Taking Both A Bird and Its Young or Eggs Is Forbidden (Deuteronomy 22:6-7).
Here what was seemly with regard to nature is in mind. Man was able to look on nature as a provider, but was not to treat it with disregard. Rather he should receive all with gratitude and watch over the provider. Compare the attitude required with regard to trees which were also providers (Deuteronomy 20:19-20). A general principle was being taught here of preserving the sources of supply.
‘ If a bird's nest chance to be before you in the way, in any tree or on the ground, with young ones or eggs, and the mother sitting on the young, or on the eggs, you shall not take the mother with the young, you shall surely let the mother go, but the young you may take to yourself, that it may be well with you, and that you may prolong your days.'
There were two principles involved here. The first was the unseemliness of taking the young or the eggs of a bird for consumption, and at the same time eating the mother, who was fulfilling her God given responsibility of ‘multiplying', thus taking the provision and eating the provider. This was seen as an offence against creation and against decency. The second was the principle of conservation. Some of what was found should be left so that it could reproduce further food in the future. To take the supplies and kill off the supplier was foolishness.
This has to do with taking eggs for food, not as an interesting hobby. The latter would have been looked on as waste. A bird could, of course, be shot down with a slingstone, and eaten, but it was not to be slain while it was fulfilling its God-given function. Thus this was very much a matter of principle. The point may also be of the impropriety of finding a bird nesting and killing the bird as well as stealing her young. It had similarities to boiling a kid in its mother's milk (Deut. 14:31).
A further thing that may be in mind could be that in normal circumstances the bird could have flown to safety. It had remained to defend its young. It was fulfilling its motherhood. Under such circumstance it was to be spared on a parallel with the fatherless and widows, as an act of compassion. It inculcated a sense of decency and fair play.
“That it may be well with you, and that you may prolong your days.” That this is especially added here would seem to confirm that this was seen as an exceptionally ‘good' thing to do, and as recognition that it was conforming to creation's purpose. It may on the one hand simply signify the benefits that would be obtained. The ready food would make it well with them, while preserving the mother would ensure future provision though their lives. But comparison with Deuteronomy 5:33; Deuteronomy 6:3; Deuteronomy 6:18; Deuteronomy 12:25; Deuteronomy 12:28; Deuteronomy 19:13 suggests that it was more because they would have obeyed Yahweh's commandment and shown compassion and thought for God's creative purposes and for living things. Thus they would benefit within those creative purposes. The phrases may have been added to emphasise the importance of what might have seemed to some, who were harder hearted, to be an unnecessary imposition.
Some might question whether a mother bird should be of such importance. But perhaps that should draw out the further fact that this was a real test of goodness, goodness towards something that would not appreciate it and would give no reward in return. This was one of many laws which taught that consideration should be given to the defenceless, whether human, beast or bird. Such behaviour revealed what true men who obeyed God were like. They were considerate and thoughtful in all their ways, people of compassion in all circumstances, even with the weakest.
In the end this was not saying that someone who just obeyed this particular commandment would have long life. It was rather pointing out that those who were like this would live long lives, while those not considerate in all their ways would in general not. For the fact is that righteousness contributes to long life just as being dissolute does not. Righteous behaviour tends towards good health. Furthermore a man who made friends was more likely to live longer (especially in a turbulent society) than one who made enemies. These are general principles which God supports. It brings out that God is with and guides the righteous in what contributes to health and happiness.