76 LIFE : OUTLINES OF GENERAL BIOLOGY 



which may, of course', lx» supplemented by visual or by some other 

 sensory acuteness. In any case, bird-migration still bristles with 

 brain-stretching problems. 



NOTE ON THE STIMULUS TO MIGRATION. If we grant 



that the migration of birds is an old-established custom, correlated 

 with or t-nrrgisten tl along with constitutional rhythms in the internal 

 economy of tlu- Inxly. and if we set aside the j)roblem of its origin, 

 an I'stimate of the advantages that it confers, and the puzzle of way- 

 hnding. there remains a question that many of us have asked: 

 What are the s^'asonally recurring factors that pull the trigger? 

 Thtre are reasons for discounting food shortage, as also the changes 

 in tcmjHraturc and pressure, for a state of readiness to migrate is 

 oftrn manifrst Infore the three factors alluded to liave Ix'come well- 

 marked. In a recent study by Mr. William Rowan, emphasis is laid 

 on the changes in the length of the day. The stimulus may be due 

 to the shortening daylight in autumn and the lengthening daylight 

 in sj)ring. To the objection that northern birds wintering in the 

 Southern Hemisj)here are subject. Ixfore their nortliward flight, to 

 shortening instead of to lengthening daylight, and that those 

 wintering on the lC(juator are not subject to any stimulus of this 

 kind, it may Ix' answered that the "photoixriodism" may operate 

 not at jxirticular moments, but rather in securing the regularity of 

 the bird's physiological cycle. 



Hut Mr. Rowan has not only argued, he has exix'rimented. He 

 capturi'd some j uncos (finches) in Alberta in the course of their 

 southward migration, and kept them in two outdoor aviaries, one 

 of wiiich was efjuipix-d with two 50-watt electric bulbs (of ordinary 

 glass), while the other was left to natural illumination. In both 

 cas<'S the birds were well fed, and were found to thrive in spite of 

 the s<*verity of a winter normally une\|xrienced. Hut the elitect of 

 artificially hngthening the illumination in the one aviary, while it 

 was naturally decreasing in the other, was to bring about a pre- 

 mature rt -activation of the reproductive organs. The result of this, 

 when the birds were lilx-rated, was interesting, though the data 

 are still too im|xrfect to allow of quite secure conclusions. The birds 

 kept in the natural aviary, with their gonads at the normal winter 

 mininunn. did not migrate. They remained in the vicinity and were 

 usually recaught. Their thwarted migratory instinct had passed 

 intf) alx van(c. a frecjuent phenomenon in animal behaviour But 

 when the birds from the illumined aviary, with their gonads in a 

 state more a|)propriate to spring, were lilxTated at the same time 

 as the others, namely, in mid-winter, they di.sappeared. The migra- 

 tory acti\ity had apparently Ix'en re-activated at an ina])propriate 

 season lx'caus<> the artificial light conditions had effected an accele- 

 ration of th<« normal jihysiological cycle. 



