A pilot study for satellite-tracking Gadwalls Mareca strepera wintering in the Lea Valley, UK

ABSTRACT A pilot study monitored four overwintering Gadwalls fitted with satellite trackers in November 2018 at Rye Meads Nature Reserve, Hertfordshire. The research assessed the feasibility of a larger study investigating the range, movement and habitat preference of Gadwall utilising the Greater London wetland areas, and the suitability of fitting trackers using a glue-attachment method. Tracked birds varied in extent of movement. Two male Gadwalls remained largely sedentary, making use of the freshwater gravel lakes in the Lee Valley Regional Park. A pair-bonded male and female moved together, spending a number of weeks in the Lea Valley before moving 20 km to Hatfield Forest, Essex. This represented a change of habitat to a mature woodland with established lakes and ponds. We speculate that differences in movement between the male–female pair and other males may be attributable to mating effort and individual differences in migratory strategy. Considerable variation in the retention of trackers was seen, with one bird believed to have gradually removed the device through preening. Retrap data for one male showed overwinter weight loss that was higher than in any of 14 Gadwalls recorded previously. The cause of weight loss was unknown and we recommend further investigating the energetic costs of having a tracker fitted. Glue attachment may be unreliable for studies that require monitoring of Gadwalls for more than a few weeks, due to a high likelihood of attachment failure.