spatial distribution of the carcasses depends on the type of obstacle they collided
with, the size of the animal, and the wind speed at the time of collision
(Korner-Nievergelt et al. 2015). Calculating the proportion of dead animals detected
in the search area relative to the total area is rarely done because of the difficulties in
estimating the variables involved (Teixeira et al. 2013).
Carcass persistence is the time a carcass stays on the rail or ground before it is
removed by scavengers or has been severely decomposed, becoming undetectable
to the observers. It depends mainly on the carcass size, the abundance and activity
of scavengers, and temperature and humidity (Guinard et al. 2012; Santos et al.
2011).
Searcher efficiency, or detectability, is the probability that an observer actually
finds a carcass in the search area. Detectability depends mostly on the survey
method used, the experience and motivation of the observer, characteristics of the
ground (vegetation density, ballast color), time of day, weather conditions, and type
and size of the carcass. The method used for monitoring (e.g., by vehicle or on foot)
greatly influences the detectability rates. For roads, Hels and Buchwald (2001)
concluded that surveys by car detected between 7 and 67% of the amphibian
carcasses that had been detected by surveys on foot and, similarly, Teixeira et al.
(2013) reported a reduction in the detection rates from 1 to 27% for small and large
animals, respectively. The observer’s experience largely determines the probability
of detectability: nevertheless, after several hours of work (>3 h), there is a “saturation effect” that affects the observer performance. Motivation is also important,
because a motivated observer (e.g., working on a thesis) may stop more often to
collect data and see details that otherwise would have gone undetected (PVMC
2003). On railways, the color of the rock ballast is one of the main factors
influencing carcass detectability. Generally, smooth terrain and light colors of rock
increase detectability as does clear verges on both sides of the rail. Additionally, if
the vegetation beyond the verge is sparse and short (or even absent), detectability
increases. Another crucial aspect is the accessibility of the rail line, as well as the
topography on its sides. Weather conditions also influence detection by altering
visibility—for instance, detectability decreases on rainy or foggy days (e.g.,
Mathews et al. 2013). Intrinsic characteristics of the species, such as, the size, shape
and color of a species, can compromise their identification (e.g., small carcasses
with cryptic colors are more difficult to detect). These sources of error can be taken
into account by mathematical estimators to recalculate a detectability function that
is then used to correct the observed mortality estimates (Bernardino et al. 2013;
Korner-Nievergelt et al. 2015), but this is not yet common practice in railway
mortality studies.
30
F. Carvalho et al.
with, the size of the animal, and the wind speed at the time of collision
(Korner-Nievergelt et al. 2015). Calculating the proportion of dead animals detected
in the search area relative to the total area is rarely done because of the difficulties in
estimating the variables involved (Teixeira et al. 2013).
Carcass persistence is the time a carcass stays on the rail or ground before it is
removed by scavengers or has been severely decomposed, becoming undetectable
to the observers. It depends mainly on the carcass size, the abundance and activity
of scavengers, and temperature and humidity (Guinard et al. 2012; Santos et al.
2011).
Searcher efficiency, or detectability, is the probability that an observer actually
finds a carcass in the search area. Detectability depends mostly on the survey
method used, the experience and motivation of the observer, characteristics of the
ground (vegetation density, ballast color), time of day, weather conditions, and type
and size of the carcass. The method used for monitoring (e.g., by vehicle or on foot)
greatly influences the detectability rates. For roads, Hels and Buchwald (2001)
concluded that surveys by car detected between 7 and 67% of the amphibian
carcasses that had been detected by surveys on foot and, similarly, Teixeira et al.
(2013) reported a reduction in the detection rates from 1 to 27% for small and large
animals, respectively. The observer’s experience largely determines the probability
of detectability: nevertheless, after several hours of work (>3 h), there is a “saturation effect” that affects the observer performance. Motivation is also important,
because a motivated observer (e.g., working on a thesis) may stop more often to
collect data and see details that otherwise would have gone undetected (PVMC
2003). On railways, the color of the rock ballast is one of the main factors
influencing carcass detectability. Generally, smooth terrain and light colors of rock
increase detectability as does clear verges on both sides of the rail. Additionally, if
the vegetation beyond the verge is sparse and short (or even absent), detectability
increases. Another crucial aspect is the accessibility of the rail line, as well as the
topography on its sides. Weather conditions also influence detection by altering
visibility—for instance, detectability decreases on rainy or foggy days (e.g.,
Mathews et al. 2013). Intrinsic characteristics of the species, such as, the size, shape
and color of a species, can compromise their identification (e.g., small carcasses
with cryptic colors are more difficult to detect). These sources of error can be taken
into account by mathematical estimators to recalculate a detectability function that
is then used to correct the observed mortality estimates (Bernardino et al. 2013;
Korner-Nievergelt et al. 2015), but this is not yet common practice in railway
mortality studies.
30
F. Carvalho et al.