[Illustration]
The crocodile bird, or Egyptian plover, has enjoyed a dubious publicity
because of its reputed habit of entering, and coming out of, crocodile
mouths. As Herodotus put it, the crocodile's mouth is infested with
leeches, and when the crocodile comes out of the water it lies with its
mouth open facing the western breeze. Then the crocodile bird goes into
the crocodile's mouth and devours the leeches, to the gratification of
the crocodile, who is careful not to harm the bird. Though there are
some more recent observations corroborating this, modern observers who
have had abundant opportunity have watched for this behavior and have
not seen it.
As one authority on African birds puts it, it is evidently not an
everyday occurrence.
But the crocodile bird has other habits that are just as bizarre and
interesting. It lives along the sandy shores of African rivers, and
when it lays its clutch of two to four eggs these are buried in the
sand so there is no sign of them aboveground. The bird sits on top of
this spot. A. L. Butler, who studied this bird in the Sudan, thought
that the sand might be scraped away from the eggs and the eggs brooded
in normal fashion by night. The young birds are very precocial, and
feed themselves on tiny insects, but they follow the parent. When
danger threatens the young squat motionless in some depression. The toe
mark of a hippopotamus is a favorite place. Then the old bird, with her
bill, throws sand over the young until they may be completely covered.
Not only does this happen when the birds are very small, but continues
up until the time the birds can fly. Dr. W. Serle in Sierra Leone once
saw a crocodile bird burying something and found the disturbed spot
fairly easily, as recent rain had beaten the sand beach smooth and
hard; a fully fledged young was unearthed. It squatted motionless until
prodded from behind, then it ran swiftly, rose, and flew away strongly.
The burying is not only protection from immediate enemies; A. L. Butler
believed it was normal for the young when not feeding to be buried for
safety or as protection from the burning sun. For a further protection
from the sun the parent moistens the sand by regurgitating water over
it.
Butler on one occasion saw a crocodile bird drink at the water's edge,
run up onto a sand beach, regurgitate water, then settle to brood.
Butler marked the spot, went to it, and, scraping away the dampened
sand, found a tiny chick about one inch below the surface.
This covering of the eggs by the parent is not unique in the bird
world. The pied-billed grebe of North America also does this. When
disturbed at the nest the incubating bird has been seen to use quick
pecking motions to draw material from the edge of the nest over the
eggs. Instead of leaving the eggs exposed the nest simply looks like a
heap of trash and may thus escape the attention of a predator. It used
to be thought that this grebe used to incubate only at night, leaving
the eggs covered during the day to be incubated by the heat from the
sun and from the decaying vegetation of the nest. However, recent
studies have shown this is not the case, and protection by concealment
seems to be the main advantage of this behavior.
Yet another species of quite a different group, the eider duck, covers
its eggs on leaving them. The eider's nest is characterized by a
blanket of down, plucked from the breast of the bird, and when the
female has time, when she leaves the nest she pulls the edges of the
down blanket over the eggs, perhaps for concealment, perhaps for the
sake of the down's insulating properties, keeping the eggs warm in a
northern climate during the parent's absence.
Here we have covering of eggs for what seems to be very different
purposes: to keep the eggs cool; to keep them warm; and to hide them
from view.