38
Order Charadriiformes
Diverse small to medium-large birds are members of Charadriiformes. The order
includes about 350 species (see for review Ericson et al. 2003 ). The typical representatives are waders (or “Charadrii”), the gulls and their allies (or “Lari”), and the
auks (or “Alcae”). Because different species of Charadriiformes gather in large
numbers during breeding, migration, or wintering seasons, they are frequently
attractive targets for hunters. Many species have been hunted for their meat, feathers,
oil, and eggs. Hunting by humans is believed to have resulted in the extinction of at
least two Charadriiformes, the Eskimo curlew and the great auk. Hunting continues
to contribute to the threatened state of many other charadriiform species.
Most marine birds rest and sleep on the water surface, while others roost on land
for a few hours a day. Principally, all of them still need to come to land to lay eggs
and raise their young (see for review Ashmole 1971 ; Schreiber and Burger 2001 ).
Below, I would like to analyze several physical adaptations that help to make a
successful seabird from the viewpoint of biological materials.
Eyes
Seabirds have excellent air as well as amphibian vision. The eyes of birds are small
and located on either side of their head. They play crucial role in seabird life because
eyes protect them from the rather bright light at sea surface and give them a
wide fi eld of view. So called nictitating membrane of diving seabirds is responsible
to close the eye when the bird is underwater, but does not play a refractive role.
Some species possess a clear lens in the centre of this membrane that enhances their
vision. The cornea of the seabird eye has an important refractive role in air but
does not usually work underwater because its refractive index is similar to that of
water. However, seabirds’ (similar to water birds) (Tyler and Ormerod 1994 ) eyes
had enhanced powers of accommodation. The iris of the eye had a very well
developed sphincter muscle. It is suggested (Goodge 1960 ), that pressure exerted by
this muscle could change the curvature of the lens, enabling the greater powers of
accommodation.
According to Sivak ( 1980 ), “the suggested mechanisms by which aquatic birds
can compensate for the refractive loss of the cornea underwater include an exaggerated underwater accommodative increase in refractive power, the development of a
fl attened cornea which is non-refractive in either medium, and use of the nictitating
membrane as an underwater refractive goggle. Recent research indicates that
certain waterfowl are capable of altering the shape of the lens dramatically by
means of the ciliary and iris muscles, and that penguins have fl attened corneas,”
(Sivak 1980 ). It was reported (see for review Katzir and Howland 2003 ), that seabirds like the cormorants, possess specifi c an iris accommodative mechanism
capable of producing dramatic lens changes in these animals. Such phenomena were
not described for non-diving birds.
1 Introduction
Order Charadriiformes
Diverse small to medium-large birds are members of Charadriiformes. The order
includes about 350 species (see for review Ericson et al. 2003 ). The typical representatives are waders (or “Charadrii”), the gulls and their allies (or “Lari”), and the
auks (or “Alcae”). Because different species of Charadriiformes gather in large
numbers during breeding, migration, or wintering seasons, they are frequently
attractive targets for hunters. Many species have been hunted for their meat, feathers,
oil, and eggs. Hunting by humans is believed to have resulted in the extinction of at
least two Charadriiformes, the Eskimo curlew and the great auk. Hunting continues
to contribute to the threatened state of many other charadriiform species.
Most marine birds rest and sleep on the water surface, while others roost on land
for a few hours a day. Principally, all of them still need to come to land to lay eggs
and raise their young (see for review Ashmole 1971 ; Schreiber and Burger 2001 ).
Below, I would like to analyze several physical adaptations that help to make a
successful seabird from the viewpoint of biological materials.
Eyes
Seabirds have excellent air as well as amphibian vision. The eyes of birds are small
and located on either side of their head. They play crucial role in seabird life because
eyes protect them from the rather bright light at sea surface and give them a
wide fi eld of view. So called nictitating membrane of diving seabirds is responsible
to close the eye when the bird is underwater, but does not play a refractive role.
Some species possess a clear lens in the centre of this membrane that enhances their
vision. The cornea of the seabird eye has an important refractive role in air but
does not usually work underwater because its refractive index is similar to that of
water. However, seabirds’ (similar to water birds) (Tyler and Ormerod 1994 ) eyes
had enhanced powers of accommodation. The iris of the eye had a very well
developed sphincter muscle. It is suggested (Goodge 1960 ), that pressure exerted by
this muscle could change the curvature of the lens, enabling the greater powers of
accommodation.
According to Sivak ( 1980 ), “the suggested mechanisms by which aquatic birds
can compensate for the refractive loss of the cornea underwater include an exaggerated underwater accommodative increase in refractive power, the development of a
fl attened cornea which is non-refractive in either medium, and use of the nictitating
membrane as an underwater refractive goggle. Recent research indicates that
certain waterfowl are capable of altering the shape of the lens dramatically by
means of the ciliary and iris muscles, and that penguins have fl attened corneas,”
(Sivak 1980 ). It was reported (see for review Katzir and Howland 2003 ), that seabirds like the cormorants, possess specifi c an iris accommodative mechanism
capable of producing dramatic lens changes in these animals. Such phenomena were
not described for non-diving birds.
1 Introduction