84 INTRODUCTION. 



inconstant character, and in but few cases can it be de- 

 pended upon. This has been used as a distinguishing 

 mark in separating some of the smaller Flycatchers. I 

 have tested it, and found it to be valueless, as there is no 

 rule relative to age or sex by which this is governed. 



Again, intensity and paleness of color have been almost 

 unanimously considered of specific value. Indeed, some 

 species have been formed wholly upon this peculiarity ! 

 (Witness Turdus Alicice.) Spots and bars on the wings, 

 arid streaks on the rump, are characters changing with 

 age and season, and should never be depended upon. 



Another thing is the difference in size; any one who 

 has collected, and carefully measured, birds of one species 

 from one locality, in any numbers, will at once be con- 

 vinced of the absurdity of paying any attention to this 

 particular in determining specific characters. As perti- 

 nent, I wish here to announce a somewhat surprising dis- 

 covery that I have made. After a careful measurement 

 of over three thousand specimens, I have been convinced 

 of the fact, that birds for a certain period increase in size, 

 after which they gradually decrease. Whether the period of 

 decrease is limited or not during the life of the bird, I am 

 unable at present to state. The period of increase may also 

 be variable, both specifically and individually, which yet re- 

 mains to be proven. Both the increase and decrease are 

 proportional ; feet, bill, wings, and body alike keep equal 

 pace. This rule is not without its exceptions, but in the 

 majority of cases it is the rule, and I offer it to my fellow- 

 laborers to prove and use in their ornithological investi- 

 gations. 



Besides those named, there are other distinctions used 

 in determining species, that in some cases are inconstant. 

 I think it advisable always, before attempting to sepa- 

 rate a supposed species from one closely allied, to procure 

 a sufficiently large number of specimens, and carefully 



