26 A NATURAL HISTORY OF THE DUCKS 



Among the Tree Ducks we find less difference in the tracheae of the sexes, and there 

 is no tracheal box in the male. We do, however, find the trachea widened into a 

 simple spacious chamber in this sex, but the voice does not seem to be affected. 



ASSOCIATION OF SPECIES 



One of the most striking characteristics of this group of birds is their sociability. 

 Among northern migrants, the Mallards, Teals, Widgeons, and Pintails mix together 

 on their winter feeding grounds almost indiscriminately, while companies of Can- 

 vas-backs, Red-heads, and Scaups, with a sprinkling of Widgeons, are a common 

 sight. But even the smaller ducks are often seen feeding side by side with geese and 

 swans. The fancier may safely confine almost all ducks in the same pond, and will 

 have no real difficulty except with Casarcas, Spur-winged Geese, Comb Ducks, and 

 allied species. 



There is some parasitism among ducks. The case of the Widgeon feeding on plants 

 brought up by Pochards and Scaups is often cited, but this is not true parasitism. 

 It is less well known that certain diving ducks, notably Canvas-backs, Ruddies, and 

 Red-heads, lay quite indiscriminately in each other's nests, and the Black-headed 

 Duck (Heteronetta) is apparently about as parasitic as the cuckoo. Duck nests have 

 been described in which four species have laid eggs. 



Association may, of course, be merely accidental, as when it is brought about by 

 similar food habits. There are some kinds of water-fowl that are quite averse to 

 mixing with kindred species, or even with others of their own kind, but such habits 

 are found mostly among the gooselike, not among the true ducks. It is this mixing 

 of so many different sorts of birds, showing off their various and characteristic 

 flights and swimming postures, that makes sitting in a duck-blind so fascinating. 

 It is not at all uncommon to bring home ten or twelve species of ducks in one day's 

 shooting in localities like Currituck Sound, North Carolina. 



Sociability almost ceases when the breeding grounds are reached, and ducks seldom 

 breed in colonies, like gulls and terns. Eiders, some Tree Ducks and perhaps some 

 diving ducks are exceptions. Many species do place their nests in fairly close prox- 

 imity to each other, but others, as Casarcas and Egyptian Geese, isolate themselves 

 and have a strictly limited "beat" from which they chase away all intruders. 



The European Sheldrake appears to have a real understanding with foxes and 

 other mammals that could very easily destroy its nests, were they so inclined. Other 

 ducks nest on cliffs among birds of prey, and yet are apparently free from harm. 



FOOD 

 It is hardly necessary to enumerate plants and animals upon which ducks are de- 

 pendent. Under each species will be found such details as I have been able to gather. 

 The more adaptable ducks of the Mallard type are practically omnivorous, and their 



