Birds, 6835 



hawking over these particular pieces, I have been unable to shoot 

 one to ascertain what is the attraction. But I may mention that fine 

 calm weather on the north-east coast is, I am assured, most un- 

 usual during these ^ north}'' months, and probably a very considerable 

 hatch of insects, whose larvae are aquatic, takes place during the 

 warm bright mornings succeeding a ' north :' they remain for about a 

 couple of hours, and then disappear for the rest of the day. But I 

 propose reserving for my next letter some notice of the birds which 

 frequent this north-eastern sea-bord during the winter months, and 

 devoting the present one to a few remarks on the flight and habits of 

 a bird an ordinary observer would not fail to remark had many striking 

 points of resemblance to Acanthylis; I mean Chordeiles Virginianus. 

 You will not, I hope, think it superfluous, if, as a standard of com- 

 parison, I first of all advert to a ievr well-known characteristics of the 

 habits and structure of the Hirundinidse. I have seen it stated that 

 the unusual dilatation of the oesophagus in these birds at the point 

 it leaves the fauces is a contrivance which serves as a crop. I do not 

 know whether this opinion is one generally received, but I do not 

 think it could be at all confirmed by observations on our swallows 

 here : it is generally quite empty, even though the stomach be 

 crammed, or sometimes there is a single insect evidently taken at the 

 moment the bird was shot, or more rarely four or five insects, but then 

 I remark that the whole are of one species. I have a note made at 

 the time where this was the case in an H. euchrysea I shot after ob- 

 serving it some time. In this instance they were all a small species 

 of Ichneumonidse. Now this would look, not as if the dilated oeso- 

 phagus served as a crop, but as if the interval occupied in snapping 

 up the five insects was so short that there was not time for swallowing 

 the first before the fifth had joined it, which can rarely be the case, 

 except in a swarm of insects. Now I think the Naturalist of Selborne 

 long ago made the remark that swallows may very frequently be ob- 

 served to take insects so rapidly one after the other that it is evident 

 the bird must have had both or all in its eye, so to speak, at the same 

 tiipd; but to accomplish this it is obvious how important it is to the 

 bird to be able to move with a certain velocity, otherwise before he 

 came up the relative positions of the floating points would have 

 materially changed. With a body moving freely through a fluid this 

 velocity could in no other way be so easily acquired or maintained as 

 by keeping for certain distances in one direction, or by impetus, and 

 accordingly, as is well known, the flight of swallows is very generally 

 a swoop in a straight line, or a curve of great length, then a rapid turn 



