ZOOLOGICAL REPORT. 113 



species, the evil which is done to us at certain times is largely com- 

 pensated by the destruction of insects which they accomplish at other 

 times. It is important, then, that we do not destroy these species, but 

 only divert them from the crops when they can injure them. Their de- 

 struction would permit, without counterbalance, the development of many 

 species of insects more fatal still to agriculture. The study of the alimen- 

 tary regimen has furnished me also some information which I believe useful 

 in comprehending the reunions, the separations and periodical emigrations 

 which are obseived so commonly among birds. If there are some animals 

 who easily accommodate themselves with food varied to the change of 

 the seasons, others desire exclusively such food as nature offers to them 

 only periodicnUy in the same country, or in a manner continues to them 

 under different climates. Many of the mammifers, whose food is of this 

 kind, sleep and remain torpid during the unfavorable season. This curious 

 phenomenon of hibernation does not exist in respect to birds, and it seems 

 to be replaced by the act of periodical emigrations much less common 

 among mammifers. I believe, in a word, that outside of the habits directly 

 instigated by the necessities of reproduction, we arrive at the demonstra- 

 tion, that birds reunite, disperse, emigrate and travel conformably to the 

 necessities of their alimentary regimen. Without citing in detail fiicts, of 

 which tlie specimens I have collected furnish proof, I feel myself authorized 

 to point out this general cause, whose demonstration can be given in a 

 longer treatise, or furnished to tiie members of a committee who should 

 wish to render the account of it themselves. There exists always a curious 

 harinony between these passages of diverse species of birds in the same 

 country; inastnuch as they succeed one another in order to subsist accord- 

 ing to the seasons of the resources which are adapted to their wants. 

 Thus, everyone knows how it is with birds, of which numerous bands 

 return in the spring into our climate in order to give themselves to the 

 reproduction of the species. It is a somewhat precise study to perceive 

 clearly, how, agreeably to the successive development of certain vegeta- 

 bles or of the hatching of certain insects, the different species of birds 

 emigrate in order to enjoy in turn food produced for them and their young. 

 The birds of the spring-time arrive from the East and the South whose 

 more gentle winter has nourished them ; but when the cold is renewed in 

 our country, if they return towards the warm climate, it is only to give 

 place to other emigiants from the polar regions. The waders and swim- 

 mers of the arctic zone have incubated in the North daring the summer, 

 and emigrate during the winter into our country to find the food which 

 the ice of the polar regions cannot offer them, 'i'he examination of tiie 

 stomachs of certain birds has made me acquainted besides with a fact 

 worthy of being noticed. At certain epochs, there are some species which 

 are submitted to prolonged fastings; their stoniiohs contain tlien no ali- 

 mentary matter, but habitually foreign bodies which are not digested. 

 More frequently there are feathers of tlie bird itself, forming a voluminous 

 ball, which keeps the stomach dilated. The d fferont species of the genus 

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