28 A NATURAL HISTORY OF THE DUCKS 



they appeared before or after sex differentiation, and whether they were not at first 

 common to both sexes, as they are in some cases now. There is a splendid field for 

 inquiry into the question of the choice of mates in birds. 



Display is seen at almost any time of year except when ducks are in full moult, 

 and it has no necessary relation to the pairing act. Ducks that are definitely paired 

 practically cease to display, and it follows that these actions are best seen, not on the 

 breeding grounds, but in early spring. Mallards and Black Ducks begin to pair in 

 late winter or very early spring, the older ones doubtless before the birds of the year. 

 At the same time, or a very little later, the reproductive organs of both sexes begin 

 to enlarge. Other species, Teals, Pintails, Widgeons, and Gadwalls, do not generally 

 pair until a little later. Flocks of diving ducks, Scaups, Red-heads, Ring-necks, 

 and Scoters, break up into pairs later than the surface-feeders. 



On the nesting grounds we see various pursuit flights, single males chasing a pair 

 of mated birds or perhaps many males chasing one female; and these flights, though 

 interpreted in various ways, cannot be considered, in my opinion, as part of a dis- 

 play. Francis Harper, who spent the whole spring season among the ducks on the 

 Athabasca Delta, seems to think that a mad scramble among all near-by males 

 occurs whenever a female who is ready for pairing appears. In order to join in these 

 pursuit flights a male will temporarily abandon his own mate who may be beside 

 him. I had always thought of the pursuit flight as having to do more with rivalry 

 for nesting sites than anything else, in other words a question of territory, but these 

 observations of Harper's put a different light on the matter. 



The time for nesting varies in holarctic species with the latitude at which the in- 

 dividual stops for breeding. As we approach the warm countries, the nesting time 

 is not only much earlier in the spring, but it extends over a longer season. In all 

 those tropical countries dominated by a definite rainy season, ducks breed directly 

 after the floods begin, when it becomes possible for them to disperse over the interior. 

 This may mean irregular flights from coast to interior, but such movements are not 

 true migration. 



It would take altogether too much space to go into an enumeration of the nesting 

 sites of ducks. Most of the true fresh-water and diving ducks nest on the ground, 

 or over the water, whereas nearly all the tropical species nest in trees, not neces- 

 sarily in hollow limbs, but quite as often in deserted nests of other species. It is cer- 

 tainly rare for ducks to build stick nests of their own, but there are such instances 

 among Tree Ducks. 



The adaptable Mallard chooses tree nesting sites much more often than is gener- 

 ally known, and probably, should elevated positions become necessary, this sagacious 

 bird would soon learn to discard ground-nesting entirely. 



Many diving ducks build nests that are almost floating, while it is not rare for 

 surface-feeders to lay their eggs a long distance from the water. The Tree Ducks 



