If the genial warmth of the spring produced the passion of love, as it expands the foliage of trees, all other animals should feel its influence as well as birds:  but, the viviparous creatures, as they suckle their young, that is, as they previously digest the natural food, that it may better suit the tender stomachs of their offspring, experience the influence of this passion at all seasons of the year, as cats and bitches.  The graminivorous animals indeed generally produce their young about the time when grass is supplied in the greatest plenty, but this is without any degree of exactness, as appears from our cows, sheep, and hares, and may be a part of the traditional knowledge, which they learn from the example of their parents.

Their Contracts of Marriage.

Their mutual passion, and the acquired knowledge, that their joint labour is necessary to procure sustenance for their numerous family, induces the wild birds to enter into a contract of marriage, which does not however take place among the ducks, geese, and fowls, that are provided with their daily food from our barns.

An ingenious philosopher has lately denied, that animals can enter into contracts, and thinks this an essential difference between them and the human creature:—­but does not daily observation convince us, that they form contracts of friendship with each other, and with mankind?  When puppies and kittens play together, is there not a tacit contract, that they will not hurt each other?  And does not your favorite dog expect you should give him his daily food, for his services and attention to you?  And thus barters his love for your protection?  In the same manner that all contracts are made amongst men, that do not understand each others arbitrary language.

Construction of their Nests.

1.  They seem to be instructed how to build their nests from their observation of that, in which they were educated, and from their knowledge of those things, that are most agreeable to their touch in respect:  to warmth, cleanliness, and stability.  They choose their situations from their ideas of safety from their enemies, and of shelter from the weather.  Nor is the colour of their nests a circumstance unthought of; the finches, that build in green hedges, cover their habitations with green moss; the swallow or martin, that builds against rocks and houses, covers her’s with clay, whilst the lark chooses vegetable straw nearly of the colour of the ground she inhabits:  by this contrivance, they are all less liable to be discovered by their adversaries.

2.  Nor are the nests of the same species of birds constructed always of the same materials, nor in the same form; which is another circumstance that ascertains, that they are led by observation.

In the trees before Mr. Levet’s house in Lichfield, there are annually nests built by sparrows, a bird which usually builds under the tiles of houses, or the thatch of barns.  Not finding such convenient situations for their nests, they build a covered nest bigger than a man’s head, with an opening like a mouth at the side, resembling that of a magpie, except that it is built with straw and hay, and lined with feathers, and so nicely managed as to be a defence against both wind and rain.