Whathasjustbeensaidappliestofishesaswellastootheranimals；andthusinsuchofthemasarecarnivorous，andmadeforbiting，themouthhasawidegape；whereasintherestitissmall，beingplacedattheextremityofataperingsnout。Forthisformissuitedfortheirpurposes，whiletheotherwouldbeuseless。
Inbirdsthemouthconsistsofwhatiscalledthebeak，whichinthemisasubstituteforlipsandteeth。Thisbeakpresentsvariationsinharmonywiththefunctionsandprotectivepurposeswhichitserves。
ThusinthosebirdsthatarecalledCrooked-claweditisinvariablyhooked，inasmuchasthesebirdsarecarnivorous，andeatnokindofvegetablefoodwhatsoever。Forthisformrendersitserviceabletotheminobtainingthemasteryovertheirprey，andisbettersuitedfordeedsofviolencethananyother。Moreover，astheirweaponsofoffenceconsistofthisbeakandoftheirclaws，theselatteralsoaremorecrookedinthemthaninthegeneralityofbirds。Similarlyineachotherkindofbirdthebeakissuitedtothemodeoflife。
Thus，inwoodpeckersitishardandstrong，asalsoincrowsandbirdsofcrowlikehabit，whileinthesmallerbirdsitisdelicate，soastobeofuseincollectingseedsandpickingupminuteanimals。Insuchbirds，again，aseatherbage，andsuchasliveaboutmarshes-those，forexample，thatswimandhavewebbedfeet-thebillisbroad，oradaptedinsomeotherwaytothemodeoflife。Forabroadbillenablesabirdtodigintothegroundwithease，justas，amongquadrupeds，doesthebroadsnoutofthepig，ananimalwhich，likethebirdsinquestion，livesonroots。Moreover，intheseroot-eatingbirdsandinsomeothersoflikehabitsoflife，thetipsofthebillendinhardpoints，whichgivesthemadditionalfacilityindealingwithherbaceousfood。
Theseveralpartswhicharesetontheheadhavenow，prettynearlyall，beenconsidered。Inman，however，thepartwhichliesbetweentheheadandtheneckiscalledtheface，thisname，（prosopon）being，itwouldseem，derivedfromthefunctionofthepart。Forasmanistheonlyanimalthatstandserect，heisalsotheonlyonethatlooksdirectlyinfront（proso）andtheonlyonewhosevoiceisemittedinthatdirection。