THE BREEDING BEHAVIOR OF ANIMALS 



187 



ment. Digger wasps, ichneumon flies, and certain other insects deposit 

 their eggs in various places and provision them with Hving or dead animal 

 food. Birds of one group, the megapodes, lay the eggs in a pile of 

 decaying vegetation, the decomposition of which liberates heat that aids 

 in development (Fig. 155). Again, many animals build nests. These 

 nests may be very simple in construction. In the fishes, for example, 

 many species merely hollow out a small area on the bottom of the stream 

 by pulling out the pebbles and heaping them up on the downstream side 

 of the nest. The eggs, when laid, drop into this hollow and among the 

 loose stones. Birds build nests of a great variety of forms, from the loose 

 collection of grass or straw put on the ground by the killdeer, or the 



Fig. 156. — Blue-tailed skink, Eumeces fasciatus (Linnaeus), with eggs. This lizard 

 buries its eggs (the white mass in the middle foreground) in decaying wood and stays with 

 them until hatched. The curved white streak to the left of the center of the picture is the 

 tail (blue in life) of the parent, and a part of the striped body can be seen to the right of the 

 center. {Photograph by A. G. Ruthven.) 



insecure litter of twigs set in the branches of trees by the mourning 

 dove, to the elaborate hanging basket of the orioles. Still other forms 

 enclose their eggs in cases, as was pointed out for the earthworm in the 

 preceding chapter and as is true also of the leeches and some insects, 

 snails, and spiders. 



Among the nest-building forms the habit of caring for the eggs has 

 usually been developed ; that is, one or both of the parents in many species 

 remain with the eggs until they are hatched. The habit of remaining 

 with the eggs may ensure incubation, or the elevation of the temperature 

 to a point at which development will proceed. Incubation by the parents 

 is necessary in most birds and is an aid in some other animals. Remain- 

 ing with the eggs does not, however, necessarily imply incubation. For 

 example the common skink is a "cold-blooded" animal which remains 

 with the eggs (Fig. 156). Its temperature is so nearly that of the sur- 



