The range of frequencies at which birds call in an environment varies with the quality of habitat and the ambient sounds. The acoustic adaptation hypothesis predicts that narrow bandwidths, low frequencies, and long elements and inter-element intervals should be found in habitats with complex vegetation structures (which would absorb and muffle sounds), while high frequencies, broad bandwidth, high-frequency modulations (trills), and short elements and inter-elements may be expected in open habitats, without obstructive vegetation.    Low frequency songs are optimal for obstructed, densely vegetated habitats because low frequency, slowly modulated song elements are less susceptible to signal degradation by means of reverberations off of sound-reflecting vegetation. High frequency calls with rapid modulations are optimal for open habitats because they degrade less across open space.   The acoustic adaptation hypothesis also states that song characteristics may take advantage of beneficial acoustic properties of the environment. Narrow-frequency bandwidth notes are increased in volume and length by reverberations in densely vegetated habitats.