they can hear the calls' echoes because, via the Doppler effect, the frequency of the reflected sounds is shifted ever upward as the bats fly toward their prey.
(they; can hear; the calls' echoes; because, via the Doppler effect, the frequency of the reflected sounds is shifted ever upward) (the frequency of the reflected sounds; is shifted ever; upward) (the bats; fly; toward their prey)
they the frequency of the reflected sounds the bats
can hear is shifted ever fly
the calls' echoes because, via the Doppler effect, the frequency of the reflected sounds is shifted ever upward
upward
toward their prey