Page:Territory in Bird Life by Henry Eliot Howard (London, John Murray edition).djvu/249

little over one hour. Now exposure at that temperature is evidently dangerous, but it would be still more dangerous if the weather were wet instead of dry, and the temperature 46° F. instead of 50° F.; and it is. I imagine, on this account that the impulse to brood is so strongly implanted in the female. No sooner, it seems, does she depart than she returns with a small quantity of food which she hurriedly distributes and immediately settles down to brood; and if forcibly prevented from returning, her attitude betrays symptoms of what, humanly speaking, we should term great distress. If, then, the conditions in the external environment were such as would make it difficult for the female to obtain food rapidly, what advantage would she derive from so strongly developed an impulse? Might it not be a disadvantage? Might it not mean that she would abandon the search too readily and be content to return with an insufficient supply, and might not that be as injurious to the young as prolonged exposure? Manifestly the impulse to brood could only have developed strength in so far as it fitted in with all the other factors that make for survival; and the principal factor in the external environment seems to be the territory. How could the young have been freed from the risk of exposure if the impulse to brood had not been so strongly implanted in the parent? How could the impulse to brood have been free to develop if a supply of food had not