___1____Maybeyouthoughtthattheywerejusthappy.Afterall,youprobablysingorwhistlewhenyouarehappy.
However,theysingmostofthetimeforaverydifferentreason.____2___Somescientistsbelievethatbirdsdosingsomeofthetimejustbecausetheyarehappy.
Doyouknowwhata“territory”is?Aterritoryisanareathatananimal,usuallythemale,claims(声称)asitsown.____3___Nootherfamiliesofthesamespeciesarewelcome.Youryardandhouseareyourterritorywhereonlyyourfamilyandfriendsarewelcome.Ifastrangershouldenteryourterritoryandthreatenyou,youmightshout.Probablythiswouldbeenoughtofrightenhimaway.Ifso,youhaveactuallyscaredthestrangerawaywithouthavingtofighthim.Abirddoesthesamething.Butheexpectsanoutsideralmostanytime,especiallyatnesting(筑巢)season.Soheisscreamingallthetime,whetherhecanseeanoutsiderornot.__4__.
Birdssingloudestinthespringwhentheyaretryingtoattractamateandwarnothersnottoentertheterritoryoftheirs.Youcanseethatbirdshavealanguagealltheirown.___5__