these occasions,
the pair bow back and forth and call at one another. It is
thought that this ceremony overcomes the female’s natural
hostility towards the male. The female is much bigger, and
she could easily kill him otherwise. Copulation takes place
on top of the cliff or on the ledge; it is frequent until
the eggs are laid, then stops.
The female usually lays 3-4 eggs in April. The eggs are oval
and smooth with a rich colouring, and for this reason they
are much prized by egg collectors. They are laid one at a
time, at intervals of 2-3 days. Incubation starts with the
second egg to last, so that all the eggs hatch at roughly
the same time. Both parents brood the eggs, though the female
does more than the male, whilst he does more of the hunting,
and brings her food on the nest. The incubation period is
between 30 and 32 days, with the eggs hatching over a period
of two or three days. After that, the female stays with the
chicks for a period, whilst the male continues to bring them
food, passing it to the female to feed the chicks. A young
peregrine falcon is covered with white down, but after about
ten days they grow another coat of down, which keeps them
warmer. After this, the female spends less time on the nest.
She may be extremely aggressive towards intruders at this
stage. The chicks begin to develop adult plumage at around
18 days, and after 21 days, they begin to exercise their wings.
The female continues to spend most nights on or near the nest
with them, and stays close to the nest in the daytime, whilst
the male continues to bring most of the food.
The chicks leave the nest at about 35 to 42 days old, but
they are not experienced enough to fend for themselves fully,
and they stay close to the nesting site for another two months.
They often accompany their parents around the territory whilst
they are still dependent upon them for food. It seems they
can learn to kill for themselves, though they are probably
taught to some extent by their parents. Their behaviour after
leaving the nest is not completely understood. The young birds
separate from their parents in early autumn, dispersing more
or less randomly, for any distance from 10 to 200 miles.
The peregrine falcon’s legal status under the bird protection
act of 1954 means that they cannot be killed, and the chemicals
that were responsible for their decline in the 1950s have
been discontinued. A survey in 1970 showed that the population
had recovered to 55% of the pre-war population, from 45% in
1963. Numbers have been increasing since then, also.
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