We described the patterns and driving factors of vertical migration for resident birds in Mt. Gongga through four surveys in two years. Consistent with previous studies in Europe and North America [12, 15, 16, 23, 27], our results show that altitudinal migration is common in resident birds, and seasonal change in temperature, food availability and fecundity may be the main driving factors for the altitude migration. Most resident birds migrate downhill after breeding season at high altitudes, while some species migrate upward and downhill in non-breeding season. Migration not only exists in migratory birds, but also in resident birds. Under the influence of climate change and human activities, montane species are facing unique challenges such as habitat fragmentation.
We found that compared to insectivores, omnivores have greater range of altitudinal migrations and exhibit multiple shifting patterns in Mt. Gongga (Table 2). Insectivores in our study did not perform uphill movement during the breeding season and tended to shift their distribution boundaries downhill during non-breeding season, which is probably correlated with the distributions of invertebrates [16, 23]. In mountainous areas, invertebrate abundance normally decreases from birds’ breeding to non-breeding season, [58–60], and this decline is particularly obvious at high altitudes. In breeding season the invertebrate resources at the altitude inhabited by insectivores are sufficient to meet their needs, but harsh climates and reduced food supplies over non-breeding season may drive them to move downhill [19, 23, 27]. The demand for various food types at different periods in a year may also drive altitudinal movements. A more diverse diet spectrum allows omnivores to use habitats at different altitudes and thus perform altitudinal migrations to obtain food resources, thus presenting varying migration patterns.
Our results show that the weakly territorial species had more diverse migration patterns, and strong territoriality species tended to move uphill (Table 2). Researchers generally accepted the idea that strong territorial species would expend more time defending their territories [25, 30], so that they are less likely to go through rapid distribution changes but may be constrained to move upwards by interactions with closely related species [61, 62], whereas species with weak territoriality would continuously move to seek access to food resources [25, 63]. Changes in temperature have great impact on bird’s behaviour, as we found that species with a narrower temprange (< 30°) migrate predominantly uphill during the breeding season, and species with a wider temprange (> 30°) and lower annualtemp (< 10°) tend to migrate downhill during the non-breeding season (Table 2). This is consistent with the results of another research carried out in Taiwan . Species with wider range of temperature tolerance and cold endurance will have greater opportunities to broaden their ecological niches in competition as predicted. Meanwhile, relying on the rich food resource in Gongga and better cold endurance allow them to breed at higher altitudes, to avoid the more intense resource competitions at lower altitudes . In addition, nearly all 8 species with wider temprange (> 30°) and lower annualtemp (< 10°) are weakly territorial species except Trochalopteron elliotii, verifing our conjecture that species with weak territoriality will put more effort in foraging resources. If temperature changes intensify interspecific competition, those species would be forced to migrate at altitudes for an 'exploration', and adjust to colder and harsher conditions for breeding season.
The relationship between nest structure and nest predation is controversial , and the nest predation risk is not necessarily higher at lower elevations . Our results show that migratory movements are associated with predation risk, with scrub-nesting birds migrating downslope and moving more widely on average than canopy-nesting birds, which we suggest is related to the type of vegetation in different seasons (Table 2,3). Nest height often determines the safety of a nest as a strategy to avoid nest predation or maladaptation [26, 31]. And the nest predation rates could generally be reduced by the increase of height-related coverage and complexity of vegetation surrounding the nest. (Holway 1991; Martin 1993; Arriero et al. 2006; Hollander et al. 2015; Buehler et al. 2017; Bellamy et al. 2018; Haohui Guan, 2018). In non-breeding season when dead scrub increases the predation risk and the negative effects of extreme conditions for both exposed nests and individuals , choosing to move downhill to find more suitable habitat increases the chances of survival. Canopy-nesting birds rarely conduct elevational migration, which is probably because their nest sites are higher and less exposed in the montane habitats of mixed coniferous or deciduous forests, thus reducing the predation risk.
When considering the absolute speed of shifts (ignoring shift direction) as well as expansions in species’ upper limits, the most important predictor was HWI [28, 30]. Our modelled results show a correlation between HWI and uphill movement (Table 2), which is not difficult to understand that the uphill movement consumes more energy in the mountain system. Dispersal distances increase exponentially with flight efficiency in resident birds of Mt. Gongga . Flight efficiency and migratory behaviour are strongly and consistently associated with variation in dispersal distances, which is consistent with the idea that the energetic cost of transport is a major determinant of dispersal distances . Moreover, the possibility of estimating dispersal ability based on morphology may have applications in conservation biology, as this can be instrumental in assessing species vulnerability to habitat fragmentation and climate change . However, due to our small sample size, it is impossible to make a more detailed analysis with the height and migration rate of HWI.
Body mass is not associated with seasonal shifts in altitudes, which is consistent with a recent study in Gaoligong . Most of the studies on the correlation between body mass and altitude migration were conducted in tropical and smaller altitude span regions [16, 28]. Claramunt also found that body mass was not important in explaining prenatal dispersal distances in British birds, and that the correlation between the two was low and not statistically significant [48, 67]. Second, there was no significant difference in body mass among our respondents and the relationship between transport and body mass in birds has been attributed to variation in wing shape, in which larger birds have also higher aspect ratios, rather than an intrinsic effect of body mass itself [68, 69]. Body mass has long been linked to environmental gradients by Bergmann’s rule , which states that larger-bodied species tend to be found in colder environments. However, evidence for the existence of Bergmann’s rule is variable [71–73], and it tend to be applied more to intraspecific variation in body mass or variation among closely related species, particularly along latitudinal gradients.