use of this wintering area. Rhizomes of shoalgrass ( Halodule beaudettei ) ac- 

 counted for 71% of the diet of 19 Redheads; three molluscs, Anachis avara , 

 Neritina virginea , and Cerithidea pliculosa were the animal foods eaten most 

 often, but they comprised only 9.5% of the diet. 



IMPORTANT BIOLOGICAL PARAMETERS 



Egg Laying The Redhead is noted for the practice of laying eggs in nests 

 of other birds, either of its own or of other species (Weller 1959). In Iowa 

 in 1938, the peak of the nest-building and egg-laying period was 19-25 June, 

 when one-third of the observed nests were constructed (Low 1940). In Alberta, 

 the mean date of nest initiation over a five-year study period was between 7 

 and 26 May (Keith 1961). Nests were begun in late April or early May in Mon- 

 tana, with first nest establishment completed by 10 June (Lokemoen 1966). 



Mean Clutch Size The parasitic or semiparasitic habits of the Redhead make 

 determination of clutch size somewhat difficult (Weller 1959). Laying by sever- 

 al females in one nest has resulted in "clutches" of as many as 87 eggs (Weller 

 1959). Various studies (cited In Bellrose 1976 and Palmer 1976b) have reported 

 average clutch sizes varying from 8.9 to 13.5 eggs, and Bellrose reported an 

 overall average of 11.1. Weller (1959) reported that unparasitized clutches 

 contained from 5 to 9 eggs in studies conducted at Delta Marsh, Manitoba, and 

 Knudtson Marsh, Utah. The average clutch size was 7.4 (n = 17). Weller also 

 reported that parasitic females laid an average of 10.8 eggs. Palmer (1976b) 

 believed that the true clutch size was 9 eggs in most instances. 



Incubation Period Incubation periods in studies cited by Palmer (1976b) 

 and Bellrose (1976) range from 23-29 days. Palmer (1976b) stated that the in- 

 cubation period is usually 24 days. 



Hatching Success In a two-year study in western Montana, Lokemoen (1966) 

 found that eggs hatched in 15.2% of 138 clutches. However, only 9.9% of the 

 eggs in these clutches hatched. Hatching success was greater than nest success 

 because some nests lost eggs by interference from other birds or contained late 

 eggs laid by parasites. In a summary of studies, Bellrose (1976) noted overall 

 that some eggs hatched in 52% of the nests observed, but that an unusually large 

 number of unhatched eggs were left in successful nests. 



Fledging Success No precise data are available. Bellrose (1976) calcul- 

 ated that the production of young per hen may vary from 1.3 to 2.7. 



Age at Fledging Palmer (1976b) reported that most young fly at 60-65 days 

 and noted that a range of 56-84 days had been reported. 



Age at First Breeding Female Redheads may breed as yearlings, but the pro- 

 portion that do so is unknown (Bellrose 1976). 



Mortality of Eggs and Young Many nests in which eggs fail to hatch were 

 deserted because of the intrusion of parasitic females. This is prevalent 

 enough that it led Bellrose (1976) to remark "The redhead appears to be its own 

 worst enemy". Mammalian (skunks and racoons) and avian (crows, magpies, and 



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