HIBERNATION OF MAMMALS — MATTHEWS 411 



soft option. The bird found in the mountain canyon was sitting inert 

 in its shelter, its respiration and heart rate reduced to so low a level 

 that they could not be detected with the portable apparatus available 

 in the field, and its temperature down to more than 40° F. below 

 that normal for its active state. A ring was placed upon this bird's 

 leg so that it could be identified, and for four successive winters it was 

 found dormant in the same niche of refuge. 



THE NOCTURNAL TORPIDITY OF HUMMINGBIRDS 



Although the poorwill is the only bird that is definitely known to 

 hibernate, there are others that go some way toward the reduced 

 state of metabolism characteristic of hibernation. The hummingbirds 

 that are the incarnation of activity during the heat of the tropical 

 day relinquish their intense metabolic activity when they go to roost ; 

 their temperature drops and their rates of respiration and heartbeat 

 fall — and this happens in some species not only when they sleep at 

 night but even when they perch for an extended period during the 

 hours of daylight. The metabolic rate of animals varies inversely 

 with their size ; the larger the slower. The daytime resting metabolic 

 rate of those species of hummingbirds in which it has been measured 

 is higher than that recorded for any other animal, and during active 

 hovering it is about six times greater than the rate at rest. But 

 during the nocturnal period of torpidity it falls to about one-twelfth 

 of the resting rate, and the birds become cold, immobile, and incapable 

 of flight. 



Tile hummingbirds are not the only kind of birds that enter into a 

 spell of torpidity when they are not in active movement. Dr. David 

 Lack has recently shown that nestlings of the common swift become 

 torpid when weather conditions are such that their parents cannot 

 obtain the supply of insects necessary for feeding their broods ; their 

 temperature falls and they enter upon a state similar to the hibernation 

 of the poorwill until more favorable conditions return. The nestling 

 swift can withstand many days of fasting that would be fatal to the 

 nestlings of most birds, and while it is denied the usual source of food 

 its weight steadily falls as it uses up the energy obtainable from its 

 tissues to maintain the bare minimum rate (the hasal rate as it is called) 

 of metabolism. It is particularly significant that the ornithological 

 systematists have, on purely morphological grounds, classified the 

 swifts, nightjars, and hummingbirds as closely allied families. The 

 ornithologist who investigates the metabolism of the common Euro- 

 pean nightjar during its daytime period of inactivity will probably 

 obtain some very interesting results. 



Very few mammals hibernate, and it is peculiar that among the ex- 

 clusively insectivorous ones most of the bats, which during the winter 



