Download Tyto alba (Barn Owl) Prey Preference Based on

Tyto alba (Barn Owl) Prey Preference Based on Species and Size
Monica DiFiori
September 18, 2013
Biology 182 Lab
Prof. O’Donnell
Introduction
Various studies have been performed in an attempt to shed light on the optimal foraging
theory. This theory analyzes animal predator behavior as a common ground between the profits
of nutrition and the costs of acquiring food (Campbell and Reece et al. 2008). More specifically,
this theory suggests that natural selection inherently selects for foraging techniques that reap
maximum nutritional and energy value from prey while minimizing time, effort and vulnerability
sacrificed by the predator. (O’Donnell, 2013). Multiple studies on the flighted predator have
shown no prey selection based on size or sex (Boonstra 1977; Vargas et al. 1988, as cited in
Trejo and Guthmann 2003). Within birds, optimal foraging is either selective or opportunistic
(Altringham 1996, as cited by Frafjord 2003). Meaning, birds could be consciously selecting
their prey based on nutritional or energy value or they could be selecting their prey based on its
abundance. Owl foraging behavior has been closely looked at in order to detect any strategy of
prey selection.
Most owls forage at night (Lyman and Power, et al.), finding their prey via sound when in
complete darkness (Payne 1962; Konishi 1973, as cited by Lyman and Power et al. 2001).
Keeping the Tyto alba, or the barn owl’s hunting habits in mind a multitude of hypotheses have
sprung from studies to explain their prey selection. Many of these studies lead to the conclusion
that some species of owls select prey based on a certain age and/or size (Janes and Barss, 1985;
Dickman et al. 1991, as cited by Lyman and Power, 2001). A different view, the hypothesis of
differential activity, suggests that the most active prey would be most vulnerable to their owl
predators (Kaufman 1974, as cited by Trejo and Guthmann, 2003). Lyman and his colleagues
further examines this theory and concluded prey activity varies by age and taxon, therefore
taxonomic and demographic structure of prey selected by barn owls is dependent on which
particular prey are active during a certain owl’s foraging time frame (Lyman and Power, et al.).
Although the size and abundance of prey may be the main limiting factors, the habitat and the
behavior of the prey may also affect their vulnerability to predators (Frafjord, 2003). More
specifically, in a study conducted by Lyman and Power, behaviors in specific age classes within
the rodent species were suggested to make them more susceptible to predation. Lyman and
Power’s work lead to the conclusion that rodents in an intermediate age group were more likely
to be selected as prey because they were more vulnerable due to the dispersal and roaming
behaviors characteristically associated with their developmental stage in their respective life
spans. Because of these characteristic behaviors, Lyman and Power deduced the rodents were
most likely to have been in a new and unfamiliar territory during their period of dispersal
(Lyman and Power, et al.), causing them to be at a more vulnerable state than older and younger
individuals within their own populations.
In order to study these questions posed by owl foraging patterns, owl pellets were
dissected and analyzed. Striving to determine how owls select their prey and what type of
selection they employ, the bones of rodents found in the pellets were categorized and identified.
Methods
The barn owl pellet was placed in warm water to help with separation and forceps were used
to dissect it, pulling out all bones and teeth. As each fragment was separated, it was placed in an
organized manner on a paper towel. Upon completion of the dissection, the minimum number of
prey was determined by looking at the number of mandibles and crania. Assuming two
mandibles and one skull are equal to one animal, this data was recorded. Following a
dichotomous key (O’Donnell, 2013), the species of each prey animal present was determined.
The first marker looked at in the identification process was the skull’s features. The upper
incisor and lower incisor were measured and compared with the accepted ranges of the
determined prey’s species as a way of confirmation via an incisor key (Hager and Cosentino,
2006, as cited by O’Donnell, 2013). When measuring the incisors, one incisor is pulled out with
forceps and measured using a circle template, finding the best fit and recording its respective
length. If unfortunately, there were no mandibles present, the skull features were analyzed and
confirmed with the length of the upper incisor. Additionally, if there was no skull present, the
species would be identified via the length of the lower incisor. The determined species of all prey
present in the pellet were recorded as well as the lengths of the incisors if applicable.
In addition to identifying the species of prey present in the pellets, the relative size of the
prey was also determined. Using a caliper to measure one mandible’s length, the measurement
was recorded.
By using the skull’s features and the lower and upper incisor length, the different species in
the owl pellet were identified. Moreover, the mandible length was recorded as a benchmark for
the size of the prey.
Results
The numbers of various types of prey differed considerably, suggesting owls selectively
forage (P = 2.86 x 10-28; see Figure 1). Specifically, within the Microtus species barn owls do
not selectively forage for a certain size (P = 0.32; see Figure 2). On the other hand, barn owls do
selectively forage when concerned with the Thomomys species (P = 1.37 x 10 -14; see Figure 3).
Number of Individuals
Observed and Expected Foraged Prey
200
150
100
observed
50
expected
0
pocket
mouse
harvest
mouse
pocket
gopher
voles
Species of Prey
Observed and Expected Selection of
Microtus Based on Size
Observed and Expected Selection
of Thomomys Based on Size
60
30
40
20
observed
0
juvenile subadult adult size
size
size
expected
Size Class of Microtus Prey
Figure 2. Comparison of the expected and observed number
Microtus rodents selected by owls based on size class.
Number of Individuals
Number of Individuals
Figure 1. Comparison of the observed and expected number of specific prey species foraged in order to determine if
owls selectively forage. Note the large difference demarcated in the graph between expected and observed values of
pocket gophers and voles.
20
10
observed
0
juvenile subadult adult
size
size
size
expected
Size Class of Thomomys Prey
Figure 3. Comparison of the observed and expected
numbers of owl selection for Thomomys rodents
based on size class.
Discussion
The purpose of this experiment was to determine what strategy barn owls employ when
foraging; whether it’s opportunistic or selective. Upon analyzing the data, it was determined that
barn owls generally forage selectively. The data in Figure 1 suggests that barn owls specifically
select for voles and to a lesser degree pocket gophers. Also seen in Figure 1, the observed values
compared to the expected values of selection for pocket mice and harvest mice suggest barn owls
tended to avoid them. In the past, a multitude of studies have explored different reasons for owl
selection of specific species and size class.
Focusing on the data concerned with voles, or Microtus, it is apparent that barn owls
actively select for this particular species when looking at Figure 1. However, when the data is
more deeply analyzed in Figure 2, we find no specific selection concerning size class. Therefor,
we cannot conclude that barn owls selectively forage for a specific size of Microtus, but instead
forage for Microtus opportunistically. The expected and observed numbers for all three size
classes are very similar. Suggesting barn owls forage for Microtus based on abundance and not
based on a particular size class. In future research concenring Microtus, a larger sample size may
lead to the discovery of a more visible trend concerning selective foraging.
Contrary to the conclusions drawn regarding Microtus, owl selection for pocket gophers,
or Thomomys, prove to be selective. Indicated in Figure 3, both the juvenile and subadult size
classes were highly selected for. These results correspond with three different explanations
explored by various other studies.
The vulnerability of the juvenile and subadult age classes with the Thomomys species is a
likely effect of increased rates of dispersal (Ambrose 1972, as cited by Trejo and Guthmann,
2003), habitat selection (Halle 1988, as cited by Trejo and Guthmann, 2003) and lack of
experience avoiding predators (Longland and Jenkins 1987, as cited by Trejo and Guthmann,
2003) associated with that size class. Focusing on the dispersal patterns in conjunction with age,
the relationship suggests that a majority of rodents dispersing from their burrow are juveniles and
subadults, which were highly selected for. When the young rodents start to forage for
themselves, they travel away from familiar territory and become vulnerable to flighted predators
in their new and unfamiliar territory (Frafjord, 2003). Moreover, when dispersing from the
burrow, rodents still have not learned how to successfully avoid avian predators and thus are at
higher risk of predation. Lastly, the hierarchy of habitat selection within rodents is defined as the
larger rodents get first pick and therefor choose areas with lower vulnerability towards predators
(Halle, 19988, as cited by Trejo and Guthmann 2003). Meaning the juveniles and subadults are
forced into undesirable areas of increased vulnerability (Dickman et al. 1991, as cited by Trejo
and Guthmann 2003). In order to determine which of the three explanations, if any, carries more
weight concerning predation of rodents by owls, future studies should focus on the geographical
and demographical details of the habitat.
With all research conducted in this manner in efforts to discover trends in owl foraging,
there is one shortcoming. If there is false identification of prey, the data collected may be wrong
and present misleading and incorrect trends. When classifying the rodents found in the owl
pellets, the skull and both mandibles may not be present. Without the skull and both mandibles,
the categorization of the individual prey may be incorrect because there is no way to cross check.
However, the checking of the incisor lengths against the skull features are a means of
confirmation and do not completely discredit the identification of the rodents with missing
skeletal features.
According to the data collected in this experiment, barn owls are concluded to generally
forage selectively for voles (Microtus) and pocket gophers (Thomomys). Further exploration of
the results in the selection of voles and pocket gophers revealed that barn owls foraged
opportunistically for voles, but selectively foraged for juvenile and subadult pocket gophers.
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