（原文網頁可以聽錄音）
很多小型鳥看到猛禽都會叫警戒聲，所以松鼠也學會要聽鳥叫
文章中提到的song bird有Black-capped Chickadee, Red-breasted Nuthatch, Steller's Jay, Tufted Titmouse
研究時是用 機器猛禽（剝皮）引起注意，有一隻是pygmy owl
In 2005, Dr. Greene was an author of an article in the journal Science that demonstrated how black-capped chickadees embed information about the size of predators into these calls. When faced with a high-threat raptor perched nearby, the birds not only call more frequently, they also attach more dee’s to their call.
Chickadee的警戒聲會因猛禽的危險等級而有變化，通常越接近牠們的體型，牠們就會越常還有結尾會帶越多聲 ‘’地（dee)‘’
a large goshawk might only merit a chick-a-dee-dee from a nimble chickadee, while that little pygmy owl will elicit a chick-a-dee followed by five or even 10 or 12 additional dee syllables
例如:Pygmy Owl出現時，chickadee會叫大約 ５聲或更多的‘’地(dee)‘’
如果是 Goshawk出現，可能只會是 Chick-a-dee-dee
That sound humans hear simply as chick-a-dee actually could contain information differentiating between a Cooper’s hawk and a pygmy owl. We know birds hear this as if it’s slowed down
其中有些叫聲我們聽起來一樣，但Chickadee可以聽出不同
more than a one-quarter decline in bird abundance when the artificial road noise was turned on; some avoided the area almost entirely.
文章最後提到，很多鳥沒辦法在太吵的地方聽到警戒聲，所以路邊的鳥要花比樹林裡的鳥更多時間聽，因此吃飯時間也比較短
有些鳥聽到人造聲會選擇離開那個區域
