Temporal and spatial variation in reproductive benefits in a partial migrant

Abstract

In partial migrant systems, where residents and migrants coexist within a population, residents are commonly predicted to gain a reproductive advantage over migrants through priority access to high-quality territories and an earlier breeding start. Annual variation in reproductive benefits has been suggested to be important for the coexistence of both strategies in a population, as differences in wintering conditions experienced by the two strategies may result in a periodic reproductive advantage for migrants. However, the importance of spatial environmental variation for reproductive output in partially migrant populations remains largely unexplored. We investigated variation in the reproductive output of migrants and residents in a population of Swiss red kites (Milvus milvus) both temporally, across and within years, and spatially, along an elevational gradient. We gathered 4 years of reproductive data combined with 183 GPS-derived full annual cycles from individuals breeding in the Swiss Alpine foothills. At low, but not high, elevations, residents produced more fledglings than migrants. We also found evidence for annual variation in the reproductive advantage of the two strategies. Furthermore, while reproductive output did decline with a later breeding start, there was no difference in the start of breeding between the two migration strategies. The results of this study suggest that differences in reproductive output between migrants and residents in partial migrant populations can vary both due to the use of spatially distinct overwintering grounds and because the strategies are differently affected by spatial variables in the breeding area, such as elevation. The study emphasizes that spatial and temporal variation in reproductive benefits must be considered when predicting how migratory species will respond to future environmental change.

Keywords: Milvus milvus; competition; elevational gradient; fledgling number; migration strategy; red kite; relative reproductive output.

FIGURE 1

Three hypotheses explaining the expected relative reproductive output between migrants (M) and residents (R). (1) Breeding habitat hypothesis: Residents acquire higher quality territories and show higher reproductive output across elevations than migrants. Harsh conditions at high elevations result in lower reproductive output in both strategies. (2) Timing of breeding hypothesis: Residents start breeding earlier than migrants and, therefore, show higher reproductive output. This effect disappears at high elevations, where conditions are favorable later. Migrants arrive later and breed at similar times across elevations, exhibiting similar reproductive output. (3) Winter carry‐over hypothesis: Harsh winter conditions at high elevations result in lower subsequent reproductive output in residents. Migrants overwinter in milder conditions, independent of nest‐site elevation, and experience higher, elevation‐independent reproductive output.

FIGURE 2

Study area in western Switzerland, spanning 576 km². An elevational gradient of 1704 m (482–2186 m above sea level [asl]) runs along a roughly north–south axis. Nests of residents and migrants are indicated by white and blue points, respectively. Extent: 7.12, 46.69: 7.44, 46.90 EPSG:4326 (elevation map: Federal Office of Topography, swisstopo; Switzerland map: ©Vemaps.com). Switzerland‐wide elevational breeding population distribution (bottom left; Knaus et al., 2018)—note that this histogram is not corrected for the actual area available at different elevations and, therefore, cannot be interpreted as red kites avoiding low‐elevation areas. See Appendix S1: Figure S1 for the elevational distribution of migrants and residents within our sample population.

FIGURE 3

Interactive effect of migration strategy and nest‐site elevation, and migration strategy and year on fledgling number, including 95% credible intervals (CrI), with raw data in the background (left row). Elevational effect on the relative reproductive output (RRO), where RRO >0 indicates a resident advantage and RRO <0 indicates a migrant advantage, plotted across the study area (right row).

FIGURE 4

Effect of timing of breeding on fledgling number, including 95% credible intervals (CrI). Julian date: 77 = March 18, Julian date: 134 = May 14. Raw data in the background.