LINKED PAPER Temporal and spatial variation in reproductive benefits in a partial migrant. Witczak, S., Kormann, U.G., Catitti, B., Scherler, P., van Bergen, V., Gruebler, M.U. 2024. Ecology. DOI: 10.1002/ecy.4451. VIEW
How many offspring an animal produces determines their contribution to the population gene pool, and how many offspring are produced on average by the animals of a population is a key component contributing to population growth and size. Certain behaviours can improve the reproductive success of animals, though those same behaviours may also have costs, such as reduced chances of survival. It is not uncommon for different behavioural strategies to develop that maximize different aspects of an animal’s overall success. However, for multiple strategies to occur in a population, they must have approximately equal overall success in the long run.
In partially migrant animal populations, we see the occurrence of two different over-wintering strategies within the same population: year-round residence in the breeding area and migration. In many such populations, there has been a recent increase in the proportion of residents, which raises the question of what might be driving an increase in the relative success of residents compared to migrants. It is often suggested that residents maximize their reproductive output by increasing their likelihood of acquiring a (high quality) territory come spring, while migrants may have higher survival chances having overwintered in a milder region.
Figure 1. A Red Kite Milvus milvus in flight © Patrick Scherler.
The Red Kite is a medium-sized European raptor species exhibiting partial migratory behaviour in many populations. Since 2015, the Swiss Ornithological Institute has been studying the ecology of Red Kites in Switzerland in a large, ongoing project. Residence was first observed in the Swiss population in the 1960’s and has become increasingly prevalent ever since, but the exact mechanisms behind this increase are unknown. Here we aimed to investigate whether differences in reproductive success between the two strategies could help explain the growing number of residents in the population, and what might be potential mechanisms behind differences in reproductive output.
We found that there were differences in the reproductive output of migrants and residents, but these were dependent on the spatial and temporal context of the breeding event. Residents had higher reproductive output than migrants, but only when nesting at low elevations. Given that most of the Swiss Red Kite breeding population breed at lower elevations, this could potentially mean that the majority of breeding Red Kites would profit more (reproductively) from using the resident strategy in Switzerland. Nevertheless, we also found that migrants had a reproductive advantage over residents in one out of four study years, irrespective of the elevation of their nest site. Given that there are evidently years when migrants can outcompete residents, this suggests that under certain conditions, e.g., particularly harsh winters in the breeding area, migration is still an important strategy for the survival of the population at large.
Figure 2. A large proportion of the Swiss Red Kite distribution will benefit more reproductively from using the resident versus migrant strategy. A map of Switzerland where red shading indicates Red Kite occurrence (Knaus et al. 2018), while overlaid yellow shading indicates areas under the elevation (m.a.s.l.) margin below which residents have a reproductive advantage over migrants.
Identifying the main environmental drivers behind the temporal and elevational differences in reproductive performance between the two migration strategies would be an important avenue of further research. This would not only further our understanding of this specific system, but also of the drivers of partial migrant population dynamics, in general. We could then use such knowledge to predict future trends in partial migration populations and their corresponding resident and migrant subpopulations using environmental projections under different climate change scenarios.
Image credit
Top right: Red Kite Milvus milvus nestlings © Valentijn van Bergen.