212 MIGRATION 



The general assumption that migration is caused by the reawaken- 

 ing of the reproductive instinct has come down from much earlier 

 times, and in more definite form has received the support of the con- 

 sensus of ornithologists during the past quarter of a century or more. 

 It was advocated by Brehm, who later turned to the scarcity of food 

 as the explanation (1896:234), and was combated by Giitke (1895), 

 as indicated in the following discussion. Chapman (1894) enunciated 

 similar views in America, and Marshall (1910) rendered them more 

 definite by turning to internal secretions for the exciting cause, while 

 Schiifer (1907) pointed out certain critical difficulties with the hypoth- 

 esis. Cahn (1925) and Bergtold (1926) regard sex hormones as the 

 primary if not sole cause; AVetmore (1926:27) also considers them 

 the prime influence, and Thomson (1926:295) assigns them the chief 

 though not exclusive role. This is essentially the attitude of Heape 

 (1931:240), as it was of Rowan in his earlier experiments (1926, 

 1929) (cf. Garner and Allard, 1920; IMarcovitch, 1924). 



Rowan's Researches (1926-1932). The chief credit for initiating 

 the experimental attack upon the causes of migration is to be ascribed 

 to Rowan, who has combined both direct and indirect methods in his 

 procedure. In the main series of experiments, this comprised the 

 artificial increase of length of day, a device to insure increased activ- 

 ity, and the liberation of experimental and control birds. On the basis 

 of the results obtained he concluded that the changes observed could 

 probably not be assigned to ultra-violet radiation. The rhythm of the 

 gonads could be readily modified by control of lighting, but compul- 

 sory exercise produced practically the same effect, and hence it was 

 suggested that the greater length of day operated through the oppor- 

 tunity for increased exercise and that the latter was the primary 

 factor in the development of the gonads. His extensive results were 

 summed up in 1931 in the two statements: "Variations in day-length 

 are assumed to be the primary external stimulus. The internal 

 stimulus is assumed to be a hormone produced by the interstitial 

 tissue of the reproductive organs." 



The most remarkable direct experiment was organized in 1930 

 with crows {Corvus brachyrhynchos Brehm), through the efforts of a 

 wide group of cooperators (Rowan, 1932). The birds liberated for the 

 purpose of determining the presence and direction of migration com- 

 prised four major groups as follows: (1) expcrimcntals receiving in- 

 creasing illumination from October 15 to November 17; (2) experi- 

 mental capons (castrated males), with exactly the same treatment; 

 (3) controls; and (4) control capons, both of which were given no 

 illumination. "Of the controls there is little to say. The high per- 



