MIGRATION AMONG SHORE-BIRDS 167 



grounds a very short time; for the spring migration 

 does not close until the end of May, while in some 

 species early migrants begin to arrive from the north 

 by the first week in July. In the case of the Wilson's 

 phalarope {Steganopus tricolor) in Utah, I have 

 definitely established that there are many that do 

 not breed each year. At the northern end of Great 

 Salt Lake this species nests in small colonies where 

 marsh conditions are suitable. During June, in 

 addition to the breeding colonies, I found large flocks 

 composed entirely of non-breeding individuals (as 

 was shewn by examination of the sexual organs in a 

 considerable number) congregated on the lake front 

 where food was abundant. On one occasion I esti- 

 mated that between 20,000 and 30,000 were gath- 

 ered about shallow pools on one stretch of alkali 

 barrens, and it was impossible to say how many 

 more were concealed in the shimmering heat haze 

 that concealed all low objects beyond a distance of 

 half a mile. Strangely enough more than ninety 

 per cent of these birds were females, which leads to 

 an assumption that there may be a preponderance 

 of that sex in the species under discussion, as mated 

 pairs at the time were on their nesting grounds in 

 nearby marshes. 



It is not improbable that some of the early mi- 

 grants which are recorded among other shore-birds 

 are also non-breeding individuals, or are perhaps 



