© Marcel Burkhardt
How do upper range limits form?
The environmental conditions in a species’ range change with elevation above sea level. They influence immigration, emigration, reproductive success and survival rates, and therefore also the upper limit of distribution.
As part of a study on Barn Swallows, volunteers examined a breeding population in the Prättigau region (canton of Grisons) over a 700 m elevational gradient. In a period of 14 years, 1,337 nestlings and 194 adults were ringed on 63 farms. Ring recoveries allowed us to estimate immigration, emigration and survival rate, all important factors in population dynamics. More than 90 % of recaptured juveniles did not return to breed on their natal farm, but dispersed to a different one, whereas only 17 % of adult birds changed breeding site from one year to the next. Both juveniles and adults from the highest-lying farms showed a tendency to disperse to sites at below-average elevation. The birds dispersing to high-lying farms were almost all juveniles.
How can these results be explained? Barn Swallows feed almost exclusively on flying insects. The activity of the insects depends on temperature and is often reduced at high elevation due to cold spells. The lower reproductive output makes high-lying breeding sites less attractive. So in spring, adults and juveniles select low-lying sites first, until all the best sites are occupied. Late arrivals or weaker birds, mainly juveniles, are forced to higher locations with less competition but more difficult conditions.
This means that the upper distribution limit can be expected to rise after years with high reproductive success due to large numbers of returning juveniles, and to descend after years when breeding success was poor.
Grüebler, M. U., J. von Hirschheydt & F. Korner- Nievergelt (2021): High turnover rates at the upper range limit and elevational sourcesink dynamics in a widespread songbird. Scientific Reports 11: 18470. doi.org/10.1038/s41598-021-98100-x.