Lastly, we come to the beautiful harmony of colouring of the eggs with surrounding objects, rendering them often very difficult to find, even by a practised eye, and the scant nest still further aiding in the chances against discovery.
The Sandpiper affords an admirable instance of the assimilation of its eggs to surrounding objects, and the extreme difficulty experienced in finding them attests to its protective utility.
Why Eggs Vary so much in Point of Number.—The number of eggs laid by birds of different orders seems to be regulated by the danger to which they are exposed and the amount of food which the parents will be able to supply.
Thus, the Eagle in its inaccessible eyrie enjoys almost perfect immunity from danger, and has only two young ones, for which, however, the supply of food is only equal to the demand, and it is probable that one more voracious appetite would seriously endanger the safety of the whole family.
On the other hand, take the Common Partridge with its sixteen or twenty eggs, the high percentage of its dangers, and the generally abundant supply of food.
Again, the Swift, on its untiring wings for sixteen hours a day, avoids the majority of dangers which threaten less favoured birds, and only lays two eggs, in a position very few other birds could adopt, yet one which secures her little household the amount of safety necessary for the due survival of the species. However, with her limited family, dexterity on the wing, and enduring powers, she seems to have no leisure time during the period her young require feeding. As an opposite, take the House Sparrow, with its five or six eggs, innumerable dangers, and easy access to food, and it must be confessed these things are ordered by a power of infinite wisdom.
Curious Nesting-Places.—The general situation and locality in which each bird's nest is likely to be found are mentioned in dealing with the bird under its separate heading; however, it may not be uninteresting to chronicle a few of the most remarkable and well-authenticated departures from accepted rules.
I have myself found a Dipper's nest on the branch of a tree twelve or thirteen feet from the water and twenty from the bank. This nest was of ordinary shape and size, its material being of the same kind as others, and securely fastened amongst the prongs of the branch like a Missel Thrush's. When the parent bird was disturbed she dived into the pool below to make her escape (a habit invariably adopted when nesting in an ordinary position). She reared her young in safety, however, in spite of the awkward situation she had selected, and I have every reason to believe got them off without mishap. I have also found a Thrush's nest on the ground, precisely in the position a Lark selects.
House Sparrows furnish many examples of curious situations adopted for incubatory purposes. Not long ago a pair of these birds built their nest, and successfully hatched a brood, in the cartridge-box of a cannon which was fired twice daily in the Gun Park at Woolwich. It is a notable fact that in some parts of the country Sparrows build extensively in trees, whilst in others such a circumstance is unknown. Some ornithologists are of opinion that it is an hereditary habit, others supposing that it is resorted to for the sake of coolness in hot weather; but a reason I incline to is that in parts of the country where houses and out-buildings are made of stone the birds find ample accommodation in joints, crevices, and crannies where the mortar has been dislodged, and are therefore not driven to the necessity of adopting trees, like birds found in districts where the houses are made of bricks, consequently closer, and affording less opportunity for nest-building. This bird, besides its noted pugnacity, is an arrant rogue, and invariably takes advantage of the House Martin's labour. I have known a house with twenty nests all close together under its eaves, about half of which were occupied by Sparrows, which had, in some cases where the nests were new, been actually watched ejecting the eggs of the original owners.
The Robin is noted for its caprice in the selection of a nesting site, and has been found hatching its eggs in nearly every conceivable situation, from the ordinary mossy bank to the pocket of a gardener's old coat which had been hanging undisturbed for several weeks in a tool-house. Old kettles, water-cans, inverted plant pots, &c., in buildings close to machinery in daily motion, and other equally curious places, are by no means rare occurrences. A case is recorded of a Robin's nest having been built in the hole made by a cannon-ball through the mizzen-mast against which Lord Nelson was standing when he received his death-wound on board the Victory.