Developing and implementing novel methods
for managing grey squirrels

Abstract

Improving levels of biodiversity and preventing further decline is a global issue. Alien invasive species have been identified as one of the main causes of global biodiversity loss. In total 1 million species of plants and animals are assessed as at risk from extinction due to the impacts of invasive species and in 2019, global annual costs of biological invasions were estimated to exceed US 3.6 billion between 1930 and 2022.

It is important to have practical and effective tools to manage wildlife populations and mitigate their impacts. Culling, traditionally used to reduce wildlife population numbers and human-wildlife conflicts, can be ineffective, inhumane, expensive or environmentally hazardous. Fertility control is increasingly used as a wildlife management tool for a range of mammal species, offering a number of potential benefits, particularly when compared with culling. Fertility control is more publicly acceptable than other methods, which is likely to increase landowner and practitioner support and facilitate deployment. Fertility control has been proposed as a method for controlling a range of overabundant rodent species, but most of the currently used fertility control agents are injectable single-dose immunocontraceptive vaccines and rodent populations are often too numerous for these agents to be delivered cost-effectively.

Oral contraceptives are currently being developed to manage the invasive non-native grey squirrel Sciurus carolinensis in the UK. Grey squirrels were introduced into England in the 19th century and it is estimated that their numbers now exceed 2.5 million. As a result, the red squirrel Sciurus vulgaris, the only squirrel native to the UK, has declined to less than 300,000 individuals and is now considered to be ‘Endangered’ in Great Britain. The grey squirrel is responsible for the decline of the native red squirrel Sciurus vulgaris, through the transmission of the squirrel pox virus and competition for resources. It is also estimated that the grey squirrel causes £37 million in tree damage per annum.

Oral contraceptives require a delivery system that is effective, practical, targeted and economically viable, so it is equally important that a suitable bait delivery system is developed to ensure that a bait is delivered to a sufficient proportion of the target population with minimal impact on nontarget animals. The development and testing of this bait system is the focus of this thesis.
In Chapter 2 I designed and tested a novel bait hopper, equipped with a PIT-tag reader and bait weighing device, that could record the frequency of feeding visits and amount of bait consumed per visit by free-living grey squirrels. In captive trials, the hopper proved highly effective at measuring patterns and quantities of bait uptake by individual grey squirrels. I also found that the bait marker rhodamine B (RB) was effective at marking grey squirrels when low amounts were consumed with no apparent adverse effects, making it suitable for measuring population level bait uptake. The adaptability of the hopper design means that it has wider applications for wildlife management; in particular, efficacy studies for bait-delivered drugs in the context of wildlife disease control and/or population reduction

Understanding grey squirrel feeding behaviour with regards to feeding devices is important so that delivery methods can be designed to ensure that a sufficient number of target individuals receive an effective contraceptive dose at a time of year that will guarantee their infertility throughout peak times of breeding. In Chapter 3, using the purpose-designed bait hoppers baited with RB bait for four days, I demonstrated that it was possible to deliver baits to the majority of grey squirrels in 10 woodland environments. Season, hopper density and squirrel density were all important factors determining bait uptake, with better bait uptake found in summer and with a higher density of hoppers.

In Chapter 4, I assessed individual level bait uptake for six woodland populations of squirrels in three seasons, with four days of rhodamine B bait deployment via purpose-designed squirrel-specific bait hoppers with integrated PIT-tag readers, developed and tested in Chapter 2. I demonstrated that it was possible to deliver multiple doses on most days to most male and female grey squirrels, with bait deployment more likely to be effective in spring, immediately before the second annual peak in squirrel breeding. Winter was also found to be a suitable month to deploy contraceptives.

Effective wildlife population management requires an understanding of the abundance of the target species. Camera traps are increasingly used to estimate animal abundance, and methods have been developed that do not require the identification of individual animals. In Chapter 5 of this study, I developed a practical and cost-effective method, based on a camera trap index, that could be used by practitioners to estimate densities of grey squirrels in woodlands, to provide guidance on the numbers of traps or contraceptive feeders required for grey squirrel control. Camera traps were deployed in ten independent woods of between 6 and 28 ha in size. An index, calculated from the number of grey squirrel photographs recorded per camera per day had a strong linear relationship (R2 = 0.90) with the densities of squirrels removed in trap and dispatch operations. This method can easily be adapted to other rodent or small mammal species, making it widely applicable to other wildlife management interventions.

It is important that species-specific bait delivery systems are designed and tested to ensure that oral contraceptives can be delivered effectively and in a targeted way without impacting other wildlife. In Chapter 6 I discuss two studies that investigated two different species-specific bait systems, that could be used to ensure oral contraceptive are targeted towards grey squirrels and the potential impact on nontarget animals minimised. From the trials conducted in this study, I found that a feeder with a weighted bait door was successful in excluding most other species of UK wildlife, in areas where there were grey squirrels, but no red squirrels present. I also found that body weight could be used to develop a feeder that allows access by most grey squirrels whilst excluding red squirrels. This was confirmed from data collected from a prototype selective feeder where both red and grey squirrels were present.

The methods designed and tested in this study proved highly effective at measuring patterns and quantities of bait uptake by individual grey squirrels. Oral contraceptives could offer suitable control methods to manage invasive non-native rodents and small mammals in the future. In addition to the grey squirrel, key species on the International Union for Conservation of Nature 100 world’s worst invasive alien species list include; the ship rat Rattus rattus, house mouse Mus muscola, small Indian mongoose Herpestes javanicus and stoat Mustela ermina. The methods and devices developed in this study could be adapted to optimise contraceptive delivery methods for the management of these species in the future.