An assessment of whether age, sex, and reproductive status affect bait uptake by grey squirrels

The grey squirrel (Sciurus carolinensis) is an invasive alien species in the United Kingdom (UK). The economic impact of grey squirrels on England and Wales forestry is estimated to be around £37 million ($47 million USD) per year, and the presence of this species has caused the decline of the native red squirrel (Sciurus vulgaris) due to interspecific competition and transmission of diseases. Oral contraceptives, delivered in baits, are being developed to control the number of grey squirrels in the UK. Bait uptake can be affected by many factors that must be understood to develop effective delivery methods. As sexually mature females drive population growth, targeting these females with oral contraceptives should be a priority to maximize the impacts of oral contraceptives on grey squirrel populations. This study investigated whether sex, age, or reproductive status affected bait uptake by grey squirrels. We fitted 138 squirrels with passive integrated transponders (PIT‐tags) in 4 woodland sites in the UK and measured bait uptake using squirrel‐specific bait dispensers (hoppers) fitted with PIT‐tag readers and baited with hazelnut paste and the biomarker rhodamine B for 5 days in 2018 and 2022. Overall, 49% of squirrels consumed bait from the hoppers and bait was most likely to be consumed by reproductively active females. Immature squirrels made more visits per day to hoppers and visited more hoppers per day compared with adults, though the results for immature animals were highly variable. Sex, age, and reproductive status were not found to affect bait uptake in grey squirrels at the density of squirrels (3.9–6.3/ha) and bait hoppers (3/ha) tested in this study. The results confirmed that it should be possible to deliver oral contraceptives equally to different individuals within populations of grey squirrels within a relatively short period. The findings of this study could be used to inform targeted interventions aimed at delivering baits containing contraceptives or vaccines to grey squirrels. Our study found that the consumption of bait from hoppers by grey squirrels is not influenced by age, sex, or reproductive status and it is likely that around half of the individuals in a population will be affected by contraceptives, vaccines, or other drugs delivered orally in baits. In addition, our study demonstrated that it is possible to deliver multiple doses of a bait to the majority of breeding females within 5 days, thus maximizing the effectiveness o