1.SS4. 1 RtCKNi'.l.l. Oil Ihf Siiiiiiiiir of Birds. 6^ 



tiiuioiis sin<;iii<4' ihroii^h the suninu'V of tlic Red-cved \'ii'co. tlic 

 Soiii^ Sparrow, the Baltimoic Oriole, the Ph(rl)e Bird, aiul tlie 

 (jreat-crested Flycatcher. In all (jt" tliese, perhaps, the silent 

 period actually occurs with the species as a whole in certain years 

 when conditions imcongenial to song prevail, but the records of 

 .several seasons taken collectively disguise any such break in the 

 singing times which may have occurred. 



We must now consider these species which, without exhibiting 

 any marked seasonal change of plumage, are vet silent during a 

 more or less protracted stay after the close of the first song-period. 



Let us first take up some matters preliminarv to the considera- 

 tion of this class. 



It is probable that extreme fatness engenders a constitutional 

 predisposition towards silence. The majorit}' of birds arriving 

 on the spring migration possess little or no obvious adipose tissue. 

 I have likewise found this to be the case with birds that are in full 

 song in midsummer. If we examine a large number of spring 

 birds some exceptions will be found, though there will be com- 

 paratively few, and very few that can be considered extreme ex- 

 ceptions. In the fall, however, the contrary will be found to be 

 the case. At this season the majority of birds are more or less 

 fat and many excessively so, fat often beginning to accumulate 

 before the completion of the moult. If, then, ex-cessive fatness 

 tends to induce silence, we have in this fact a reason for the 

 absence of a second song-period with many species : singing 

 is first checked by the moult, and the adipose condition directly 

 succeeding suppresses all inclination to resume it. In many birds 

 which remain with us long after the second moult, without 

 decided change of plumage, vet with no second period of 

 song, we find illustrations of this sequence of physiological con- 

 ditions. 



A striking instance, afibrded by the Red-e^■ed Vireo. may be 

 cited. This Vireo is one of our most persistent songsters, and 

 forms one of the exceptions to the rule that birds are not gen- 

 erally in song when the moult is in active progress. It is in full 

 moult in August, in which luonth a silent period, although indi- 

 cated, is obscured by individual variation in the time of discontin- 

 uance and resumption of song. In the month of August this 

 species may be found in an active stage of moult. Though its 

 vocal vigor is at low ebb during this time, especially in sultry sea- 



