28 FLEAS, FLUKES AND CUCKOOS 



theless no really adequate theory has been produced to cover all the 

 facts relating to the close association between nesting birds and vicious 

 insects. 



We have described how starlings destroy the ecto-parasites of cattle : 

 there are some mites and insects which perform a similar task for the 

 starlings. Foremost among these minute "de-lousers" is a group of 

 predacious mites, the Cheyletidae, which live permanently on the body 

 of birds and prey on feather mites (Analgesidae) and possibly also eat the 

 eggs of feather lice (Mallophaga). 



In the previous chapter attention has been drawn to the fact that 

 many of the insect inhabitants of birds' nests are beneficial to their 

 hosts. Many rove beetles (Staphylinidae), some species of which are host 

 specific, are regular inhabitants of birds' nests and mostly prey on 

 insects or mites, including ecto-parasites. Certain fly-larvae and mites, 

 and a few moth larvae found in nests are coprophagous and feed on the 

 birds' excrement. 



In tropical countries a true symbiotic relationship has been 

 developed between birds and certain flowering plants. Thus, hum- 

 ming-birds (Trochilidae) with their long slender bills and tube-shaped 

 tongues imbibe nectar and pollinate and fertilise the flowers as they 

 pass from bloom to bloom. Various species show a marked prefer- 

 ence for flowers of certain colours, especially scarlet, and the ruby- 

 throated humming-bird {Archilochus colubris) migrates northwards 

 across America and Canada as different red flowers open in succession. 

 The Honey Eaters (Meliphagidae) have an elaborate brush on the tip 

 of the tongue which acts as an efficient pollen-collecting device. In 

 Britain red berries attract birds and a much more casual and ill- 

 defined relationship exists between berry-eating thrushes, for instance, 

 and plants like the hawthorn and yew. The berries are eaten and 

 the fleshy portions digested, while the seeds which they contain pass 

 through the birds' bodies and are disseminated in a condition in 

 which germination can take place. In the plant world however, there 

 are copious examples of very strict dependence among symbionts. 

 A classical relationship is that between fungi and orchids. The seeds 

 of the latter cannot germinate without certain chemical substances 

 which are supplied by the former. The fungi on the other hand 

 can only live on the plants from which they derive their own nourish- 

 ment. Hence the relationship takes the form of a close and powerful 

 alliance, but there are rather similar associations in which the situation 



