452 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



As is well knowTi, the continental shelf bordering the coast of 

 Peru is very nari'ow and the sea bottom decliaes from the shore so 

 precipitately as to leave few bays or harbors and no really large is- 

 lands. The cold Humboldt current, with its steadily and visibly 

 flowing waters, is thus brought so close to the mainland shore as to 

 preclude the occurrence of warm seas or the development of typi- 

 cally "tropical" conditions. 



The temperatures of the surface waters have been treated in a sepa- 

 rate paper.^ It may be sufficient here to say that surface tempera- 

 tures as low as 17° C. (63° F.) were noted in the bay of Paita (5° S.) 

 in midsummer, while a temperature of 16° prevailed at Mollendo 

 (17° S.) in midwinter. Excluding observations in protected waters, 

 the observed range of temperature in any locality did not exceed 3.4° 

 C. and was usually 1° to 1.5° C. The air temperatures exhibit some 

 variation, but, about the islands, at least, they are so generally gov- 

 erned by the water as to make the seasonal changes relatively small. 

 That such uniformity of temperature conditions may have a marked 

 influence upon the breeding habits and breeding seasons of the bhds 

 is to be inferred, and in the following pages, in reference to several 

 species of birds, there will be noted a tendency to prolong the breed- 

 ing season, if not to extend it over the entire year. 



Since breezes from a cool sea must pass over lands that are warmed 

 under a tropical sun, no opportunity for precipitation occurs and 

 neither rainy seasons nor occasional rains may prevail. It is true 

 that in the very northern portion of Peru heavy rains may occur at 

 intervals of years, and at no great distance from the coast, and that 

 the mainland coast generally becomes sufficiently cooled in winter to 

 suffer a settled condition of fog and mist, or garua; but the several 

 islands a little offshore are, absolutely or practically, free from rain- 

 fall, and conditions of atmospheric moisture sufficient to permit of 

 the growth of ordinary vegetation can be found only upon the high 

 peaks which are ^vrapped with clouds. The most interesting and 

 economically significant consequence of the arid climate is that the 

 nitrogen of the guano deposited by the birds can not become con- 

 verted into ammonia to be lost by evaporation, but is permanently 

 preserved in a form readily available for the purposes of agriculture. 

 The absence of rains and storms must have no little effect, too, upon 

 the abundance of available food and upon the successful propagation 

 of the birds. 



Notwithstanding the general correctness of the preceding state- 

 ments, a distinct difference may be noted in the atmospheric condi- 

 tions prevaihng over the northern islands. Nearer the equator and 

 the upper limit of the Humboldt current the sea breeze comes some- 



> Coker, U. E. Ocean tamperatures off the coast of Peru. The Geographical Ee\iew, vol.5, New York, 

 February, 1918, pp. 127-135. 



