Wing span, and head and body length These are often quoted in books, but they are not useful field measurements because too much variation in measuring technique is possible. Their main use is in conveying the size of bats to the public. Ear length Take the longest measurement from the notch at the anterior base of the pinna to the tip. Ensure that the ears of Plecotus are fully extended. Tragus width Take the greatest width. In Plecotus it maybe necessary to eliminate curvature by slightly bending the tragus at its widest point. Tragus length Record the maximum length from base to tip, ignoring curved edges. Forearm With the wing folded, take the maximum measurement from elbow to wrist. With the elbow of the bat resting on the movable jaw of the callipers, the callipers are adjusted to the correct maximum length when you can see or feel slight movement of the skin of the wrist against the fixed jaw as the forearm is rotated within the jaws of the callipers.
Fifth digit Measure from the inside (posterior) of the wrist to the tip of the finger. This is best done on a flat surface. This measurement is needed to distinguish Pipistrellus pipistrellus from P. nathusii. This is a difficult measurement to take accurately and some authors use a measurement from outside the wrist to the fingertip as an easier and more reliable measurement. Calcar The length of the calcar compared with the total length of the edge of the uropatagium is an aid to identification of some species (e.g. whiskered and Daubenton’s). The calcar will not be perfectly straight, but spread the tail membrane and measure the chord from the base of the calcar at the ankle to the tip and similarly from the tip of the calcar to where the membrane joins the tail. 5.4 Weight The weight of an individual bat can vary by as much as 50% seasonally and by a considerable, although lesser, percentage over a hour period. These factors must be borne in mind when comparing weights of individuals or population samples. Weights are of greatest value in long-term studies of growth and body condition, and there is little point in amassing data in a casual way. Two convenient types of spring balance are available. A g Pesola (long scale) balance can be used for all British species and can be read to an accuracy of g. Other Pesola balances (including the 30-g) and the much cheaper Salter balances should only be read to an accuracy of 0.5 g. The balance should swing free from the ring at the top either from a fixed hook or from a well supported hand. Ensure that the balance and bat are free from obstruction and that weighing is carried out in a draught-free environment. Torpid bats can be hung by the feet onto the clasp of the balance. Active bats should be enclosed in a small cloth bag and the difference between the weight of the empty bag and the weight of the bag plus bat recorded. The bat must be confined sufficiently to discourage movement without injury. Small cones of cloth open at both ends with the bat's head inserted down into the narrow end of the cone can also be used. If very light cones or bags are used, the balance can be adjusted to give a reading of 5.3 MEASUREMENTS 53 zero when they are empty, but their weight should be regularly checked, as it will vary. Remember that a bat’s weight will vary greatly during the course of 24 hours when bats are active. This will put great limits on the use of weight data. Record the time of weighing, using the hour system. 5.5 Rare bats If a bat, which cannot be identified with a good key or which is clearly a rarity, is found seek expert advice immediately. Help can be obtained from an SNCO, The Bat Conservation Trust or through local bat group contacts. If it is not possible to arrange fora second opinion from a bat worker or an expert to seethe bat within a reasonable time, take detailed notes before releasing the bat at the site of capture. Weigh the bat and measure the forearm and any other characters used in the identification of species of its genus. Note other non-measurable characters, such as colour, texture and extent of bare areas and of fur the fur colour may change from the base of the hairs to the tip, and such colour banding should also be recorded. Take colour photographs and make sketches if possible. Make detailed notes of the circumstances in which the bat was found. These details will not only help your record to be accepted, but, by virtue of the fact that such bats are rare, the data will be useful to add to the limited amount of data available on the species. 5.6 Parasites The word parasite is a loosely used term and here applies to any organism living in close association with a bat for at least part of its life cycle. Parasites fall into two groups – those that live within the bat’s body or in the gut (endoparasites) and those that live on the surface or only very superficially below the skin (ectoparasites). The animal used by these organisms as a means of transport, for food or for shelter is called the host. Usually the bats live in harmony with these parasites, but occasionally (especially with the endoparasites) disease or debilitation may occur.