First records of complete annual cycles in water rails Rallus aquaticus show evidence of itinerant breeding and a complex migration system

In water rails Rallus aquaticus, northern and eastern populations are migratory while southern and western populations are sedentary. Few details are known about the annual cycle of this elusive species. We studied movements and breeding in water rails from southernmost Norway where the species occurs year-round. Colour-ringed wintering birds occurred only occasionally at the study site in summer, and vice versa. Geolocator tracks revealed that wintering birds (n = 10) migrated eastwards in spring to breed on both sides of the Baltic Sea, whereas a single breeding bird from the study site wintered in north Italy. Ambient light records of geolocator birds further indicated that all but one incubated 2–4 clutches per season. By combining information on incubation and movement, we found evidence for itinerant breeding in three individual birds: After a first breeding attempt (one did not incubate), all moved 129–721 km to breed again. This behaviour is rarely recorded in birds and was unexpected because the water rail is described as monogamous with both parents caring for eggs and chicks. The study greatly improves our knowledge about the annual cycle and reproduction in water rails. However, more studies are warranted to evaluate the generality of our findings and causes of breeding itinerancy.

Methods
The table includes data on the migration (timing of departure/arrival, estimated stationary periods and estimated coordinates of stationary sites) and timing of incubation (first to fourth clutch; one individual did not show any signs of incubating eggs) for 11 individual water rails. This information was estimated from light-level data collected by geolocators and processed in R package ‘TwGeos’ and using the ‘invChanges’ function from the R package ‘GeoLight’, ver. 2.0.1. For more details about methods, please see Lislevand et al. 2020: doi: 10.1111/jav.02595.