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Birds Of Prey - Background Feature (1 Viewer)

P

peter hayes

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Courtesy of the BBC

What is it that makes birds-of-prey such good hunters; especially those
species that catch live victims sometimes greater than their own weight?
Tenacity is one quality that marks them out.

When a hunting sparrowhawk 'locks on' to a target there is no turning
back. A robin picking up crumbs around a picnic site has been ambushed
in a surprise attack that required the assailant to dodge under tables,
chair legs and around the feet of people enjoying lunch. In the heat of
the chase sparrowhawks have collided with windows, struck cars and
become trapped in confined spaces ranging from garden sheds to lobster
pots.

If birds could talk, every species from wrens to wood pigeons would be
able to describe the feared silhouette of a hawk to a tee: short rounded
wings and a long manoeuvrable tail. The wings provide propulsion and the
tail acts as a rudder. Caught out in the open a hawk is powerless and is
mobbed by small birds. However, the boot is on the other foot when cover
is available. Sparrowhawks pounce on their prey by accelerating over
short distances. They are a high speed, close-quarter assassin. Tools of
the trade include lanky, unfeathered legs and an extraordinarily long
middle toe for catching quarry and holding it fast. Sharp talons and a
flesh-tearing bill complete the weaponry. Grip is all-important because
many captives resist and attempt to struggle free.

Ospreys specialise in catching slippery fish. Unlike other
birds-of-prey, they have four powerful toes of about equal length, while
the outer one is reversible and can swivel backwards or forwards. As
well as needle-sharp claws, the undersides of the feet are covered with
short, spine-like scales, which ensure a firm grasp. Indeed, so
vice-like is the grip, that it can be difficult for the bird to release
caught fish, especially monsters that prove too big to handle. There are
a number of instances of ospreys being dragged under and drowned, unable
to relinquish their hold on a large fish. A carp was netted in a German
lake and attached to it was the embedded skeleton of an osprey.



Falcons have exceptionally strong talons that prevent escape by
piercing flesh. Most powerful is the rear talon, the killer claw, that
can exert a force capable of cutting through a human wrist (which is why
falconers wear thick gloves). They also have a notch along the inner
edge of the bill, which is used to dispatch victims quickly by biting
through the neck vertebrae and severing the spinal cord.

Peregrines are undoubtedly the swiftest birds on Earth - but not
necessarily in level flight. Their top attacking speed of almost 200
miles-per-hour is derived by stooping. Accelerated by gravity, a bird
closes its wings and plunges at quarry. Sometimes it overshoots. Then,
almost unbelievably, it may pull out of a dive and strike from below on
the way up again. Quite how it copes with the gravitational forces
experienced in these acrobatics is a mystery. Without oxygen a fighter
pilot would black out in similar circumstances.

For perfect poise and control on the wing the golden eagle is arguably
the most impressive hunter. Instead of diving directly on wily prey such
as mountain hares or grouse, the eagle continues circling before turning
and gently losing height. Then it flies back at speed, only a few feet
above the ground, and comes out of nowhere to seize its target. Golden
eagles use air currents to soar and may glide up to 15 miles at a
stretch. Gliding speed is controlled by the amount of wing flexing.
Against the wind, a bird slants downwards and rakes it wings back.
Flight with the wind requires a shallower gliding angle on flatter
wings. The weather in upland areas where the birds live is often poor.
Undeterred, there are records of individuals flying quickly through hill
fog in visibility down to 20 yards. How do they avoid obstacles? The
answer lies in their skill at riding air currents which keep them clear
of the ground.

The power of sight is of paramount importance to birds-of-prey, all of
which have forward sight with overlapping 'binocular' fields of vision.
Acuteness of sight is difficult to define but if we were endowed with
the eyesight of a kestrel it is estimated that we could read a newspaper
at a range of 25 yards. Better still, if we had the vision of an eagle
we would be able to detect the twitch of a rabbit from a distance of two
miles. One explanation for hunting birds' phenomenal eyesight is the
sheer size of the retina. This is the screen at the back of the eye on
to which the lens casts an image. Compared to ours, an eagle's retina is
physically larger. The retina is composed of rods and cones, two
different kinds of light-sensitive elements. Rods register shape,
whereas cones discern colour. The retina of a human eye contains 200,000
rods. An eagle has about a million. However, you ain't seen nothing yet.


In 1995 the mind-boggling discovery was made by Finnish researchers
that some birds-of-prey see a wider colour spectrum than we do:
including ultraviolet light. How might this be useful? Kestrels are a
familiar sight hovering along motorway verges and grassland. They are
watching for small rodents. Their quarry is fast and nimble and ranges
over habitat that is often uniform and extensive. At times, the
kestrel's task must seem like a search for the proverbial needle in a
haystack. However, rodents mark their runs with urine and faeces, which
are visible in ultraviolet light. In tests, wild kestrels brought into
captivity were able to detect vole and mouse latrine scents in
ultraviolet settings. This ability enables them to screen large areas of
vegetation in a relatively short time and to concentrate hunting at
'busy intersections'. Does this mean they possess the world's best
traffic camera?
 
No offence taken. I merely pass on the info for discussion on the forums. It's all good debate!
 
Fascinating stuff, Peter, a very interesting read.

Spar, please do let us have more information. Some people may know all this extra detail you're giving, but many more won't and may appreciate your knowledge.
 
Peter,
I think that Spar is hinting that birds respiratory systems are very different to mammals and much more efficient but I will let him explain.

As far as hunting goes, the article states that the Golden Eagle is the most impressive hunter. Yes, indeed. Mike Tomkies who spent over 20 years studying eagles in Scotland including the prey that they brought to the nest included Peregrine as one of the birds being preyed UPON by the eagles. For information, the two 'eagle' books that he wrote among many others on natural history are 'Golden Eagles Years' and the follow on 'On Wing and Wild Water'.


Eddie.
 
Spar,
That is good stuff. It is not boring at all. I would bet you that most people think that birds have a lung system like ours and its just an 'in and out' situation. Your explanation is great. Where did you learn so much?

Yes, Mike Tomkies did make a movie film as well as his many stills photographs but like yourself, I have not seen it. Although a video is advertised on the back of 'On wing and wild water' it is sold out. I believe that he went to live in Spain, then moved back to the Borders (hence the contact address for the video) but has gone back to Spain again. All his natural history books are great reads and his patience and fieldcraft skills were honed over many many years of being in the field(wilderness). A very interesting man.


Eddie
 
I agree with Eddie - you appear to be a walking encyclopaedia. And, don't apologise, it's definitely not boring - it's very interesting information and I hope you continue to educate us (me).

:t:
 
Spar,
Sexual dimorphism. The textbook answer is that if the two sexes of a species vary significantly in size, it means that they, as a whole, can take a wider variety of prey than if they were the same size. Thus, for example, a female Sparrowhawk could take passerines the size of House Sparrow, Chaffinch etc and also Starling and Thrush sized species whereas the male may have difficulty in capturing the latter ones mentioned. Now, this greater variety and therefore abundance of food comes in handy, especially in the breeding season when the female needs lots of energy reserves for forming eggs, incubating them and extra flying to get food for the young even though the male may help in some of these activities. That really is the stock answer.

The next question may be why does this dimorphism manifest itself in all species. Well, it probably does in many species but it is not so obvious. It depends on the life history of the species. Where a species needs to spend time defending a nest or feeding area, it may be advantageous for the male to be big and strong to be able to do the defending successfully and with for example, geese and swans, the male is generally slightly bigger than the female in terms of length etc but often, the female is carrying more reserves that she will need for nesting and this shows in the amount of fat stored on the body. In these species, the fat is stored in the vent area and can be noted when the bird is standing sideways on to the observer. The females which are best prepared for nesting will have very convex vents whereas those that are not will have either less convex or flat or even concave vents. Thus, birds with big bums will probably have more young and be better at raising them all other things being equal!!! Fat is also stored in passerines but as ringers will tell you it is only noticable when the bird is in the hand and the feathers are gently blown to reveal the skin which has a different colour to it when appreciable fat is directly underneath.

I am sure the above will generate some discussion

Eddie
 
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