Because the vast plateau at such altitudes absorbs a huge amount of solar radiation, the atmospheric layer above it in summer is much warmer than air at similar elevations over lower land or the oceans. (scientificamerican.com)
The songs also differed from those at similar elevations on a nearby peak. (sciencemag.org)
If there are any differences in song structure among these four locations, the acoustic adaptation hypothesis predicts that song should differ between but not within elevations, because the composition of the mixed - conifer forests in Sierra Nevada are fairly standard and do not vary significantly between sites at similar elevations [30] and should therefore shape the song in similar ways in order to optimize signal transmission. (rsos.royalsocietypublishing.org)