MORTALITY AMONG MIGRANT BIRDS 123 



age. Some are restricted to one annual family, 

 many species rear two, and a few of the more com- 

 mon ones three broods a season. In spite of their 

 apparent prolificness, these birds do not increase 

 appreciably in numbers unless there is some change 

 in conditions that affect them. The family in the 

 song sparrow, for example, which begins the season 

 with two individuals, by August, if there has been 

 no summer mortality, with an average family of 

 four to the brood has increased to ten or to four- 

 teen, depending upon whether two or three families 

 have been produced. In other words, unless deaths 

 begin early, the summer season closes with the origi- 

 nal pair of adults augmented by from eight to twelve 

 young. This increase represents the margin of safety 

 in this particular species, and illustrates the tremen- 

 dous annual mortality among song sparrows as a 

 group. Ducks rear a single brood, with six to twelve 

 or more young, but maintain themselves in spite of 

 the vicissitudes of travel and the slaughter of the 

 hunting season. Most shore-birds, with four young 

 or less as the annual addition, are unable to with- 

 stand destruction by man in addition to the other 

 perils that they encounter, and have to be carefully 

 protected to avoid actual extermination. The slow 

 production to offset death in the passenger pigeon, 

 which laid but a single egg, was unquestionably the 

 cause of extinction of this species, as excessive hunt- 



