144 Transactions. — ZooJogy. 



is that set out by Darwin — that the occasional laying 

 of an egg in another bird's nest may reduce the work of 

 tending a very large brood, and be of service to the parent 

 in enabling her to migrate earlier ; and at the same time the 

 mistaken instinct of the host may make the young more 

 vigorous than if fed by its true parent, and thus prove of 

 advantage. That the eggs are laid at considerable intervals of 

 time was thought by Darwin to have increased the difficulty 

 of self-hatching, and to have helped to advance the onset of 

 parasitism. On carefully looking into the habits of all of 

 these parasites the most important point that strikes one is 

 the universal prevalence of promiscuous breeding and poly- 

 andry — that is, of the occurrence of small flocks of five or six 

 cock birds and only one hen (118). What is the cause of this 

 preponderance of males is the key to the mystery. The only 

 reason the writer can find is the fact that durmg migration 

 the strongest birds, usually the males, obtain a lead, and are 

 known always to arrive first in any one locality. Male 

 Cuckoos, silent for days or weeks, are seen m Australia and 

 elsewhere, and it is only on the arrival of the females that 

 they begin their calling. That there are non-migrating para- 

 sitic Cuckoos in India and many non-pai-asitic migrants every- 

 where makes it certain that there are other causes as well for 

 this preponderance of males, but that migration is one of them 

 seems feasible. Now, taking these birds with their peculiar 

 habits of breeding, there is no pairing in the true sense of the 

 word, though it is undoubted that the assistance of both male 

 and female is necessary in the construction of a nest. Wal- 

 lace tells us that the male bird of a pair, often a young one, 

 may learn from his mate, who has had previous experience of 

 nest-building, and, vice versa, a young female is often helped 

 bv an old male bird and a verv neat nest constructed. On 

 the other hand, a pair of young birds new to the business 

 often construct a very poor habitation indeed. The female 

 Cuckoo or Cowbird, whose companions roam about the bush, 

 has no mate to help her, her feet and bill are ill-adapted for 

 nest-construction, so she is either content with a few poor 

 sticks on which she lays her eggs or else she drops them one 

 by one into the nests of other birds. In order to make this 

 the more easily effected she has acquired the faculty of ir- 

 regular egg-laying, so that her chicks are hatched out at 

 intervals of several days, a habit also seen in our Native 

 Harrier {Circus gonkU) and in our Shags (Phalacrocorax). 

 Male Cuckoos and Cowbirds ai'e always in the majority, 

 especially at the beginning of the breeding season, and, for 

 reasons before mentioned, this may be partially due to the 

 long flight of migration. From lack of domestic habits and 

 assistance of the male bird the female has not acquired the 



