Individual, sexual and temporal variation in the winter home range sizes of GPS-tagged Eurasian Curlew Numenius arquata

In a collaborative study led by the University of Hull, BTO scientists aimed to find out more by establishing the overwinter home range size (the size of the space used by the birds during winter) of Curlew in the Humber Estuary, North-East England, UK. Curlew visit both estuarine and agricultural habitats during winter, but this study may be the first to examine how this habitat use changes throughout the non-breeding season. As Curlew display sex-differences in bill length which impact foraging technique, this study also wanted to determine if males and females used habitats differently. 

Over the course of four winters, GPS tags were deployed on 18 Curlew from two sites (Welwick Marsh and Long Bank Marsh) on the Humber Estuary. An in-depth analysis of these GPS data allowed the researchers to estimate the home range size of the individual birds and, for the first time in a wader species, infer their behaviour from their movement patterns. These analyses revealed where and how the Curlew were spending their time, both on a daily basis and across the season. 

The study uncovered a number of surprising results. The Curlews’ average home range size was 76.1 ha, which is considered small when compared to other wading species such as Knot and Dunlin. Furthermore, contrary to expectations, a slight decrease in Curlews’ home range size was detected as the winters progressed. These results imply that the high-quality habitats of the Humber Estuary had a plentiful supply of food, meaning the birds were not forced to travel long distances or expand their home ranges in response to resource depletion. Unexpectedly, although the Curlew spent more time resting at night (31% compared to 13% during the day), their nocturnal home range was often larger than their diurnal one.      

Home range characteristics and use also differed between individuals. For example, some birds travelled up to 3.5 km inland to forage on farmland, while others stayed exclusively on tidal mudflats. Contrary to predictions, these differences were not explained by sex. Instead, Curlew foraging behaviours varied between groups of birds wintering at different locations on the estuary. The drivers behind these individual differences remain cryptic, but it is probable that Curlew employ specialised foraging tactics to avoid competing with one another. As conservationists aim to support their survival, they should account for this variety of strategies when maintaining habitat on the Curlews’ behalf.

Although these findings may be site specific, the valuable knowledge that Humber Estuary Curlew maintain relatively small home ranges and employ individualised foraging strategies will inform management responses to sea-level rise and habitat conservation. Crucially, this study also demonstrates there is still much to learn about Curlew habitat use, paving the way for future work.  

Notes

The study formed part of a PhD studentship undertaken by Lucas Mander at the University of Hull and supported by Humber Wader Ringing Group (HWRG), the British Trust for Ornithology (BTO) and the generous contributions made to the BTO’s Curlew Appeal. The study also received financial support from IMMERSE - IMplementing MEasuRes for Sustainable Estuaries, an Interreg project supported by the North Sea Programme of the European Regional Development Fund of the European Union. We would like to thank all ringers and helpers from the Humber Wader Ringing Group (HWRG), Spurn Bird Observatory (SBO) and other ringing groups who participated in the wader captures. The authors thank David Turner and the Humber Nature Partnership (HNP) for initiating and supporting the project, and Matthew Stone at the University of Hull for help with the fieldwork. They received invaluable support from Andrew Gibson at Yorkshire Wildlife Trust (YWT) who provided access to capture sites and Tim Page at Natural England (NE) for giving permissions to capture at the sites. The authors thank Lucy Mitchell (University of Hull) and Chris Thaxter (BTO) for providing valuable comments and analytical advice to an earlier version of the manuscript.