XX INTRODUCTION. 



tribes, which present a marked difference in the plumage of the old 

 and young, we observe that the latter are expelled by their parents 

 from the breeding-places, and appear both in summer and winter in 

 districts which none of the old birds visit. From a consideration of 

 these and similar facts, we are inclined to believe that, of the species 

 which are found all the year within certain parallels, the younger 

 individuals make the widest excursions in search of food or proper 

 breeding places, and that, as their strength is matured by age, they 

 fill up the casual vacancies which occur in the districts best adapted 

 for their constant residence. 



A number of species, which rear two or more broods within the 

 United States, raise only one in the Fur-countries, the shortness of 

 the summer not admitting of their doing more. The Passenger 

 Pigeons do not visit the Fur-countries, where they breed, until after 

 they have reared a brood, and quitted the breeding-places in Ken- 

 tucky. It is probable that some other birds also breed in succession 

 in different districts ; and it is even possible that a few of the Falco- 

 nid(E and some species of certain families of Grallatores, after 

 spending the short summer of a high northern latitude in rearing one 

 family, may gain a similar climate in the southern hemisphere for 

 the purpose of rearing another brood, passing, of course, nearly one 

 half of the year in the transit to and from these breeding-places *. 

 This hint is thrown out as simply conjectural ; but, in the families 

 to which I allude, seldom more than two young are produced at a 



* I have several times, when cruizing between Minorca and Sardinia, seen large flocks of Swal- 

 lows, attended by great numbers of Hawks and a very small species of Owl, holding a direct course 

 from the coast of Africa towards the Gulf of Lyons. From the direction of their flight, it did 

 not appear that they could have rested on any part of the islands I have just mentioned; so that 

 the traverse they attempted was full seven degrees of latitude. Both Swallows and birds of prey 

 appeared much tired, and settled for several days on the masts and yards of all the ships of the fleet, 

 from whence vast numbers were taken by the crews. Having no acquaintance whatever with Orni- 

 thology at the time, I do not know the species of the small Owl, but I have a distinct recollection of 

 its being no bigger than a Sparrow ; and in this I can scarcely be mistaken, as I kept one for nearly 

 a week before it made its escape. It was fed on Swallows, which it invariably strangled by grasping 

 them by the neck with one foot. If a bird of this size can cross such an expanse of sea at one flight, 

 the greatness of the distance to be traversed, according to the supposition in the text, does not seem 

 to be of itself a sufficient reason for rejecting the hypothesis, particularly if the length of time occupied 

 in the passage and the long halts at the various resting-places be considered. 



