Bill,
It would be difficult (if not impossible) to accurately quantify and rank one species above the other in terms of predation risk by raccoons. One would have to first ensure all other variables are equal and this would require a study site where wood turtles and spiny softshells nest with the same frequency. Since habitats are VERY different for each species, it is unlikely to find such a place. The logic is correct but it speaks more to the fact that raccoons are significant nest predators.
Nesting in areas that are most likely to be encountered by a raccoon would put any nest at a greater risk of predation. Thus, any individual animal (not necessarily the species as a whole) that nests along a stream may be at a greater risk of predation than a nest further inland or in an area with low raccoon density. Since wood turtles and spiny softshells both often nest along streams, and streams are favoured foraging areas for raccoons, then it stands to reason that these nests would be affected. Stating that one species is more affected than the other might be a bit speculative. What must be taken into account is that the raccoons are not there for turtle eggs since raccoons are also found foraging along streams in areas where there are no turtles present.
In fact, spiny softshell nests may be at a greater risk when all other variables are equal since there is a greater disturbance in the immediate vicinity of the nest during nesting activity. This is due to a combination of the comparatively larger body size and the significantly greater clutch size in spiny softshells (which one would assume is easier to locate by a predator that is relying on its olfactory and tactile senses).
Here is an example of some new methodology that I have been testing to determine the primary species involved in nest predation (identifying animal tracks is not a reliable indicator, particularly when numerous species are involved - which species actually excavated the nest?).
Click here
