388 NURTURE OP THEIR YOUNG. 



but their form and arrangement vary considerably, as we have 

 elsewhere seen (ANIM. PHYSIOL., 704.) Nearly all Birds line 

 their nests with soft substances, which they collect with care, or 

 even with a thick and soft down, which they tear from their own 

 breasts. The warm and light substance employed in domestic 

 economy under the name of eider-down, comes from a kind of 

 duck named Eider (Fig. 293) ; which inhabits the isles of the 

 Arctic seas, and which thus strips itself to line its nest with the 

 down torn from its breast. 



35 1 . Birds lay their eggs generally once, sometimes twice, a 

 year; in a state of domesticity their fecundity becomes greater. 

 The number of eggs is greater in small species than in large 

 ones ; Eagles lay only two or three at each season; Tomtits and 

 Wrens from fifteen to twenty. The constancy with which Birds 

 sit on their eggs is admirable ; sometimes the two parents divide 

 this labour between them ; in other cases the male is satisfied 

 with providing for the wants of the female, while she is sitting ; 

 and in other species, the whole charge of the incubation rests on 

 the mother alone. In general it is only with reluctance, and 

 when urged by hunger, that she quits her offspring for a few 

 minutes ; and when her young ones are hatched, her maternal 

 affection leads her to lavish on them the most tender cares ; 

 she covers them with her wings to protect them from the cold, 

 and brings themrcarefully-selected food, which she often disgorges 

 into their throats, after having half-digested it, to render it 

 more suitable to their tender stomachs. She guides their first 

 steps ; teaches them to use their wings ; and when danger 

 threatens, shows in saving them as much courage as devoted- 

 ness, we may almost say intelligence. There are however some 

 Birds, that lay their eggs in nests which do not belong to them, 

 in order to have them hatched by strange nurses : such as the 

 Cuckoo, which lays its eggs in the nests of Linnets, Yellow- 

 hammers, Blackbirds, or any other insectivorous Birds, accustomed 

 to feed their young with what would be suitable for the young 

 Cuckoos ; and (which is a remarkable circumstance) the foster- 

 mother becomes a tender and indefatigable parent to these 

 intruders, although they deprive her of her own offspring. Some 

 naturalists assert that the old Cuckoos take care to destroy the 



