HOST RELATIONS OF PARASITIC COWBIRDS 23 



winged) and found evidence for 5 eggs to a clutch (or set) for each but 

 no histological evidence indicating whether there was but one clutch 

 or several per season. 



Jones (1941, pp. 117-119) studied one cowbird that laid a very 

 distinctively colored egg, and he found 8 eggs, ostensibly of this one 

 bird, from May 5 to June 15. How many others may have been 

 laid by this hen and were not found by the observer cannot be stated. 



As described in my accomit of the field sparrow (pp. 164-165), Walkin- 

 shaw (1949) reported that a single cowbird laid no less than 25 eggs 

 between May 15 and July 20. Nice (1949, p. 233) showed that the 

 dates in Walldnshaw's records suggested clutches of 5 eggs at daily 

 intervals, with a break of several days between clutches. 



That the individual cowbird studied by Walkinshaw was not as 

 exceptional in its fecundity as it might seem is suggested by evidence 

 sent me by Dr. and Mrs. Daniel McGeen, who found two cowbirds 

 that had laid at least 18 and 19 eggs, respectively. The first bird laid 

 its eggs in clutches of 3 (one each day from May 23 to 25, inclusive), 

 of 7 (one each day from May 31 to June 6, inclusive), of 4 (one each 

 day from June 9 to 12, inclusive), of 1 on June 18 (this is probably 

 an incompletely recorded clutch that may have extended beyond 

 June 18 by as many days as there were eggs laid), and of 3 (on June 

 27, July 1, and July 2). The other individual's 19 eggs similarly were 

 grouped in "clutches." 



In his general survey of the biology of avian populations and 

 fecundity, Gibb (1961, p. 422) pointed out that the number of eggs 

 laid by parasitic birds could be assumed to be governed, or at least 

 limited, by the number of suitable nests in which to lay. This 

 is an overly simplified statement inasmuch as there is evidence 

 that parasitism at times may actually increase the number of nests 

 provided by certain host species. 



Thus, aside from providing evidence that a cowbird could produce 

 as many as 25 eggs in one summer, Walkinshaw's data indicates a 

 comphcating factor which may well affect the total egg production 

 of the parasite, which may well occur occasionally, and for which 

 there is no adequate method of evaluation. He found that 25 eggs 

 laid by one cowbird* were deposited chiefly in nests of one species, 

 the field sparrow. These eggs were laid in 20 nests, 15 of which 

 were deserted when the parasite laid an egg in them and removed 

 a host's egg. A good many years earlier, Chance (1940) found that 

 the European cuckoo, Cuculus canorus, could and did adjust the 



• The evidence as to the Identity of the parentage of all the eggs is as follows: all were found within a 

 limited area; all were very similar in coloration, the length of 11 similarly colored eggs had significantly 

 less variability than the length of 22 not similarly colored eggs; and no 2 of these eggs were laid on the same 

 day. 



630590—63 3 



