Part 15

In circles that discuss evolution the idea is current that food competition is important between species. It may even be stated as a rule: two species with the same food habits cannot live in the same place. Competition drives one out, unless they have different food habits. These differences seem especially evident when you look at closely related species, and they are accomplished in a variety of ways. A habitat difference is very common. The long-eared owl hunts in the woods--its cousin, the short-eared owl, hunts the meadows; the song sparrow favors the drier shrubbery while its cousin, the swamp sparrow, lives in wetter shrubbery.

THE SIZE FACTOR Sometimes the difference is accomplished by size; take the downy and hairy woodpeckers of our wood lots, very similar except that one is larger and is adapted for larger prey, the other smaller and adapted for smaller food items. Sometimes they feed differently, as the Baltimore oriole, which picks flowers and pecks through their sides, while the orchard oriole probes into flowers as they hang on the branches. Thus more individuals of several species live in an area.

When a pair of birds "sets up housekeeping" and starts "raising a family" they can no longer drift about, looking for easy living and places where food is plentiful. Their wanderings are restricted by having a fixed point, the nest, as their center of interest. Two individuals must draw on the food supply from an area about the nest. Competition would be extreme, and, if there were a scarcity, perhaps critical.

We know how different the sexes may be; how different the rooster is from the hen in our domestic fowl, or the drake and the duck in the mallard, or the red male and the green female of the scarlet tanager. These sexual differences have mostly correlated with display and mating. But logically there should be differences in feeding behavior and adaptations between the sexes.

The basic idea is contained in the old nursery rhyme:

Jack Sprat could eat no fat, His wife could eat no lean; And so between them both, They licked the platter clean.

The two birds of a mated pair, limited to a single area, could be expected to have different food preferences or adaptations for getting it. And we find that there are cases of this. The most striking is that of the huia from New Zealand, of which I've written in a Chicago Museum bulletin. Both sexes have similar food preferences, especially wood-inhabiting insects, but they get them in different ways. The male has a short, straight, stout bill for digging out the wood-boring grubs, woodpecker fashion; the female has a much longer, slender, and curved bill for probing into holes for them, creeper fashion. The female may get grubs in wood too hard for the male to chisel. They supplement each other.

DIET VARIATION BY SEX It is possible that further study may show more sexual differences to have a feeding advantage; the larger size of female hawks fitting them to take larger prey; the smaller size of certain female songbirds fitting them for smaller prey, the smaller bills of female hornbills, the straight bill of the male western grebe, and the upturned bill of the female. Perhaps all are of advantage to the species in giving each sex slightly different advantages in getting food.

Selection could have its effect in the populations with most sexual difference in feeding habits being most successful in raising and leaving progeny. Thus, slowly, differences between the sexes would accumulate. However, it must be kept in mind that this sort of evolution would be limited. The drifting apart of the sexes would be checked by the necessity for their coming together periodically for at least a short period, at nesting time.

WATER IN THE DESERT

Water is a precious thing in the desert. Without it no life is possible. When rains come plants spring into vigorous growth. During the long stretches without rain they rest, some as seed, while some plants store water in root systems, or in large trunks. Animals have developed a number of ways of surviving long dry spells in arid country.

Among mammals the kangaroo rat of our Southwestern desert seems able to get along without water. This is caused by an arrangement within the body whereby the necessary water is manufactured within the animal from other foodstuffs: metabolic water.

The accessibility of drinking water in a desert may be the determining factor in whether or not some birds can survive there. The nests of Gambel's quail must be close enough to drinking water for the newly hatched young to walk there, else they perish of thirst. It has been said that newly hatched chicks of the related valley quail of California cannot travel more than a few hundred yards from their hatching places without water. Broods hatched farther away are doomed to die.

Sand grouse, relatives of the pigeons that have adopted the general appearance and habits of quail, live in the Old World, primarily in arid or even desert areas. Where they occur their daily traveling to water is a well-marked phenomenon. Their flight is swift and powerful, and though they may traverse long distances of barren, inhospitable country to watering places, their punctuality in arriving at water, morning and evening in some species, is remarkable.

But what of the young of these desert dwellers that need water? A most unusual situation exists; indeed it seems to be unique. The old birds bring water to the young! This has long been recorded, but as recently as 1921 it has been questioned. However, Mr. Meade-Waldo's observations on birds in captivity seem to definitely establish the custom, and its methods.

PARENTS CARRY WATER Both birds incubate the eggs, the male by night, the female by day, and both parents care for the young. But it is the male only that brings water to the young. He rubs his breast violently up and down on the ground, and then, his feathers awry, he gets into his drinking water and saturates the feathers of his under parts. Then, in captivity, he would run to the hen, make a demonstration, whereupon the young would run out from under her, get under him, and suck the water from his feathers. This they did by passing the feathers through their bills, continuing and changing about until the supply was exhausted. It was found that until the young can fly they take water in no other way.

This was in captivity. Presumably in the wild the process is the same, the adult flying with wet under parts from the water hole to the resting place where the young are under the care of the female.

The similarity of the young sucking water from the feathers to young mammals suckling their mother has been pointed out. But another and a truer similarity exists: that of the young sand grouse getting water from the feathers, and young quail getting water from dew-wet leaves in areas where dew is heavy and there is but little surface water.

BIRD GRAVEYARDS

The best-known stories of animal graveyards are those of elephants. But when I asked the curator of mammals about them the answer I got was little better than a snort. Apparently the evidence for them is so vague that it's little better than a myth.

But in birds we have a few bits of evidence from far-scattered places that occasionally such things as graveyards exist.

In the antarctic Dr. Robert Cushman Murphy found on the island of South Georgia a place where Johnny penguins went to die. It was in a lake in a coastal range of hills. The lake bottom was thickly strewn with scores of penguin bodies, all of which had apparently died a natural death. The icy water, Murphy thought, might preserve them for years. The hills, away from the sea, seem a surprising place for the graveyard of such aquatic birds as the penguins, but it correlates with another peculiarity of their mental makeup. They like to nest on high land, or at least far from the sea. The blind instinctiveness of much penguin behavior is well shown by these birds when there is no high land on their nesting island. Then they may nest so far from the beach on which they land that they are close to the water on the other side. Yet they always returned to the sea by the long route, never taking the shorter route.

Another aspect of this preference for land distant from the sea is shown by their behavior when threatened with danger from man or dog. They flee away from the sea, back onto the land, when safety for them actually lies in the sea. Presumably this fixed behavior dates back to the time when the seal that is called the sea leopard was the penguin's main enemy. Then the sea held their only danger. With man's arrival the situation changed, but only after considerable experience with man do the birds change this behavior.

Apparently, when the time came for the penguins in this South Georgia graveyard to die, they followed their age-old pattern, climbing to the high country and away from the sea.

IN A HOLLOW TREE In a hole about eighteen inches in diameter and twelve feet deep in the trunk of a wych elm in Hants, England, Ursula M. Grigg reports finding the bones of at least ninety jackdaws, thirteen starlings, six green woodpeckers, and twenty-five stock doves. All the remains were clean, and not much broken or decomposed. The idea that these bones were the remains of owls' or other predators' feasts was discarded for a number of reasons; as was the idea that this had been a natural trap, the birds entering to roost or nest and being unable to escape. The most tenable idea seems to be that this was a favorite roosting place in winter, and that during the severe weather old and weakened birds, roosting there, succumbed and added their bodies to this communal grave.

ON AN ISLAND Another instance comes from the little Cape Verde Isle of Cima in the South Atlantic. A photograph in the National Geographic magazine for 1927, Vol. 52, P. 27, has the caption that this island is unique and uninhabited and covered with the tiny bones of millions of petrels which in ages past have come here to die. Certainly the plate shows an amazing litter of bird bones on the tiny plateau of this islet.

Petrels are mostly pelagic birds, coming to the land only to nest on isolated islets. Can this "graveyard" be merely the normal accumulation of the bones of the nesting season mortality, or can it be that the birds actually come here to die?

ANIMAL GARDENS