DISPERSAL AND SURVIVAL OF PLANTS 191 



our area, and there is no doubt that they are active as transportation media 

 for both ingested and externally carried plant parts. Over short distances their 

 role is unquestionably of very great significance (Samuelsson. 1934; Ridley, 

 1930). But how important is it, when it comes to long distances? It is necessary 

 to consider not only which birds can act as carriers in our area, but also what 

 they eat and where and how they travel. Due regard must be given to such 

 phenomena as behavior, food intake, rate of metabolism, flight speed, 

 migration dates, etc. 



Most of the migratory birds in our area are shore- and waterbirds. Many 

 subsist on a diet of plankton, fish, or other animal matter, augmented only 

 occasionally by a nibble of berries, grain (e.g. from a shipwreck), and garbage 

 (Witherby et al.. 1939). But there are also some, which are mainly vegetarian, 

 like swans, geese, and ducks. Smaller, vegetarian landbirds, as well as gulls 

 and terns, which occasionally eat vegetable matter, can be disregarded because 

 their metabolism is too fast in relation to their flight speed to permit them to 

 carry anything internally between our landmasses (Ridley, 1930). 



Experiments with captive ducks, geese, and swans have shown that the pas- 

 sage of food in these large birds from the time of ingestion to evacuation takes 

 from 3 or 4 hr up to, in extreme cases, 7 hr (Ridley, 1930). Their flight speed 

 during migration is not too well known. Over land it seems, according to Hoch- 

 baum (1955). to be about 80 km hr at an altitude of 300-1000 m. Over open sea 

 speed as low as 50 km hr and altitudes of only 50 70 m have been measured 

 by radar (Buss, 1946; Yocom, 1947) for mallards (Anas platyrhyncha L.) 

 and pintails {Anas acuta L.). If, therefore, one of the large birds consumed 

 a seed just before take off, it is possible that, flying at maximum speed, it 

 could carry the seed as far as 600 km in 7 hr before depositing it. But it is 

 likely that the rate of metabolism is considerably increased during a strenuous 

 flight, and if the flight speed over oceans really is slower than over land 

 (Buss, 1946; Yocom, 1947), the distance covered will be considerably shorter, 

 down to as little as 150 kni. 



Furthermore, it has been observed that ducks, swans, and geese do not 

 eat immediately prior to taking off on a migratory flight (Ridley. 1930; 

 Hochbaum, 1955). There are even reports that they start on an empty 

 stomach (Andersson in Ridley. 1930). 



The question must also be asked whether seeds will pass unharmed through 

 the viscera of these large birds. The action of their crop is very energetic, and 

 most soft seeds will inevitably be crushed although hard-shelled seeds may 

 escape damage. This has been observed in domestic geese (Ridley, 1930). 

 Too little is actually known about the diet of wild swans, geese, and ducks 

 (cf. Table 2; also Ridley, 1930; Schaaning, 1933 a, b; Witherby era/., 1939; 

 Durango, 1953; Lowenskjold, 1954; Hochbaum. 1955; Sladen, 1960) to 

 evaluate the chances for a substantial part of consumed seeds to escape 

 unharmed through the digestive channel of these birds. 



