2020托福听力练习：公路噪音掩盖鸟类求救信号（下载）

　　It may not be the most melodious bit of birdsong you'll ever encounter. [Titmouse alarm call]. But this particular call, issued by a tufted titmouse, conveys important information about the presence of potential predators. But only if other birds can hear it.

　　Unfortunately, near a highway, traffic sounds can drown out such alarm calls. Scientists have long observed a decrease in wildlife populations in habitats adjacent to major roadways. And many figured it was the noise that drives creatures to hightail it someplace more remote. But it could be more than mere annoyance that keeps animals away. The inability to hear potentially life-saving calls could also be a factor. That's according to a study in the journal Biology Letters.

　　To test that theory, Aaron Grade and Kathryn Sieving at the University of Florida recorded that titmouse alarm call, produced by a captive individual that had spotted an owl. They then played this alarm call from speakers mounted in different locations in Florida state parks. Some of the spots were noisy, along Interstate 75 or U.S. Route 441. Others were more secluded and relatively quiet.

　　The scientists then observed the reactions of cardinals to these broadcasted declarations of danger. Because cardinals and other birds often eavesdrop on the talk of titmice for news of impending peril.

　　In the quiet areas, cardinals clearly responded to the titmouse alarms. They froze in place, stopped singing and scanned the area for predators. But in the noisy places, cardinals paid the warnings no heed and continued their regular activities.

　　It's not completely clear whether the cardinals could not hear the calls or if they were simply too distracted by the traffic to respond. Either way, looks like some birds should seek the road less traveled—or travel the regions less road-ed. Whereas hungry owls should hit the highway if they're looking to sneak up on an unexpecting snack.