INSTINCT OP THE CUCKOO. MIGRATION. 389 



eggs, which they find in the nest where they place their own ; 

 but other observers state that the young Cuckoo itself throws 

 them out of the nest, or drives them away immediately after 

 their birth . The illustrious Jenner, the discoverer of vaccination, 

 tells us that he has often seen the contrivance, by which this 

 little intruder gets rid of its feeble companions ; the young 

 Cuckoo slides under one of the little Birds whose nest it shares, 

 places it on his back, where he holds it by the help of his 

 wings, goes backward to the edge of the nest, and throws it over ; 

 then he recommences the same manoeuvres with a second, and so 

 on, until he has made himself master of the dwelling. 



352. The instinct which leads Birds to sit on their eggs, is 

 generally one of the greatest power ; yet this impulse, which in 

 some respects is a blind one, is capable of being modified by 

 external circumstances. Thus, Ostriches sit on their eggs, when 

 they inhabit temperate climates ; but leave the incubation to the 

 heat of the sun, when they live in the torrid zone. It would 

 appear also, that several of these large Birds often collect their 

 eggs into the same hole ; and take it in turns to sit on them. The 

 Mound-Birds of Australia and the Eastern Archipelago con- 

 stantly save themselves the trouble of actual incubation, although 

 they bestow the greatest care upon their eggs until the young 

 are hatched. They deposit their eggs in large mounds of decay- 

 ing vegetable matters collected by themselves, the heat produced 

 by the decomposition of which is sufficient to hatch them. 



353. The care which birds bestow on their young, is a sub- 

 ject of observation full of interest ; but it is a more singular, 

 and consequently more remarkable instinct, which leads certain 

 kinds to change their climate according to the season, and to 

 make, at fixed periods of the year, journeys more or less distant. 

 Some kinds migrate in order to avoid the cold, or to seek a less 

 elevated temperature, and go to the south or to the north, to lay, 

 or to pass the period of moulting ; others change their country to 

 procure more easily the means of subsistence, as happens with 

 most insectivorous Birds ; but there are some Birds which take re- 

 gular journeys, without being influenced by any obvious cause, and 

 without their change of place appearing to produce any appre- 

 ciable change in their condition. Whatever may be the circum- 



