144 Observations on Periodical Birds. 
plenty of nourishment. Is it not, rather, 
occasioned by a propensity to moult, and the 
want of a suitable degree of warmth to en- 
able them to change their feathers? Our 
domestic fowls begin to moult in July, the 
hottest month in this latitude, and birds in a 
state of nature, usually moult when they have 
done breeding : if, therefore, the temperature 
of July is not sufficiently high to promote 
the moulting of the periodical summer birds; 
cuckoos, as they leave the care of their pro- 
geny to strangers, and, of course, are at 
liberty when they have deposited their eggs, 
should be the first birds that withdraw: 
swifts also, having only two young ones to 
rear, should be the next birds that retire; 
the periodical warblers, and those birds that 
have five or six young ones, ought to quit in 
the next place: and swallows and martins, 
usually depart, and nearly a fortnight after the last swallows and mar- 
tins had left us; and in the year 1818, I saw one at Chester, on the 18th, 
19th, and 20th of October. I bad opportunities of observing both 
these birds attentively for a length of time, and I remarked that they 
always seemed to be in the active pursuit of their prey. White, in his 
Hist. Sel. p. 264, mentions an instance of a swift being induced by 
attachment to its young, to remain till the 27th of August ; and 
though deserted by its mate early in the month, it reared a second 
brood (the first having been destroyed) without assistance: a convine- 
ing proof, that however disagreeable it may be for swifts to prolong 
their stay, they are not compelled to quit so early as they do, by any 
difficulty in obtaining food. 
