COMMENSALISM I3 



"hills" the robin watches intently from some convenient bush and 

 quickly seizes any invertebrates which are exposed on the surface. 



Certain petrels and the great shearwater follow in the wake 

 of whales and porpoises and devour their faeces. In the South 

 Seas, the sheath-bills (Chionidae) enjoy a curious relationship with 

 various colonial nesting birds, which is part parasitic, part commensal 

 and part symbiotic. They haunt their breeding sites and steal many 

 eggs and scraps of food, but they also act as scavengers and obtain a 

 considerable part of their nourishment by eating the remains of 

 Crustacea found in the faeces of birds, especially of gentoo penguins 

 [Pygosceles taeniata). When the Weddell seals are bringing forth their 

 young the sheath-bills, like the ivory gulls in the arctic, will follow them 

 out to the pack-ice and feed on the placental blood and afterbirth of 

 the new born cubs. One observer saw several of them attempting to 

 eat the umbilical cord while it was still attached to the baby seal. 



In Africa, the carmine bee-eater {Merops nubicus) rides on ostriches, 

 bustards and certain large mammals using them as perambulating 

 perches. It catches the insects which pass within reach of the "host" 

 or are stirred up in the grass. Once Myers saw a bee-eater fall off when 

 the bustard broke into a run but it flew after it and soon settled again. 

 This bird will also exploit bush fires fearlessly and darts in and out of 

 the flames to catch escaping insects. The native Mandigo name 

 means "nephew of the burning." 



In the tropics there are numbers of spectacular examples in which 

 certain birds habitually nest in close proximity to dangerous social in- 

 sects such as ants, termites, bees and wasps. At first sight it would appear 

 that the benefit is always one-sided and while the birds obtain protection 

 from the presence of the insects, the insects themselves derive no advan- 

 tage from the association whatsoever. However, increased knowledge 

 about the birds concerned suggests that often mutual benefit is involved 

 and such examples will, therefore, be considered in a subsequent 

 chapter (p. 26). A case in which the insects appear to gain no advantage 

 at all concerns a South American parrakeet [Eupsittula canicularis) which 

 is an obhgate commensal of certain termites. It breeds exclusively in 

 their carton nests which it hollows out for the purpose. The parrakeet 

 never uses unoccupied termitaries, although plenty are available. When 

 building operations begin the termites rush to the scene of action and 

 try to repair the damage. They sometimes appear to make half-hearted 

 attempts to drive ofl^ the adult birds but they soon desist and leave them 



