32 NATURE STUDY IX SCHOOLS, 



These hatch in from eleven to thirteen clays. The young remain in the nest 

 about two weeks before leaving it, nearly fully fledged, thus we have about 

 six weeks between the laying of the first foundation of the home, and its final 

 abandonment by the young. 



My question in regard to the robin is ; why do they put mud in their 

 nests ? 



THE BLUEBIRD. 



Our common bluebird is remarkable in several ways. It is usually 

 placed by ornithologists in the great thrush family, but personally I consider 

 that it should be separated from this family, and placed in one of its own, 

 but grouped with an old word form, the Saxicolidae, or rock inhabiters. 



My reasons for doing this are, first, that our bluebird and allied species 

 differ in form and structure from the thrushes. The wings are longer in 

 proportion to the body, the bill is proportionately shorter, the feet differently 

 formed, the tarsus being more flattened than that of the thrushes, and the 

 internal structure is different; the, sternum of the bluebird is proportionately 

 wider. 



Second, as a group, the rock inhabiters have different habits from the 

 thrushes. 



Third, probably the most important reason of all, yet one which will not 

 be so readily appreciated by ornithologists, who as a class are, I am sorry 

 to say, not apt to give such matters sufficient thought, is, that the lluebirds 

 belong to a far more ancient race 'than the thrushes. My reasons for this 

 statement are based upon the following facts. Species in this group are 

 not numerous, nor do they appear to be increasing. For example, we 

 have three species only of bluebirds in America, and two so-called sub- 

 species. In form, these three species are very similar, the differences be- 

 tween them being about wholly those due to color, or even shades of color. 

 Individuals of the species do not appear to be very abundant anywhere in 

 comparisons to members of the thrush family. New species and forms do 

 not appear to be coming into existence, nor is there any great range of 

 individual variation among the species. In short, the bluebirds are no 

 longer a plastic race. 



It is now a quite well established fact that species and groups among 

 animals grow old and die, just as individuals do. That is, they have 

 their seasons of youth, vigorous middle life and pass into the d crepitude 

 of age. In youth, both species and forms often hav2 to struggle for ex- 

 istence, and are then often not numerous as individuals, nor do they pro- 

 duce any species or forms, but this youth is indicated by a considerable 

 amount of individual variation. In vigorous middle life species are often 



