l8 FLEAS, FLUKES AND CUCKOOS 



One of the most curious and unusual types of commensalism is 

 demonstrated by tits which have recently developed the habit of stealing 

 milk from the bottles left by the milkman on his rounds (See Plate III). 

 This remarkable phenomenon was first noticed about thirty years ago 

 in England and it has since spread rapidly through many parts of 

 Britain. Mainly great tits and blue tits, but also several other species, 

 rob the bottles and they systematically remove paper or metal caps to 

 get at the milk. Although this habit has undoubtedly arisen spon- 

 taneously and independently in different parts of Britain there is no 

 reason to doubt that its spread has been greatly accelerated by the tits 

 learning from their parents and by mimicking one another. It is a good 

 illustration of Elton's transmission of new ideas and new behaviour 

 which does not involve "any organic inheritance or mutation in the 

 ordinary sense." The kea's behaviour (see p. 23) is probably another 

 example of the same type of learning. 



The word "phoresy" was coined to describe the passive transport of 

 one insect by another, but the term is now used to indicate any regular 

 association of this type, not necessarily between insects. Occasionally 

 phoresy and commensalism are difficult to distinguish and almost 

 merge into one another. In the case of remoras we know that the small 

 fish are not mere hitch-hikers but get free meals during their travels. 

 The larvae of certain mites (hypopus stage, Plate V), on the other 

 hand, which are carried about by a great variety of insects, and have 

 like the remoras developed special suckers for the sole purpose of 

 clinging to their transport hosts, have no mouth parts. Therefore we 

 can assume their journeys are no mere joy rides, but entail long fasts. 

 This is the classical type of phoresy — a highly specialised method of 

 dispersal with the object of finding new hosts. It is more difficult for 

 instance to define the relationships between those mites which destroy 

 the eggs and larvae of certain insects, and the adult insects upon which 

 the mites hitch-hike. The unsuspecting female carries them around 

 until she lays, whereupon the mites quickly terminate their trip and 

 transfer to the eggs, which they eventually consume. Certain curious 

 nest-dwellers, the pseudo-scorpions (p. 248) are great hitch-hikers and 

 use birds as well as insects as a means of transport. 



There are a few curious records of snails being found in the plumage 

 — especially under the wings — of newly arrived migrant birds. Several 

 plover have been found with the same species of bladder snail {Physa) 

 both in the crop and among the feathers. It was suggested that these 



