If we suppose this flight of martins to represent a party emigrating from a sand quarry (there were three such quarries within a mile radius), where the population had overflowed, it seems possible to trace the motive which animated them. I imagine that the old birds drive the young ones away, when the young return to this country with their parents after the annual migration. This is particularly the case after a very favourable breeding season, when more than the usual proportion of young birds survive. After such a season, upon returning next year to the sand quarry, the older birds drive off the younger; and if these are so numerous that they cannot find room in another part of the quarry, they emigrate in small parties. I think the same thing happens with rooks. The older rooks will only permit a few of their last year's offspring to build near them. If a gentleman has an avenue of fine elm trees in which he desires to have a rookery, but cannot contrive to attract them, though perhaps now and then a nest is partly built and then deserted, an experiment founded on this idea might be tried. It would be necessary to ask the assistance of the proprietors of the nearest rookeries, and beg them for one year to refrain from shooting the young rooks, after the well-known custom. An unusual proportion of young birds would then survive, and next building season the larger part of these would return to the old trees and be immediately met in battle by their older relatives. Being driven away from the hereditary group of trees, they would resort to the next nearest avenue or grove; if they attempted to mix with a strange tribe, they would encounter a still fiercer resistance. In this way possibly the avenue in question might become stocked with rooks.