Determinants of nesting and nest success in saltmarsh breeding Common Redshank, Tringa totanus, in North West England

Abstract

Abstract
Breeding populations of Common Redshank, Tringa totanus, Redshank hereafter, on British saltmarshes halved between 1985 and 2011, with the North West of England experiencing some of the largest losses. Understanding the drivers of these declines and exploring conservation management options to redress losses is the primary focus of this thesis.
I first evaluated the reliability of a standardised survey method (SSM) for estimating Redshank nesting density on saltmarshes. This involved multiple walked censuses on four saltmarshes in North West England. Estimates of peak nesting density derived from the SSM were compared to detailed nest monitoring information gathered at the same sites. The SSM was found to overestimate nesting density by 42% across the study sites. Reasons for this discrepancy were considered to be, (i) the presence of non-breeding birds, (ii) differing causes of nest failure across different habitats and areas, and (iii) geographical variation in, and temporal changes to, nesting phenology, the latter likely related to ongoing climate change.
I then examined the temporal and spatial distribution of wildfowl and livestock, and their effect on saltmarsh vegetation height in relation to Redshank nesting attempts. To do this, I used observational and experimental exclusion approaches on Banks Marsh, part of the Ribble Estuary National Nature Reserve, over a 30-month period. Cattle usage was variable spatially across the site with higher usage of the inner (landward) marsh compared to the outer marsh, but with a consistency of areas with greatest and least use between years. The number of nests trampled by cattle was relatively low (15%), occurring in areas of shorter vegetation and higher cattle use. Winter wildfowl herbivory played a crucial role in reducing saltmarsh vegetation height, with wildfowl grazed vegetation typically one third of the height of ungrazed vegetation (wildfowl excluded) during the peak Redshank nesting period. Redshank selected nest sites in taller vegetation and successful nests were in significantly taller vegetation than nests that failed. Using the data collected from Banks Marsh, I developed a logistic regression model based on key biotic (cattle, duck, and goose herbivory) and abiotic variables, including elevation above sea level, to predict where Redshank nest on saltmarshes. Winter grazing by Eurasian Wigeon, Mareca penelope, had a strong negative impact on Redshank nesting, whereas light, late summer grazing by cattle had a positive impact. A modelled reduction in Wigeon use whilst maintaining light cattle grazing optimised the availability of suitable nesting habitat. Under such a regime, Redshank numbers were projected to remain relatively unchanged in future under a scenario of a sea level rise of 0.25m. Under other scenarios of sea-level rise and management, Redshank populations were much reduced. The model developed provided a framework for simulating potential ‘trade-offs’ between wildfowl and breeding wader populations, where a conservation conflict could occur, and for long-term conservation management planning for future climate change impacts.
Finally, I investigated the effects of livestock and wildfowl herbivory, flooding, and predation on Redshank nest survival. I analysed self-collected and long-term nest-record data from Banks Marsh between 1969 and 2018, using Program MARK. Redshank nest survival was most strongly negatively affected by dramatic increases in winter duck herbivory. Increasing cattle grazing intensity during the Redshank breeding season also negatively impacted nest survival but to a lesser extent. Identifying the key environmental variables that influence Redshank nest survival should assist conservation managers to reflect on strategies to maintain this vulnerable species whilst also conserving other target species.

Overall, this project highlights, (i) the benefits of retesting established survey methods and developing improved population estimates, (ii) the need to address the impact of both wildfowl and livestock grazing in future research and conservation management for Redshank, and (iii) the value of long- term data that permit new insights into population dynamics of species.