DISPERSAL BY BIRDS 71 
ported in the alimentary canal of birds. Some . 
families, like the Finches and Tits, which eat great 
numbers of seeds, are inimical instead of helpful to 
dispersal, because the seeds which they devour are 
crushed and afterwards digested. But in many cases 
the seeds are swallowed whole, and are usually in no 
way injured by their passage through a bird’s body. 
Frequently, indeed, the seeds have not, to run the 
gauntlet of the digestive juices of the alimentary 
canal, being disgorged from the stomach along with 
other hard material prior to digestion. Birds which 
live on berries or other juicy fruits are the most im- 
portant in seed-dispersal. As Barrows says: “The 
seed-eaters are not the seed-planters; on the contrary, 
the insectivorous birds more often sow seeds than 
the true seed-eaters.” “Seeds which simply contain 
nourishment are eaten and destroyed, while seeds 
which are contained in nourishment are eaten and 
survive.”* It is for this reason that, if we look under 
a tree on which Blackbirds or Thrushes perch, we 
shall often find young plants of Bramble (Rubus), Ivy 
(Hedera), Holly Ulex), or Yew (Taxus). There can 
be no doubt that birds eat and subsequently eject vast 
numbers of seeds still capable of germination; many 
observations and calculations might be quoted. But 
when we come to apply the facts to the problem of 
long-distance dispersal, or the passage across serious 
barriers, we find that important limiting factors must 
be taken into account. The digestion of birds is re- 
markably rapid, food being ejected from a half to 
three hours after being eaten, so that a bird eating 
* W.B. Barrows: ‘‘Seed-planting by Birds.’’ Report of the 
Secretary of Agriculture, U.S.A., 1890, p..281. 
