If a bird's nest chance to be before thee. This is a beautiful instance of the humanizing spirit of the Mosaic law in checking a tendency to wanton destructiveness, and encouraging a spirit of kind and compassionate tenderness to the tiniest creatures. But there was wisdom as well as humanity in the precept; for, as birds are well known to serve important uses in the economy of nature, the extirpation of a species, whether of edible or ravenous birds, must in any country be productive of serious evils. Palestine, in particular, was situated in a climate which produced poisonous snakes and scorpions, and between deserts and mountains from which it would have been overrun with them, as well as immense swarms of flies, locusts, mice, and vermin of various kinds, if the birds which fed upon them were extirpated (Michaelis).
Accordingly, the counsel given in this passage was wise as well as humane, to leave the hen undisturbed, as the privation of her young would be affliction enough without the additional calamity of the loss of her liberty. Besides, left in her native haunts, she might have the pleasure of rearing another family in their stead; while the taking of the brood occasionally was permitted as a check to too rapid an increase.
Harmer ('Observations,' 4:, p. 31) institutes an inquiry into the reasons that might have induced Moses to issue this prohibition to the Israelites while encamped on the confines of the promised land, and made no previous mention of the subject, although birds were and are undoubtedly inhabitants of the desert. Quails, partridges, pigeons, ostriches, are met with by travelers in that part of the wilderness through which the Israelites passed. As to the ostrich, he answers that their eggs, deposited in the sand, are hatched by the heat of the ground alone, without incubation (Job 39:13); and as to the other birds which are found in the desert, and sit on their eggs, 'they were too few, perhaps to require a law, and of too wild and shy a disposition to run any considerable of being taken by those that might find their nests; or had their nests out of reach, as the dove, which builds in crevices of the rocks, when in a wild state (Jeremiah 48:28) - not to say the old ones are unfit to be eaten, being too tough for food.
This may sufficiently account for the silence of Moses on this point in the first years of their wandering in the desert. But what occasion, it may be asked, was there to mention it at all? What eggs were they likely to meet with, when residing in Canaan, of use to human life? Or young birds whose dams were in danger of being taken through their attachment to their eggs or to their young? Some eggs might possibly be useful for food, and esteemed among the Jews, which were laid by wild fowl or birds; but the beauty of the shell might make many of the younger people fond of taking the eggs of many of the birds of that country, which are numerous.' Then there is the providential reason assigned by Michaelis, and already quoted.