6 PROCEEDINGS OF THE NATIONAL MUSEUM vol.80 



of the redhead and canvasback are able to meet such a situation is 

 immaterial; the fact that such a habit is practiced indicates that 

 it may well be expected in the ruddy ducks as well, and probably was 

 a stage in the development of the present parasitic habit of the black- 

 headed duck of Argentina. In the ruddy ducks the eggs seem to be 

 able to meet such a thermal situation. The large size of these eggs 

 is possibly correlated with their heat-adapting abilities, and may 

 therefore be an indirect correlative of the parasitic habit. It has 

 not yet been demonstrated whether the eggs of Heterorietta also 

 possess these thermal abilities ; there are two alternatives to be here 

 considered. If the eggs of this duck are heat adaptive, their sur- 

 vival in strange nests is easily accounted for, as the victims un- 

 doubtedly do take care of them to some extent. If they are not 

 heat adaptive, then the parasitic habit is the sole cause for the sur- 

 vival of the species, for if eggs unable to cope with thermal diffi- 

 culties are laid in nests where they are uncared for, as in the " dump- 

 ing " nests, the chances are all against their survival. If, however, 

 they are laid in nests where they are given incubation by some other 

 bird, they will have a good chance to hatch out. Therefore, if the 

 eggs of Heteronetta are not thermally adaptive, the fact that they 

 are laid in cared-for nests is the secret of their survival. If they 

 are able to cope successfully with the thermal difficulties attendant 

 on the lack of regular incubation, then the parasitic habit is due 

 merely to the loss of the nest-building instinct, as the eggs would 

 get along in uncared-for nests just as well as in others where they 

 receive incubation. If the eggs were laid in uncared-for nests, that 

 is, old or abandoned nests, the birds could hardly be called parasitic. 

 Whichever of these two possibilities is correct, however, the fact 

 remains that Heteronetta appears to have lost the nest-building 

 habit. The problem of the parasitic habit then must be considered 

 unsettled until this angle is delved into more fully. 



We have seen that the ruddy duck sometimes uses old nests of 

 coots; at other times it builds very poor nests; we have seen, inci- 

 dentally, that the concept of territory is not well maintained, as the 

 nests are often close together ; the mere fact that females with nests 

 of their own lay in one another's nests shows how poorly established 

 are the territorial limitations of the actions of the individual birds. 

 The diminution of the territorial and nest-building instincts are 

 seemingly correlated with the diminution of incubating activity on 

 the part of the female parent ; if the bird comes to the nest relatively 

 little, it follows that it will not be apt to respect its territorial bound- 

 aries as much as a bird with a strong instinctive attachment (through 

 incubation) to the nest. In the case of ducks the territory as such 

 vanishes after the eggs hatch, as the parents take their offspring into 



