Bird Mind


I HAD been fond of birds since a child, but it was when I was about fourteen that I became a real bird watcher. The incident which precipitated the change was this. One morning of late winter, crossing the laundry yard of my aunt’s country house, I saw a green woodpecker on the grass only a few yards from me; I had just time to take in the sight of it before the bird was off to the wood beyond the hedge. The green woodpecker is a common bird enough, but I had never seen one close. Here I saw every striking detail — the rich green of the wings, the flash of bright yellow on the back when he flew, the pale glittering eye, the scarlet nape, the strange moustache of black and red; and the effect was as if I had seen a bird of paradise, even a phœnix. I was thrilled with the sudden realization that here, under my nose, in the familiar woods and fields, lived strange and beautiful creatures of whose strangeness and beauty I had been lamentably unaware.

Most bird watchers, I should imagine, get started off on their hobby by some such sudden glory. The next step in their career is generally the same new realization of beauty and strangeness in other common birds; and only then, as familiarity dulls the edge of novelty, do they turn to look for rarities to give them new excitement.

That certainly was the course of events with me. I can recall to-day with extreme vividness the pleasure of discovering the detailed loveliness of a cock bullfinch busy on a fruit tree, — glossy black of head, soft but vivid brick-red of breast, delicate white of rump, exquisite bluish gray of back, — or watching for the first time the mottled brown creeper mousily climbing a tree and prying into the crevices with its long curved beak, and realizing that here was a new kind of existence which I had not previously suspected.

In the same way I remember feasting my eyes on the astounding beauty of the plumage of teal on seeing a flock of these on a pond in Surrey; and being spellbound by the fantastic ventriloquial note of the grasshopper warbler the first spring I heard it.

A year later I can recall with equal vividness the excitement of coming over the crest of a heathery common (not forty miles from London) and seeing an enormous bird of prey get up fifteen or twenty yards away, leaving half a pheasant carcass on the ground; and of discovering that this was no less a personage among birds than a young white-tailed eagle. That was my first experience of the thrill of rarity; and it set me off deliberately trying to collect glimpses of rare birds as the philatelist collects rare postage stamps. I could take a map and mark down just where I saw my first Dartford warbler, my first smew, Montagu’s harrier, and so on.

For revelation of the beauty of flight, I commend a scene once witnessed in North Wales. I was high up on the mountains above Ogwen. A buzzard was mewing overhead. I looked up, and saw it circling upward, very high, a mere speck over the crags of the Glydyrs. Suddenly it set its wings at an angle and started to glide steeply down. Without once changing direction or speed, it kept its invisible track until it disappeared far to westward behind a distant ridge against the late afternoon sky. In one act of flight it had traversed half Snowdonia. No other creature in the world but a large bird could achieve anything like this speed or singleness of sweeping motion.

Many of the bird watcher’s rewards come like this one, unexpectedly. About five months ago I was standing by the side of a road in the Belgian Congo in the quick-falling equatorial twilight, the lorry on which we were supposed to be traveling having stuck in the mud for the fourth or fifth time. Just ahead I saw shapes fluttering against the sky: they turned out to be nightjars, thirty or forty of them, sweeping round and back over one spot with quick, swift-turning flight, ghostly because so absolutely noiseless, but beautiful in their grace and controlled motion. I slipped down over the bank to be nearer, and then suddenly realized why they were there. A big white ants’ nest stood on the slope and from it were issuing hundreds upon hundreds of the winged king and queen termites for the one sortie of their lives, the nuptial flight.

The nightjars had discovered this, and as the termites fluttered up looking rather like caddis flies (I could just see their long winged shapes against the twilight sky), the birds swooped at them, with their huge mouths opened wide. This social but silent meal, taken on the wing, with twistings and turnings and every subtlety of flight, this banquet for the birds which for the white ants was a massacre of all their emigrants and potential founders of new colonies — the whole scene, though I watched for only five minutes, gave a sudden insight into the strange details of the birds’ life, a sense of unexpected intimacy, such as comes in some foreign country through a glimpse of a family seen at their lamp-lit evening meal through a window left uncurtained.


But it is not only the unexpected that is memorable. One may be rewarded for deliberate search or patient watching. On the Oxford University Expedition to Spitsbergen, three of us set out one day to visit one of the celebrated bird cliffs of the arctic. The vast numbers of sea birds which come north to the arctic to breed must find security for their nests from marauding foxes, and most species take advantage of ledges on precipitous cliffs. As such cliffs are few and far between, they are crowded in an astounding way.

It was a long way from camp to the birds — five hours across snowy tundra to where a heavy ship’s boat had been left a day or so before, and then four hours more in the boat. Our goal was the cliff bastion at the north end of Prince Charles Foreland — a narrow prow of rock, each side of it more than a mile long, and two thousand feet high. When we were still nearly two miles away, we began to hear the birds; and by the time we were half a mile off, the noise was like the parrot house at the zoo heard from just outside. The chattering and screaming of the hundreds of thousands of birds blended into one continuous high-pitched roar. Though it was 11 P.M. when we at last reached the foot of the cliff, the midnight sun made bright daylight, and there was no abatement of the birds’ activity. Innumerable kittiwakes, guillemots, and razorbills, with a sprinkling of puffins, crowded the cliff face, and, near the top, colonies of the huge and predacious glaucous gulls, the robber barons of the arctic bird world. Against this background of deafening noise, hundreds of tiny winged specks whirred into the high ledges with food, whirred out again to fish for more. It was an amazing concentration of vital activity.

I found it an effort of imagination to recall to mind that this bird city was but a seasonal affair, and that in the winter the bird cliff was wholly untenanted, the rich supplies of food that teem in the arctic seas made unavailable by a crust of ice. But the efficiency with which the birds exploit the riches of the sea was vividly brought home by a simple concrete fact. Spitsbergen boasts no trees or even bushes; and over most of it the prevailing grass and lichens are low, the few flowering plants nestling in crevices or growing in the form of dense cushions. All round the bird clilf, however, where plants were nourished by the birds’ droppings, the grass was tall, the weeds were high and rank, growing a couple of feet high instead of two or three inches, so that the whole complexion of the surroundings was changed.

We had twelve miles still to do, two of us, in the slow heavy boat, and had been out already for fifteen hours. So I lay down on the grass, and, though less than seven hundred miles from the Pole, was able to rest the better part of an hour in perfect comfort, so windless and balmy was the air, tempered by the midnight sun. The birds’ clamor filled the place; their forms shot to and fro at every level, up to nearly half a mile above my head, hundreds every minute. It was a complete revelation of the abundance of Nature and her unconcern with man on this arctic uninhabited island.

I was staying for Whitsun one year at a little inn on the Upper Thames. Under the eaves a number of house martins were building their mud nests. I was interested to find out something of their courtship, but there had been no courting in evidence during the Saturday afternoon and evening. So, knowing that such activities are often most pronounced in the early morning, I got up before dawn on the Sunday. To my surprise, there were no martins to be seen — none in or by their nests, none flying round. I walked round the place, and up and down the towpath, along which spread a faint mist from the river, and still saw nothing of my birds. Then from the barn came a single swallow, and flew steeply up into the sky. I followed its flight with my eyes, and suddenly saw why I had failed to find the martins. They were all up there in the blue, circling round in company with some barn swallows and chimney swifts, from which I could just distinguish them at the height they were flying. The sun had not yet risen where I stood on the solid earth; but he already reached the birds high above my head. And as the earth spun and the sun’s rays approached its surface, the birds sank with them, twittering all the time; until finally the light struck the inn and flooded the meadows, and the birds dispersed to the duties of the day.

There can be little doubt that when I first came out, half an hour before I saw the birds, they had already flown up to greet the sun and make themselves a longer day; but how far above the earth they flew before they reached the light and began to circle in it and sink with it, I do not know. Nor do I know if martins and swallows and swifts do this regularly. I have not had the opportunity of setting myself to find out, and although I published a little account of the incident at the time, and asked for more information about such sun-greeting habits, none was sent me. It may be that the birds only ascend thus on radiantly fine mornings; or only during a short space of the early breeding season; or that it is a local habit of Oxfordshire birds. That remains to be discovered. All that I know is that on this particular day I saw the birds anticipate the dawn; and that the sight is still something to be remembered.


The routine of birds’ lives we are a little apt to take for granted. They feed, sleep, make love, disport themselves in song and play, rest, look after their young; they grow, age, and die; their life, in fact, more or less mirrors our own. That is what we for the most part vaguely assume, and then, without troubling our heads overmuch about the matter, proceed to enjoy the obvious sights and sounds with which bird life provides us.

But in reality there are very great differences between the life of birds and the life of men. Perhaps the biggest single difference, and yet the one we are least apt to remember, is that all birds, except a few in the tropics, are more or less unsexed, made neuter, during half the year. The finches or buntings that fought and courted and bred so vigorously all summer, keeping together so closely in pairs, each bird jealous of rivals of the same sex, the pair jealous of intruders on to their nesting area, cock and hen full of amorous devotion to each other, then of parental devotion to the young — these same birds in winter lead a wholly different life. Then they are gathered together into flocks, in which there is neither sexual jealousy nor sexual attraction, no desire to sing, no parental feeling. Sometimes, indeed, the only imprint of sex upon winter life is the existence of flocks all of males, or all of females, the sexes shunning instead of seeking one another, as is often the case in chaffinches.

This neuter phase of behavior is caused by a physiological change within, the reproductive organs shrinking in autumn to a twentieth or even a fiftieth of their full size, to swell again during early spring.

In some species, the unsexing process does not go as far as this, and mated birds remain constant to each other throughout the year. On a mild midwinter day one may see the separate pairs crystallize out, as it were, within a flock of jackdaws, the birds sitting about very obviously two by two. But even with these birds, the emotional state is wholly different in winter and summer, and each spring the cock bird will renew his courtship just as if it were for the first time of asking, and he had not been mated for years.

We have only to reflect how extremely different human life would be if men and women were only to attract each other in the summer, and parents ceased to care for their children with the approach of winter, to see how remote a bird’s life is from our own.

With migrant birds, there is a further complication. Twice a year they are seized with an impulse to leave their haunts and fly south or north as the case may be, often thousands of miles. This impulse is not a mere desire for change, such as sends human beings to the seaside in summer, or to Switzerland or the Riviera in winter: it is the result of some deep-seated alteration in the vital chemistry, and the urge itself is as purely instinctive and automatic as the instinct of a kitten to pounce on mice or of a worker bee to build hexagonal cells of wax. Of the nature of this change in physiology, we know very little. Professor Rowan of Alberta has recently made some very interesting experiments, showing that with some birds at least it is the changing length of night and day, and not temperature or shortage of food, which is the first link in the chain of causes that brings on the migrants’ impulse to move south in autumn. Kept in an aviary until midwinter in their Northern home and then released, the birds with which he experimented simply hung about the neighborhood; if their physiology had been tampered with, and they had previously been kept under a régime of a day progressively lengthened by artificial illumination, they would when released even move northward toward the arctic and almost certain death.

How instinctive it all is comes out still more clearly when we recall that there are some kinds of birds, like our familiar English robin redbreast, which are not constructed so as to feel this impulse at the turn of the seasons, but stay all the year close to where they were hatched and bred.

Most, however, of our familiar temperate birds that we are apt to think of as resident are not really resident at all; they are migratory in an irregular way, on a small scale. The thrushes of our winter hedgerows and gardens, the peewits of our winter fields, are for the most part not the same birds which bred there in the summer. Those have moved a hundred miles or so south; these are birds from the more northerly parts of Europe. This partial and irregular migration is halfway between the stationary residence of the robin and the full migration of the cuckoo and nightingale, and shows how the latter might have evolved out of the former.

The discovery of the prevalence of partial migration was made possible through the practice of banding birds attaching a light numbered and dated ring of metal to their legs, either when still in the nest or after being caught in a special and harmless trap and subsequently set free again. A few of the hundreds or thousands of banded birds will be subsequently shot or trapped or otherwise recovered, and so we can keep track of their main movements.

This banding method has also been used to shed light upon other sides of bird life. For instance, a Mr. Baldwin in the United States put up in his big orchard a great many nest boxes, most of which were occupied by pairs of the American house wren. When they were snugly asleep in the boxes, he caught them and banded them. Now these birds always raise at least two broods in the season; and when the first brood was done with and the business of rearing the second brood was at its height, Mr. Baldwin caught and examined all his wrens again — to discover the surprising fact that about 80 per cent of them had changed partners between broods, as human beings do between dances! They are very constant in their monogamy for the duration of one brood; but after that they separate, and it seems to be a matter of chance how they pair up again for the next spell of married life.

Still another method, in which banding is combined with intensive watching, has been tried on robins. The birds are banded with bands whose distinctive mark can be recognized through good field glasses; and a number of birds are thus followed all through the year, with surprisingly interesting results. The possibility of recognizing individual birds gives a new concreteness of insight into their life. Similar schemes of individual marking have enabled naturalists to discover startling facts about other creatures, notably bees and ants; and there is no doubt that here there lies ready to the hand of the bird watcher a new and valuable tool which will enrich his watching if he has but the patience to use it.

Birds when we notice them are generally well-fed or at least active. We rarely see them when they are ill or starving; we miss most of the violent deaths by hawk or owl. And so we are apt to forget the terrible mortality which all the time is thinning their ranks. But a little calculation will show how enormous this must be. A pair of blackbirds, for instance, has an expectation — barring accidents — of at least ten years of reproductive life; and each year will produce at least two clutches of about five eggs each — say a hundred eggs in all.

Accidents do of course happen, so that perhaps three seasons is more like the average number to which a blackbird, once arrived at maturity, can still look forward. This means thirty eggs; yet the race of blackbirds does not increase, which means that of those thirty, twenty-eight fail to reach the reproductive age again. Mr. Nicholson, from his observations, finds that more than half of these are eliminated even before the time comes to leave their nest. For robins, Mr. Burkitt estimates that at the beginning of autumn a pair will have only one surviving young bird left to show for all their season’s labor. And all this is before the rigors of winter have set in.

For migrants, especially the smaller kinds, the worst losses seem to be during the long migratory journeys themselves. The birds may be blown out to sea and drowned in thousands, or are so exhausted after long struggle with adverse winds as to fall an easy prey to their enemies on landing.

For residents, on the other hand, occasional years of great severity cause a real massacre. Afler the hard winter of 1917, for instance, long-tailed tits were actually wiped out over large areas of England, and it took the overplus of several favorable seasons to fill the empty spaces again. Famine and flood can take heavy toll of human populations; but man is never living so near the edge of security that one severe winter will exterminate all the human beings over a considerable area.


We all of us quite naturally begin with the assumption that other living things have the same sort of minds as ourselves — they happen not to be able to talk, to do mathematics, or to frame philosophies or religions, but in general, we take for granted that they feel and think, remember and plan, in the same sort of way as we do. Closer observation, however, and especially deliberate experiment, quite destroy this assumption. The lower animals do not talk and calculate and philosophize just because their minds are not like ours — they are on quite a different and lower level.

Many people are apt to resent this conclusion and to think men of science cold and soulless for pronouncing it. It always seems to me, however, one of the most exciting and encouraging of ideas to reflect that our minds have been perfected by slow steps from the most rudimentary beginnings, and that there is no reason to suppose that further evolutionary progress is not possible, toward minds which would stand in the same sort of relation to our imperfect instruments as do our minds to the still more imperfect instruments of a shrewmouse or a newt. It is from this point of view that I like to think of the minds of the birds I watch as mental instruments forged out of the metal of life on the anvil of circumstance by the impersonal but inexorable agency we call evolution. They are in all essentials more primitive mental instruments than ours, though in this or that particular they may reveal some advantage of sense, some novelty of instinct.

Of course the actions of an animal must on the whole achieve their end, otherwise the species could not continue to exist; but it is by no means necessary that the end be perceived in the mind of the animal, much less deliberately planned and purposed by it. The usual procedure of Nature is this — that the brains of animals shall be so constructed that in a given set of circumstances impulses will be aroused which impel them to actions adapted to the circumstances, in just the same blind and automatic way in which our bodies are constructed to do the right adaptive thing quite irrespective of our minds knowing anything about it. Our stomach is constituted so as to pour out gastric juice when wanted, and to digest food — it achieves its end admirably. But nobody ever consciously planned to make his gastric juice just of this right composition, or even purposed to liberate it just when it is needed.

As a very simple example of adaptive but wholly unintelligent instinct, take the crouching of many young birds, such as those of various plovers, when danger threatens. In their natural haunts, this is an admirable method of escaping detection, for their colors blend with the surroundings. But they will crouch just as readily on a lawn or a carpet, against which they are conspicuous in the extreme. Their instinct to crouch is in fact as automatic and unpurposed as their color. Another piece of adaptive but apparently quite instinctive behavior we may take is the actions of many ground-breeding birds when an enemy threatens their eggs or young. They trail their wings on the ground and shuffle along as if badly wounded, only to spring into the air when the enemy has been lured far enough from the nest. Here again, all the evidence is against the bird having any conscious purpose or knowledge of what it is doing; the shamming wounded is an inborn pattern of behavior, like sneezing in ourselves.

As a curious example of lack of intelligence we may take the well-authenticated fact that some birds, when they begin a nest against one rung of a ladder or on one of many similar rafters, seem wholly unable to keep the situation distinct, and proceed to build a whole series of nests against all or most of the rungs or rafters. In nature, every situation is a little different from every other; here, man’s artificiality has been too much for them with its repetition of sameness. How remote from ours is the mind capable of this lack of discrimination!

Equal evidence of difference from a human type of mentality, though in a rather different way, is provided by the numerous recorded examples of comparatively clever birds like jackdaws which do not know when to stop in cases where mere bringing of more nest material is useless. Jackdaws breed in holes, and drop sticks into them to make the foundations of their nest. Sometimes it happens that a nice-looking hole communicates with some bigger space below, and the sticks simply drop through. But once the birds have chosen a hole they may continue bringing and dropping in sticks for days and days until a really enormous pile accumulates in the hollow trunk below, although after a few hours’ work it should have been obvious to the meanest intelligence that they were wasting their time. The reason they never realized it was simple — that in respect of nest building, birds do not and are not required to use intelligence, being endowed by Nature with instincts which normally are quite good enough.

Mr. Kirkman has recently conducted a number of interesting experiments on black-headed gulls and finds that they, while the brooding urge is on, can be made to accept stones or even sardine tins and sit on them apparently quite happily — a complacence rivaled in nature by that of the emperor penguin, whose passion for brooding something will induce it to incubate lumps of ice if eggs are not available.

It is this perverse acceptance of substitutes for normal eggs and young which makes possible the parasitism of the common cuckoo. It is surely the very nadir of intelligence for a pair of wretched meadow pipits or hedge sparrows to go on caring for a young cuckoo just because he happened to be hatched in their nest, although he eventually grows four or five times as bulky as his foster parents, and to feed him at all they have to perch on his shoulder or head.

In Mr. Chance’s remarkable film, ‘The Cuckoo’s Secret,’ occurs an illuminating incident which forcibly reveals the deficiencies of bird mind. The young cuckoo is first seen, in the absence of the old birds, heaving one of the fledgling meadow pipits, his foster brothers, out of the nest. It caught its foot not a foot from the rim of the nest, and remained there squeaking. After a little, the mother bird returned with food. Her unfortunate offspring was in full view and hearing, yet she did not attempt to get it back into the nest or even to feed it, but went straight to the young cuckoo and put the food in its capacious gape.

It would thus seem (and there are many other bits of evidence in support of the idea) that birds react much more to whole situations than we do, and are much less capable of distinguishing single objects as separate. What appeals to the mother bird and touches her parental instinct is not a young bird as such, not her young bird as such, but a young bird — more or less any young bird — in the proper situation, which is within the nest. It is as if a human mother were perfectly willing to adopt a young gorilla if she found it in her nursery, and to pay no attention to her own baby though it was howling in full view, but on the other side of the street.

This is not to say that birds cannot and do not learn; but they are only capable of limited learning, and do much more of the business of life untaught, instinctively, than we do. Flight, that most amazingly complex of all physical activities, comes untaught to birds; they do not have to learn how to migrate, or how to build a nest (though a certain improvement in this may come with repetition), and the great majority of them will sing the characteristic song of the species even if kept out of hearing of all others of their kind.


But to make up for their relative lack of intelligence, birds are extremely various and intense in their emotional natures. As cause of this, it is natural to look first to their high temperatures. The rate at which chemical processes take place, including the chemical processes of life, goes up with temperature, being roughly doubled for each rise of ten degrees centigrade, and the normal temperature of birds is keyed up to what in men or any other mammals would be dangerous fever heat. Besides this, birds are constructed to undertake that most arduous of all vital activities, flight. Thus they must have a reserve of energy and power which readily bubbles up, and can express itself in vivid and striking ways.

It is from a combination of these two causes that birds provide us with such amazing exhibitions of combined physical and emotional energy as the skylark singing his way up to the sky, the unceasing duels and wild dervish-dancing of ruffs or blackcocks on their assembly places in the breeding season; that a pair of herons or egrets will burst out with a wild expression of mutual affection each time they see each other again after a few hours’ absence, all through the long months of the nesting season.

Birds are thus, on the whole, lower than mammals in pure intelligence, higher in pitch of emotion and intensity of living. This is the background against which we must remember to interpret their actions. But what I want to discuss is the insight we get into the mind of birds by watching them in the field.

The first point which will strike even the most casual observer is that different kinds of birds are endowed by Nature with different kinds of temperament. The common whitethroat is a restless, excitable creature, always fluttering up and diving down again into the hedgerows, while the hedge sparrow is sober and retiring; the English robin, for all his charm, is exceedingly pugnacious; the house sparrow we all know as impudent and cunning, with a nature quite different from that of his close relative, the tree sparrow.

But after we have given a character to each familiar species of bird, we find that this is only a very general character, and that within the species there is great individual variation of temperament as well. There is enormous difference in the timidity or courage of birds within a single species. One hen will shoot off from her eggs as soon as a human being appears in sight; another will sit tight and let herself be watched and photographed at close range. The red-throated diver, for instance, is not usually a very close sitter; but one bird we came across in Spitsbergen had actually to be forced off her nest by gentle pressure with my boot (her sharp beak precluded the use of hands) before we could see what she was sitting on.

Such individual variation is often very noticeable in respect of the dawning æsthetic impulse which prompts many birds to adorn their nests. This in itself is a strange phenomenon of bird mind. It is strange that one of the American flycatchers should always decorate its nest with a strip of a snake’s cast skin — though it has been suggested that this may perhaps frighten egg thieves; but why should buzzards and eagles and other birds of prey bring green leafy branches to lay on their nests and renew them from time to time during incubation? Why should various plovers put shells and bright pebbles round the edge of the little depression that serves them as nest? We can only suggest that it is, as I said, an early germ of the æsthetic impulse, alien to that which prompts magpies and crows and jackdaws (including the celebrated bird of Rheims) to carry off and hoard bright, shining objects. But whatever the precise nature of the impulse, it is nearly as variable in its manifestations as the artistic impulse in human beings. Of a dozen Kentish plover nests, most will be moderately decorated, one or two will be very richly garnished, one or two will have no decoration at all.

Another very curious side of bird mind is one which was first stressed by Edmund Selous — namely, the way in which hostility between two rival males so often finds outlet in strangely formal posturings and mock combats instead of in genuine fighting. A male swan will ruffle up to an invader of his territory in fear-inspiring pose — breast puffed out, neck curved back, wings arched; the trespasser will adopt the same pose; but instead of coming to blows the two will circle round each other grandly but harmlessly, until honor, it seems, is satisfied, and the invader swims off or the two separate as if by mutual consent. It is the rarest thing for a genuine fight to develop between two swans. The same is true for very different kinds of birds, such as stockdoves; and even the constant duellos of ruffs on their assembly grounds are much more in the nature of sparring practice than of dangerous fights. Here again possibilities of great biological and psychological interest lurk behind the facts; but for the moment what we need is more watching and more facts before we can try to generalize.

Another strange bit of bird psychology is the mobbing of hawks, owls, and cuckoos by many kinds of small birds; and still another is the extraordinary talent for mimicry of other birds’ notes which is indulged by quite a number of species in a state of nature. The mocking bird of America is perhaps the supreme example, but the blue jay, the common starling, and the sedge warbler are masters of the art, and I have been deluded by a blackbird mimicking a nightingale. Here again we can see no utility attached to the practice, and it seems to be a mere by-product of their nature. But why is it found in some species and not in others, though closely related? Why does one bird practise it by nature, and others, like ravens, only when taught?

As you see, a great deal of what I have been talking about consists of facts which pose unanswered questions. This is at least a challenge to the bird watcher to go on with his watching and produce new observations. Puzzling facts are rarely to be cleared up by speculation alone; almost always they need the illumination of new facts for their explanation. It is also a reminder that bird mind, if not especially characterized by high intelligence, is yet complicated enough, and a study full of interest. Let us remember that, in the long history of life, mind has evolved as well as body, and that in studying birds we can be studying a particular phase in the evolution of mind, that strange and mysterious property of living creatures with which — let us face the fact frankly — the present scientific scheme of things, coherent though it be, and ever more embracing, is still very incompletely linked up, and whose eventual incorporation in that scheme will cause upheavals of thought as great as those due to Copernicus or Darwin or Einstein.