Individual study: Nest box use by red squirrels Sciurus vulgaris at Formby National Trust Reserve, Lancashire, England
Shuttleworth C.M. (1999) The use of nest boxes by the red squirrel Sciurus vulgaris in a coniferous habitat. Mammal Review, 29, 61-66
Red squirrels Sciurus vulgaris in the UK have declined markedly over the last 50 years, with the introduced North American grey squirrel S.carolinensis replacing it throughout most of England and Wales. Nest boxes have been advocated as a conservation measure for red squirrels and have proved useful in some reintroductions. However, there was little information concerning the benefit of nest boxes for red squirrel populations, therefore a 4-year study was undertaken to assess the effect of box provision at Formby National Trust Reserve, Lancashire, northwest England.
The study was carried out from 1994 and 1997 in woodland dominated by 40 ha of Scots pine Pinus sylvestris and Corsican pine P.nigra. The reserve contained a high density of red squirrels (3.5-4/ha in the spring).
Nest box design: Two sizes of box (internal dimensions which could contain the average drey size) were erected to investigate effect of cavity size. The ‘small’ (internal dimensions 27 x 30.5 x 48 cm deep) and ‘large’ (32 x 35.5 x 56 cm deep) boxes were constructed from 5-ply or three-quarter inch (~19 mm) timber, with a coat of linseed oil-based waterproofer applied to the exterior. A 7.5-cm-diameter entrance hole was cut in the top corner on one side of the box next to the tree trunk (following Gurnell & Taylor 1989) to allow the squirrels easy access. The box lid could be removed for inspection.
Nest box installation: Three groups of 10 boxes, (5 small, 5 large), were erected in February 1994, each fixed to a pine trunk at a height of 5-8 m. Boxes were placed in a rough grid pattern about 50 m apart i.e. each group of 10 spread across approximately 1 ha. In 1995, a further eight large boxes were put up in other areas. The boxes were filled to the top with hay as bedding material.
Monitoring: Boxes were checked once a month and data on litter size, sex ratio, preweaning survival and parturition dates recorded. Litters less than 2 weeks of age were not sexed as it was felt that it would be prudent not to touch very young animals. The litter data from boxes were compared with litters in natural dreys within the reserve.
Nest box use: All boxes were used to some extent within the first 3 months of erection. Use was highest in summer (61-79% occupied) and autumn (65-73%), declining through the winter (51-52%) and spring (36-49%). Box use was not correlated with seasonal changes in squirrel population density. Boxes were used by adults, subadults, lactating females and their young. A total of 31 litters and 85 young were discovered in boxes. Breeding females more frequently used large boxes (18 in large, 10 in small). There was no difference in the average size of litters in the different box designs (large 2.7, small 2.9). There was no significant difference between natural drey (average 2.5, n = 8) and nest box (average 2.7, n = 31) litter sizes. The number of red squirrel litters, litter size and litters moved out and into nest boxes is summarised in Table 1 (attached).
Box and drey inspection and litters moved: Box inspection did not prevent lactating females from continuing to use them as long as the mother was absent during inspection. If a female was present the litter was always subsequently moved. Twelve litters were moved out of boxes in the interval between monthly checks. Such moved young were taken to dreys rather than alternative boxes with one possible exception of a litter probably transferred to a neighbouring empty box. Litters were moved either as a result of box checks or for unknown reasons with the exception of one in 1995 (that contained a colony of bumble bees Bombus hortorum, the litter being moved as the colony expanded). Natural dreys were difficult to reach and easily damaged during inspection, therefore data from only eight drey litters were obtained over the four years.
Conclusions: The nest boxes provided an alternative refuge or breeding site, with between 16% and 26% of the available boxes used for breeding in any single year, and with lactating females favouring the larger box size. Boxes may offer a safer nest site than natural dreys (vulnerable to saturation during rain, blowing apart in strong winds, or collaping under the weight of adults and young). In this study there was no evidence of predation of squirrels within nest boxes.
Gurnell J. & Taylor J. (1989) The conservation of red squirrels in relation to forest management. Unpublished Report, Forestry Commission, Farnham, Surrey.
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