Recommended method(s): Hand collecting, Road survey, Trapping.
The most direct way to determine the number of species of snakes present in a specific area is to systematically survey the natural habitat cover and appropriate microhabitats when the weather conditions are warm. This is more complicated than it appears, as snakes are cryptic in colour and secretive in nature, making them difficult to find even under the best conditions. Therefore, not finding a particular species at a site, even after extensive searching, does not necessarily mean that the species is absent from that site. However, if the appropriate microhabitats are searched when the weather conditions are favourable and searches are done several times in a season, as the cumulative number of unproductive searches increases, it becomes more unlikely that the species is present. Species may also be detected using repeated road sampling; however, this may be inadequate for certain rare species which may require intense trapping effort.
In most instances, the preferred method for determining presence is hand collecting. This method is labour-intensive, but it is more versatile and likely more productive than other techniques that tend to require much more equipment and set-up time.
Cover for snakes can take many forms. For example, snakes may hide at the bases of clumps of long grass. However, snakes frequently use rocks and logs to hide under, and flipping such cover objects can sometimes be a productive way to find snakes. The cover object must always be carefully replaced the way it was found, to minimize disturbance of the microhabitat under it. Furthermore, the same cover objects should not be flipped repeatedly (e.g. every day), to avoid disturbance of the site and the snakes. It is recommended that a cover object remain undisturbed for a minimum of at least two weeks between examinations. Also, if venomous snakes are in the area, the safety of the field workers should be considered when lifting cover objects (for example, pull the cover object up towards you by grasping the far side of it to keep the object between you and the snake). Hand searching may also turn up the shed skins of snakes. These can be identified to species using scale counts and can lead to the detection of a particular species in an area without having to capture it.
Road surveys have been used in many studies to sample snake populations (e.g. Klauber 1939; Fitch 1949). By travelling stretches of roads (by car, bicycle, or on foot), it is possible to record at least the presence of particular species and perhaps a rough index of relative abundance of a species for a particular area over time (if one makes appropriate assumptions about vagility and other habits). Road-cruising for snakes may be quite biased as a sampling technique as studies such as Fitch and Shirer (1971) report that some snakes avoid crossing roads. Because of this, the absence of snakes on roads does not mean that they are not present in the surrounding habitat.
Despite the biases, many of the B.C.'s snakes can be found along roads, basking at the edges during the day (e.g. garter snakes) or on the road itself in the early evening (e.g. gopher snakes, rattlesnakes). When searching roads, it is important to keep in mind the reason why snakes are apparently attracted to the road. The presence of snakes on roads is likely associated with reptilian thermoregulation. Because of this, it is unlikely to find snakes sitting on a road on a hot sunny day, as the road surface will be much too hot. However, pavement generally heats up quickly in the morning, making it potentially attractive to snakes as a basking spot until they reach their optimal temperature. The same is true in the evening as the pavement retains heat relatively well and can provide a warm surface for basking. It should noted that this circadian pattern may vary depending on whether a species is diurnal or nocturnal; sampling for a particular species must be conducted at an appropriate time.
Regularly travelled roads will also yield road-killed snakes. These animals make valuable voucher specimens and may provide information on food habits, reproductive status, parasite loads, etc. The direction of travel by snakes may provide information with respect to habitat use and location of denning sites; however, the efficiency of this approach is unknown.
Funnel traps are recommended. Other trapping methods have been proposed, but their use is limited. Trapping is often a labour-intensive project and is therefore not suitable for casual sampling. As different species are attracted to different habitat features and may have unique movement patterns, traps are likely to give a biased estimate of community composition. For example, racers are readily caught in traps, as they actively forage, while rattlesnakes tend to be more sedentary and are found in traps less often (Shewchuk, unpubl. data). Traps in which animals are restrained can lead to mortality when weather conditions are extreme, when incompatible animals are trapped together (small mammals will kill snakes when they are left in a trap together), or when snake predators encounter a trap.
To effectively trap snakes, natural barriers or drift fences will need to be used in conjunction with traps. Drift fences provide a method of directing the movements of snakes towards a trap. A drift fence is simply a barrier which is set-up where snakes are likely to move. Snakes that encounter a drift fence will likely follow along the obstruction looking for an alternative route around it. Traps are then placed at gaps in the drift fence to capture snakes moving through the openings.
Funnel traps have been used in many different studies, with many different designs (Fitch 1951, 1987; Campbell and Christman 1982). They typically consist of a wire mesh cylinder with a funnel entrance constructed of the same material. They are similar in design to many commercial minnow traps, except that the optimal proportions may vary with respect to the species of snake sought. They can be constructed with a funnel at only one end of the cylinder, but better results are obtained from a double-ended design. As mentioned earlier, traps are best used in conjunction with a barrier, either natural (the base of a cliff) or artificial (the foundation of a building or a drift fence). The funnel trap should be placed tightly against the barrier to prevent the snakes from squeezing between the edge of the trap and the barrier. No baiting is required.