Food & Cover 
 
Extract from "The Role of Environment in the Life of Birds" by
S. Charles Kendeigh. 
(iled in Ecology Box.) PaRe 383. 
"Stevenson (1933) made a special study of the food requirements of some

passerine birds at the Baldwin Bird Research Laboratory. He found that passerine

species of birds during the breeding season usually have some food in their
stomach 
during all the daylight hours, and so digestion goes on continuously. However,

during active migration, Groebbels (1930) has found that after a long flight
the 
stomachs of birds are empty. They then rest until they can replenish their

resources before renewing their Journey. 
"The rapidity of digestion is indicated by the fact that a morsel of
food may 
pass through the alimentary tract and the undigested portion eliminated in
one and a 
half hours. A full stomach in a song or field sparrow may be entirely digested
in two 
and a half hours. Seed-eating birds eat an amount of air-dried food daily
during the 
summer equal to about 10% of their own body weight. Ninety per cent. of the
vegetativ 
food eaten by seed-eating species is absorbed, so there is little waste.
Insectivor- 
ous species may consume an amount daily up to the equivalent of 4O% of their
body 
weight, but much of this is water. Rorig (1905) made a special study of several

native European species of birds kept in cages during the winter. He allowed
these 
birds all the meal-worms that they could consume and found that they ate
an amount 
daily equivalent in dry weight to 8 to 14% of their own body weights. The
dry weight 
of meal-worms he takes to be 40.24% of their fresh weight. Therefore, both
insectiv- 
orous and herbivorous birds appear to consume in equivalent dry weight the
same 
relative amount of food. According to Groebbels (1931), small birds require
propor- 
tionally more food than larger ones, due apparently to higher rates of metabolism.

In a later work, Groebbels (1932) has exhaustively treated the role of nutrition
in 
birds and the bearing that this has upon bird migration and distribution."