DISPERSAL AND SURVIVAL OF PLANTS 193 



It must also be remembered that most migratory flights take place early in 

 the spring, before there is a fresh crop of seeds and berries, and that the birds 

 at this time of the year usually eat the soft, fresh vegetative parts of the plants, 

 like young shoots of grasses, waterplants, etc. Occasionally, however, old 

 seeds from the preceding fall must be consumed, but they do not make up a 

 large part of the diet. In the autumn, when the migration goes from north to 

 south, it starts in many places so early in August that during some years seeds 

 and fruits may not yet be fully ripe. In other years a substantial part must have 

 ripened and then no doubt fruits and seeds make up a considerable bulk of 

 the bird diet. 



A number of Arctic seeds, particularly those that ripen early, are able to 

 germinate at once. Others which ripen late need in most cases a rest period 

 and exposure to cold before they can germinate (Bliss, 1958). We do not know 

 what effect the passage through a bird can have on these seeds, but it seems 

 likely that those seeds which need a rest period would have a better chance of 

 survival. 



It is worth mentioning in this connection that 45 km to the north of Iceland 

 there is a small island. Grimsey. It supports large colonies of birds (gulls, 

 terns, ducks, ravens, etc.) and many of them fly back and forth to the main- 

 land. In spite of seemingly perfect ecological conditions this island still has no 

 Empetnim, though the mainland abounds in this genus and birds are known 

 to greedily devour the fruits in the fall. 



It must also be remembered that birds theoretically are able to carry seeds 

 and other plant parts capable of vegetative reproduction externally, attached, 

 for example, to feathers, feet, or beaks. At Delta, Manitoba, Canada, it has 

 been observed that birds had Lemna in their feathers when shot down 

 (Hochbaum, pers. comm.). Especially when the birds are suddenly scared 

 away from their feeding grounds, the accidental transport of plants or parts of 

 plants from one pond to another is likely to take place (Samuelsson, 1934; 

 Hochbaum, pers. comm.). Farmyard birds also have been observed with 

 lumps of clay, containing live seeds, sticking to their bodies (Ridley, 1930), 

 but it is a fact that wild birds (as all other wild animals) keep themselves much 

 better groomed than captive ones. Wild birds usually preen themselves 

 meticulously before taking off" on a flight, and it seems especially so before a 

 migratory flight (Hochbaum, pers. comm.). 



It is therefore highly unlikely that any larger bits of plants are carried by 

 migrating birds. Even if this were the case, these plant parts would have to 

 endure several hours of drying winds, during spring migration often combined 

 with temperatures below freezing. It is hard to guess what effect the wind- 

 chill factor can have on thus exposed plant parts, but most likely it would kill 

 them within few hours. Seeds, on the other hand, would probably survive, 

 and such genera as Potamogeton, Zannichellia, Sparganium, etc., could 

 theoretically have been spread in this manner (Samuelsson. 1934). However, 



