Abstract
Vigilance is the ability to maintain alertness and attention over a long period of time. Variables influencing the vigilance behaviour of Passer melanurus, cape sparrows occurring around the University of Pretoria main campus were examined. The study found that vigilance decreases with increase in the number of individuals in a flock, number of females, number of males, ratio of females and males, cover distance, and the flock distance from the observers. A positive correlation was found between traffic (number of disturbances) and vigilance percentage occurrence. Furthermore, the study also found that there was no significant difference between vigilance in females and males, as well as vigilance between different times of the day (morning, afternoon, and evening). Different variables affect this behaviour in different ways. Our results supported hypothesis one (vigilance decreased with increasing number of individuals in a group) and hypothesis three (vigilance increased with increasing number of disturbances). However, hypothesis two (males were not more vigilant than females).
Keywords: vigilance, behaviour, disturbance, cape sparrow
Introduction
Animals spend much time feeding but they often stop foraging to scan surroundings, and such behaviour is referred to as vigilance (Wang et al. 2010). Parasuraman (2009) define vigilance as the ability to maintain alertness and attention over a long period of time. The function of vigilance is one topic still under much debate. Karuse and Ruxton (2002) propose that vigilance is an anti-predatory adaptation that evolved to allow individuals to detect and flee from predators before attack. Vigilance may also be used as a way to detect food and companions (Beauchamp 2001). Many studies relating to vigilance have come up with different hypothesis as a means to explain this behaviour. In animals that live in groups, the amount of time allocated to vigilance decreases with growing group size given that many eyes and ears are available to detect predators, referred to as “many-eyes hypothesis” (Pulliam 1973). In such groups, more time is then allocated to other fitness-enhancing activities such as foraging (Caro 2005). Foster and Treherne (1981) furthermore proposed that individuals in a group dilute individual risk of attack by predators, and referred it as the risk-dilution hypothesis.
An alternative hypothesis to the predation hypothesis is the “competition hypothesis”, which states that vigilance group size effect reflects scramble competition for limited resources (Clark and Mangel 1986). In simple terms, in an area where food is limited and group size increases, an animal will increase its feeding rate (i.e. reduces time allocate to vigilance) to gain maximum amount of food supply (Clark and Mangel 1986). In their study on finches, Beauchamp and Livoreil (1997) suggested that such competitive effects may be the primary driving force behind vigilance group size effect. However, “predation hypothesis” still provides the capital explanation of the pervasive group size effect (Roberts 1996). It is also important to remember that searching for food and vigilance are mutually exclusive as most animals need to look up to detect predators or competitors and need to lower their heads to detect food and forage (Proctor et al. 2006). The trade-offs have been studied to a vast extent in many bird species (McNamara & Houston 1992, Proctor et al. 2006), making birds ideal study species of such topic.
While predation is viewed as activity of natural enemies, other disturbances (not necessarily natural) may play a role influencing vigilance (Frid and Dill 2002). Take a group of birds for example, apart from the obvious predators; the time allocated to vigilance may be affected by human presence, traffic, and weather. To address these issues, we looked at vigilance behaviour in Cape sparrows, Passer melanurus (Passeridae: Müller, 1776). These birds are common species in Southern Africa, occurring in arid savannah, woodlands, farmlands and human habitations (Kopi 2013). The aim of the current study is to describe the effects of different disturbances to vigilance in P. melanurus populations occurring around University of Pretoria main campus. Our hypotheses are that; (1) vigilance will decrease with increasing number of individuals in a group, (2) males should be more vigilant than females (Burger and Gochfeld 1994), and (3) vigilance will increase with increasing number of disturbances.
Materials and methods
Study species
Cape sparrow, Passer melanurus was the study species (Passeriformes; passeridae). The Cape Sparrow has a height of 16 cm and weighs around 29 grams. Male and female differs slightly. The male has a black head and a black with broad white semi-circles on either side, from behind the eye to the side of the throat. The mantle is grey and the under-parts white. The eyes are brown and the bill black. The female is almost similar, but duller with the head and breast dark grey. It is a social bird usually seen in small family groups or in large flocks. The Cape Sparrow is primarily a seed-eater, though it also consumes small insects such as butterflies, bees, wasps, locusts and ants. It feeds mostly on the ground.
Study site
The experiment was carried out at the University of Pretoria (25.7536° S, 28.2297° E), South Africa. The area is composed of different trees with the jacaranda species (Jacaranda mimosifolia, Lamiales: Bignoniacea) being the dominant, shrub species, and a lot of high buildings. Six sites were chosen to represent low, medium and highly disturbed areas (two sites per each criterion) (Table 1). Sampling was done at three replicates for each study site.
Table 1: Different study sites that were chosen for the study that looked at different variables affecting vigilance of cape sparrows, Passer melanurus, at the University of Pretoria Hatfield campus, South Africa.
| Criterion |
High disturbance |
Intermediate disturbance |
Low disturbance |
| Study site |
Grass area in-front of Client Service Centre (CSC) |
Grass area around Theology building |
Grass area around Drama building |
| Student centre |
Grass area in-front of Chemistry building |
Grass area around Admin building |
Experimental protocol
Surveyors were divided into different groups ranging from one to six, with three to six members in a group. Each group was assigned study areas to sample during the day. Sampling was conducted in the morning (between 7:00-8:00), afternoon (between 12:00-13:00), and evening (between 17:00-18:00). The survey was divided into three categories namely, scan, focal and approach sampling. Before the sampling began, the observers had to record distance (m) from flock, distance (m) from cover, the numbers in the flock and the number of males and females in the flock. Following that, observers had to wait for at least two minutes to allow the birds to settle and get familiar with observers’ presence.
Scan and focal sampling were done simultaneously by different observers of the same group. The objectives of scanning were to record the number of females and males of the flock, as well as how many of those individuals were vigilant and how many were not vigilant. Vigilance in this study is defined as rising of the head and moving it sideways (left and right). Focal sampling focused on one individual of the flock and recording the time spend vigilant, time spent non-vigilant, total time of observation, and reasons for flying away (if it happens that the bird flees away before observation time (two minutes) is over). In flocks where it was possible, focal sampling was done for one male and one female of the flock.
Approach sampling is where observers approached the bird and recorded the distance at which the bird flees away. In addition to that the number of disturbances (traffic, number of people, other animals such as cats, etc.) was also recorded. Any other variables that observers felt influenced the birds’ vigilance and reasons for flying away were also recorded.
Data analysis
All data from all groups was captured in Microsoft Excel (Microsoft 2010). In addition, amount of time spent being vigilant and being non vigilant were calculated as a percentage, and the ratio of males and females was also calculated. Using Statistica ( StaSoft, Statistica 12), a correlation analysis was performed between vigilance percentage and the following variables: flock number, number of females, number of males, ratio of females and males, cover distance, flock distance, and traffic. All correlations were under case-wise deletion of missing data. Furthermore, an ANOVA two-way test was performed to analyse the amount of traffic for each time period across the five day sampling period. Using data generated by the ANOVA test, a bar graph was constructed in Microsoft Excel. A T-test independent by groups was performed to analyse time spent vigilant between females and males. Another ANOVA two-way test was performed to see how vigilance differs during different times of the day.
Results
Normality tests showed that all data used in analysis were normally distributed and hence basic statistics (not non-parametric tests) were used further to analyse the data. The strength of association between vigilance percentage and the following variables, flock number (Correlation test: N=184, p=-0.088586), number of females (Correlation test: N=184, p=-0.0759), number of males (Correlation test: N=184, p=-0.066004), ratio of females and males (Correlation test: N=184, p=-0.012488), cover distance (Correlation test: N=184, p=-0.104420), flock distance (Correlation test: N=184, p=-0.116075) was negative. In other words, each variable was inversely proportional to percentage of time spent being vigilant (Fig 1-6). However, correlation between percentage vigilance and traffic showed a positive relationship (Correlation test: N=184, p=-0.020517) (Fig. 7).
The study also found that more traffic was observed during the afternoon, followed by evening, and morning with the least amount of disturbances (Fig 8.)
| Fig.8: The number of disturbances observed during the study of vigilance in cape sparrows, Passer melanurus, and populations occurring around the University of Pretoria main campus. Observations were done in the morning (07:00-08:00), afternoon (12:00-13:00), and evening (17:00-18:00) for a period of five days across six different sites with three replicates for each site. |
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Vigilance between females (T-test: (N=20) T-value= 0.172452, df= 16, P>0.05) and males (T-test: (N=20) T-value= 0.390345, df= 16, P>0.05) showed no significant difference (Fig. 9). Vigilance showed no significant difference across different times of the day (Fig. 10)
Discussion
The study revealed interesting results regarding the influence of different variables on vigilance in cape sparrows. An inverse relationship was observed between different variables and percentage of vigilance observed. These results indicate that vigilance decreases with increase in the number of individuals in a flock, number of females, number of males, ratio of females and males, cover distance, and the flock distance from the observers. The “many-eyes” hypothesis stipulated by Pulliam (1973) is being supported in this case. In a flock where there are a lot of individuals, prey detection is easier and becomes a shared duty. And thus individuals will resort more time into fitness-enhancing activities such as foraging (Caro 2005).
Furthermore the study found that there was no difference in vigilance between females and males. According to Betram (1980), males are expected to be more vigilant than females. This is explained by Artiss and Martin (1995) where bird species showcase mate guarding or protection of paternity. Correlation between percentage vigilance and traffic (number of disturbances) showed a positive relationship. The results indicate that as number of disturbances increase so does vigilance. This was expected because of the number of people present on campus and the amount of other disturbances (cars, small mammals, sounds, etc.) have been shown to disturb birds and trigger vigilance behaviour (Bekoff 1995). However, we do not consider human disturbance as a major role in this study because cape sparrows are considered to be well adapted to human habitations (Hockey et al. 2005).
There was no significant difference in vigilance behaviour across different times of the day. At all times, birds need to maintain alertness because it is not known when predators might attack.
Errors that might have affected the results of the study include consistency in approaching the birds. Different observers used different methods, some walked to the birds slowly, whereas others literally tried to scare the birds away, and in many events than most, succeeded. Furthermore, the some of the observers of the study were not very familiar with the overall appearance of the bird species under study, and therefore might have confused it with other sparrows occurring around the study area. And lastly, vigilance might have been confused with the behaviour where a bird is trying to swallow bolus of food, to detect the feeding rates of its companions, to look for nearby food, or to look for conspecifics (Betram 1980). And due to these errors, the data was not easy to analyse and interpret.
In conclusion, vigilance is not an easy behaviour to detect. Different variables affect this behaviour in different ways. Our results supported hypothesis one (vigilance decreased with increasing number of individuals in a group) and hypothesis three (vigilance increased with increasing number of disturbances). However, hypothesis two (males were not more vigilant than females).
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Appendix
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| Fig. 9: Summary ANOVA two-way test for vigilance of cape sparrow (Passer melanurus) populations around the University of Pretoria main campus grounds. The study lasted for five days with observation done in the morning (07:00-08:00), afternoon (12:00-13:00), and evening (17:00-18:00). |
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