BEI: AND SIZE OF EGGS. The number of eggs that constitute a 'clutch' or 'set' is fairly uniform with each species, but differs greatly among different species. l'elagie water-fowl, and the larger birds of prey usually lay only one egg, rarely more than two; many game birds :tin] lake and river fowl lay a dozen; wood land and field birds average five. though in species two or three are prevailing num bers, and in others, for example titmice, the clutch may consist of ten eggs. The number of eggs laid, except in eases correlated with peculiar ciremnstances in life history are so nearly constant among species of similar habit and bear so evident a relation to the average comparative danger to which the specie: is ex posed. that naturalists recognize evidences of a seeming general principle which governs the size of the brood, and if any peculiarity should appeal- in the nesting or the rearing habits that might accrue to the welfare of the individual species. it is offset by a reduced reproductive power.
Another noteworthy fact is that great diversity exists in the size and weight of eggs as compared with the size and weight of the mother bird. Ilewitson states, for example. that the raven and guillemot are of about equal size. hut their eggs vary as four to one, the latter's being as big as those of an eagle. Still more remarkable for
disproportionate bigness are the eggs of the Aus tralian mound-birds, which measure by inches, though the hen is only about the size of a common fowl; and of the extinct moan. epi ornis and the existing kiwis (Apteryx), that of one of the kiwis weighing nearly one-fourth as much as the hen which produces it. Much indi vidual variation exists, and several factors seem to enter into the explanation; but in general eggs much larger than ordinary, relatively to the size of the mother, belong to birds whose precocial young will receive little or no help and care from their parents after birth, and hence must remain within the protection of the egg until they are considerably advanced toward maturity. This requires a very much larger egg than is needed for the ea•ly-born, parent-supported. altricial birds, since a much greater quantity of food (yolk) must he stored up for them, and they must have room to grow. It is plain that few such eggs can be produced from the resources of the mother's organization, and as a matter of fact these huge eggs are laid singly: as a rule. relatively large eggs mean few in a brood.