The long-term mark-recapture study in a small Alpine population of barn swallows revealed clear demographic patterns over a 700 m elevational gradient. First, annual apparent survival of adult breeding birds decreased with increasing elevation towards the upper range limit. Second, breeding dispersal probability of adult females, but not males increased strongly towards the upper range limit. And third, adult and juvenile barn swallows at high elevations dispersed to farms situated at lower elevations than expected by chance. By considering more than one demographic parameter at the elevational range limit 4,29, we show for a highly mobile passerine bird that not only reproduction and survival is reduced at the upper range limit, but that also breeding dispersal probability is increased and dispersal is directed downwards. Thus, this study provides evidence for a higher turn-over rate of breeding individuals and increased spatial dynamics at the upper range limit.
Unfortunately, we lack reliable long-term data on reproductive output in our Alpine study population of barn swallows. However, in a recent study over 13 Swiss barn swallow populations including our study population we show that though fecundity is increased at high elevations, nestling survival is considerably reduced and start of breeding delayed 27. A delayed start of breeding is shown to result in a decrease in both, the annual number of successful broods and the number of fledglings in successful broods 30, and thus, in a reduced annual reproductive output 31. Moreover, since the activity of aerial insects, the main food of barn swallows, strongly depends on temperature 32, we suggest that spells of cold weather have stronger effects on the reproductive output at high elevations than in lowlands 33,34. We therefore have good evidence that barn swallows breeding at the upper range limit in the Swiss Alps experience reduced reproductive output.
As expected, within-study area dispersal probability was high for juveniles (natal dispersal) and low for adult birds (breeding dispersal), confirming that adult barn swallows are highly faithful to their breeding site 28,35−38. However, this was only the case at low elevations: within-study area dispersal probability of females strongly increased at elevations approaching the upper range limit. A likely underlying mechanism at least partly responsible for this pattern is the decline in reproductive success at high elevations shown to provoke increased dispersal probabilities of females 28. In contrast, male dispersal probability within the study area was independent of the elevation of the breeding site. These results suggest that the environmental gradient towards high elevations negatively affecting reproduction results in a spatial gradient of female breeding dispersal and in increased turn-over rates of females at the upper range limit.
Dispersal at high elevations was directed downwards. Thus, barn swallows preferably selected breeding sites at lower elevations either due to climatic or other environmental gradients changing with elevation. Since in this study all nest sites at both low and high elevations were located in the preferred cowsheds hosting cattle 37,39, small-scale quality of nest sites can be excluded as a reason for the observed pattern. Settlement decisions towards low elevation might be affected by an increased availability of patches with high density of aerial insects 32,40,41 32,40,41 or by the prolonged daily and seasonal duration of high insect activity due to temperature gradients 32. We suppose that these nest site preferences are not only the reason for directed downwards dispersal, but also prevent settlements at farm buildings with cattle at even higher elevations. The preference for breeding sites at lower elevations suggests that immigration of juvenile birds into the study area first occurs at low elevation until a critical breeding density is reached. Later arriving individuals, often individuals of lower quality 37, then start to select less preferred breeding sites at elevations over 1000 m 42.
At low elevations, apparent survival showed the well-known sex- and age-specific patterns of small passerines in continuous habitats. While apparent survival of juveniles (i.e. recruitment) was considerably lower than that of adults also due to reduced first-year survival and higher rates of natal than breeding dispersal 43–45, males showed higher apparent survival than females 28. The latter can be explained by higher dispersal rates out of the study area by females than by males after brood loss or reduced reproductive success 28. However, at high elevations, apparent survival of adult breeding birds declined for individuals of both sexes. This pattern can arise due to either increased breeding dispersal out of the study area or reduced true survival at high elevations.
The increased within-study area dispersal rates of females at high elevations suggest that part of the female decline in apparent survival is due to increased dispersal out of the study area. However, the elevation-independent breeding dispersal probability of males does not fit to this explanation for the male decline in apparent survival and higher male dispersal rate outside of the study area but not within the study area seems unlikely. This suggests higher mortality at high elevations. A decline in true survival in both sexes could be due to higher reproductive efforts at higher elevations 27 potentially bearing higher reproductive costs, or because low quality individuals that were outcompeted in the lowlands settle at high elevation. Consistently, delayed start of breeding is shown to be associated with lower annual survival 30. The sex specific difference then might be due to the fact that males arrive earlier at breeding sites 37,38 and therefore are more prone to adverse weather conditions in early spring 46–48.
The demographic gradients in combination with the downwards directed dispersal shown in this study revealed that the population covering an elevational gradient of 700 m shows characteristics of source-sink dynamics resulting in a dispersal-extended upper range limit 18,49. Similar to source-sink dynamics between distinct populations or patches 29,49,50, dispersal allows the section of the population at the range limit to persist although it could not persist in the absence of dispersal. The dispersing and dead breeding birds at high elevations must be replaced to maintain population size at the upper range limit. Since recruitment rates of juveniles remained unchanged and low, only immigration can maintain the number of breeding pairs at high elevations. This is also the case in study areas of continuous barn swallow populations at low elevations 28,30,45. However, immigration at the upper range limit in this study must be considerably higher than at low elevations, but immigrating birds may come from further away.
Increased immigration at high elevations can have several consequences. First, the location of the range limit does not only depend on the environmental gradients, but also on factors affecting the immigration rate to high elevations, i.e. density-dependent effects at low elevations 17,18. Thus, in years after low reproductive output or annual survival i.e. in years with growth rates λ <= 1 at low elevations, we expect low numbers of immigrants to high elevations. After several years of such conditions, we predict a descending upper range limit. In contrast, several years of λ > 1 at low elevations might result in a rise of the upper range limit extending the limit even more upwards to elevations with low nest site preference and low reproductive output. Second, the within-population elevational source-sink dynamics is likely to result in spatial structuring of the population by sorting individuals with different traits to different elevations: late arriving immigrants are more likely to end up at high elevations than early arriving immigrants. As immigrants are predominantly first-year breeders 37, we expect an altered age-structure with higher proportion of first-year breeders at high than low elevations. Moreover, late arriving individuals are often of low quality or in bad body condition 37,46,51. As first-year breeders and individuals of low quality and body condition show reduced reproductive success and survival 37 the accumulation of first-year breeders and individuals of low quality at high elevations will further reinforce the demographic gradients towards the upper range limit. Thus, environmental gradients at mountain slopes in combination with within-population source-sink effects leading to spatial structuring can result in steep gradients of demographic rates.
In conclusion, this study provides evidence that the formation of the upper range limit of barns swallows is based on two mechanisms: preference for low-elevation breeding sites and the immigration to high elevations associated with source-sink effects. We therefore suggest that within-population elevational range shifts of barn swallows and other mobile vertebrates can occur due to factors affecting both habitat selection and immigration to high elevations. The occurrence and speed of the expected shift of the upper range limit depends not only on the improvement of high-elevation habitats due to climate change (i.e. changes affecting environmental gradients 21,52), but also on the effects of environmental changes (climate and land-use change) on reproduction and survival of the population sections living at low elevations. One of the reasons for the high variation in shift directions and the smaller upwards shifts than expected from regional increase in temperatures in Alpine bird species 20,23,53 might therefore be the declining population sizes of many bird species at lower elevations.