A new study led by the University of Utah and the Utah Division of Wildlife Resources (UDWR) has found that in dry years, rivers and streams in the American West become a lifesaving yet limited resource for a large number of bird species. The riparian (adjacent to rivers) environments witness an increased diversity that is accompanied by overcrowding, which may cause increased competition for habitat and resources, and an overall decrease in native populations of birds.
This study is based on data collected by UDWR since the early 1990s, when bird populations in riparian areas in Utah began to be monitored in order to estimate the size and health of the population.
“These are some of the most beautiful places in an already beautiful state,” said study co-author Russell Norvell, the coordinator of the Avian Conservation Program at UDWR.
“The bird life is tremendous. It can be a cacophonous sound and a little daunting if you’re trying to count them all. First you’re trying to identify individual species, then how many individuals of each species and finally their position – all while these birds are flying around. You’ve gotta be on your game, for sure, to collect these data.”
By analyzing UDWR’s bird monitoring dataset, Norvell and his colleagues documented over 31,000 birds from 148 species during a period of 15 years.
Riparian birds comprised 58 of these species and 90 were non-riparian, with the numbers and diversity being higher in the northern mountains of Utah than in the southern deserts. This proves the fundamental importance of the rivers as a stop-over, even for non-riparian species.
This multi-year data showed that in hotter and drier years there were more birds close to the rivers than usual. “This suggests that the wider landscape is unable to support migrants and so they are forced to use greener areas,” said study lead author Monte Neate-Clegg, a postdoctoral fellow at the University of California, Los Angeles.
However, the influx of non-riparian birds in Utah’s river environments can be damaging to native populations: in warmer years, population growth rates slowed for 47 percent of riparian bird species.
“If you’re a bird watcher in Utah, the prospect of more diverse birds in riparian areas might sound exciting! But it’s a sign of general stress across the landscape, and you may start seeing fewer local species,” Neate-Clegg concluded.
The study is published in the journal Animal Conservation.