Relationship Between Flock Size & Vigilance Time Essay


Discuss about the Relationship between flock size and vigilance time.



The living organisms surviving in groups has many advantages, one of them is that when in groups they need to spend less time in vigilance and thus can allocate percent of time in foraging and other essential behaviours (Beauchamp 2015). Evidential studies have reported that in many foraging birds and mammals, there exists negative correlation in between the size of the group and vigilance known as “group size effect”. The effect of detection or the many eye hypotheses explains this inversely proportional relationship (Li et al. 2012). According to the many eyes hypothesis, the increased number of eyes can detect a predator more easily and hence the individuals reduces their self vigilance and gains from the members of the other group as because the predator have the tendency to predate one prey at a time during their time of attack (Beauchamp 2013). This shows that the time for conducting other activities increases when in a group of larger size. Thus the foraging efficiency increases with time (Golabek, Ridley and Radford 2012). The “many eyes” hypothesis relates that vigilance plays an important role in detection of predators and thus is known as predation effect.

Hypothetical view:

It has been seen that several birds in flocks benefits through foraging. The flocking tendency helps them to avoid predator either by dilution effect or by confusion them explained through “many-eyes” hypothesis (Zoratto et al. 2014). It has been made evident in some of the cases that even the vigilance of the individual declines, the whole vigilance of the flock increases. This type of relationship observed in the vigilance level and size of the group is considered as an advantage while surviving in flocks (Olson et al. 2015). This observation based idea is sometimes correlated with the “selfish herd “hypothesis”. According to this hypothesis, the animals and the birds present on edges try to secure position in the middle of the flock (Pincock 2012). This hypothesis was observed in the macropid marsupial and in mixed sparrow flocks, in which the spatial distance affected the vigilance between the members of the flocks.


Behavioural studies on columbid wild laughing dove had shown that groups with larger size had more chance in detecting any possible threat approaching towards them as compared with the groups with smaller size (Forshaw 2015). Based on this study, it was found that the information must be transmitted quickly within the members of the flocks in order to make the group size effective. As the foraging decreases the individual efficiency in vigilance as seen in birds, the size of the group is thought to influence the benefits and the individual costs (Marshall et al. 2012). Hence, the foraging by flock can decline the risk of predation during enhancing the intake of food. Some studies alternately show no such reduction in the individual vigilance with the increase in the size of the group. Vigilance is considered as the primary function in the protection of anti-predation (Fichtel 2012). Other factors that influence the rate of vigilance are various competitions such as interference competition and scramble, covering distance in concern, the age, gender, dominating status and observer proximity.

Vigilance is said to be positioned towards predator and conspecifics eventually (Jayne 2014). Hence, the other components of behaviour and spatial factors of flocks are investigated that will suggest new vigilance effect. Studies on varied vigilance based on spatial positioning in flock showed that individuals present in the peripheral region spends more percent of time in comparison to the individuals present in the central position in vigilance (Darrah 2013). This type of effect is known as the edge effect and explains that the individuals present in the periphery have higher risk to get exposed to predation and acts as defender in providing prevention to the central placed individuals.

In this study we will investigate how the size of flocks affect the foraging behaviour in an individual with varying size of the flocks and in which the vigilance also varies with the flock size. Thus in order to study whether the rate of foraging increases in flocks with larger size, we need to examine the relationship effect between the flock size and vigilance behaviour. We took the Australian Wood Ducks as the model system to determine the flock size effecting both time required in foraging and also competition level. We examined that whether increase in the group size positively affected in decreasing the risk of predation and hence helps in gaining more time in foraging or the increase in competition decreases the predation risk.

The species mentioned in this discussion are the Australian Wood Duck, Chenonetta jubata, a grazer by nature spending more than 30% of whole day in grazing with frequent bouts (Ndlovu Hockey and Cumming 2017.). These birds are distributed in Australia, involving Tasmania too. The habitat of Australian Wood Duck is open woodlands with grasslands, pastures usually flooded and the bay coast. They inhabit the dams with farmland with sewage ponds and urban parks. These birds prefer foraging on lands. These birds are usually found in flocks ranging in between 100 to 2000 individuals in flock (Murray et al. 2013). The feeding habit includes grasses, herbs, grains and sometimes insects. These are found to forage in shallow water or grasslands. we will analyse that the vigilance by an individual decreases depending on the size of the group by predicting that increased in group size would increase the time required for foraging purpose with increased competition. Finally, we enhanced the fact that, the overall rate of intakes would increase with increased time for foraging till there is no such negative impact of competition interference on successful foraging. The size of the flock is found to be related with seasonal factors, disperse while breeding and congregates after breeding (Camacho 2012). All the above stated factors will be analysed by taking the Australian Wood Duck as the model system.


The site under study:

The study was performed in a shallow grassland taking Australian Wood Ducks as the model for studying the vigilance effect with respect to time. The vegetation of the area is mostly composed of grasses and some trees. In the dry season, from July to August, the observations and recording data corresponding the foraging activities was done in the morning time. The estimation of the population density was done by noting the individual count per flock that encountered in various regions in the study area. The flocks when discovered, the individuals were chosen randomly from the centre and the edge of flock when the size of the flock exceeded two. The individuals were positioned in the middle of there was one bird in between the individual in focus and the flock edge. But if no individuals were found in between the flock edge and the one in focal region, the individual was positioned in the edge. The recordings were made by the use of 10x25 binoculars with 5 minutes time interval with the help of the focal-animal sampling methodology (Schlacher Nielsen and Weston 2013). The data collected were related to size of the group, scanning rate and the total duration in vigilance. The varying distances between the individuals, shifting the position and behavioural duration such as interactions, resting and foraging were also recorded. The behavioural scanning was recorded whenever an individual from the flock raised head from a horizontally marked line of the body. The individual distance was marked according a relative position of a tree which was noted as the marker.


The size of the group was recorded at the beginning of the study and if the group size got altered due to any kind of dispersal, that observation session was ended, with the start of a new session. The foraging activity of the individuals was, walking with head-down and searching, thereby probing and substrate handling that covered most of the time in observing. This time was obtained by subtracting the time taken in doing other tasks from the time recorded in total observation.


In the area under study, six animals/ha with 150 flocks of wood duck was recorded. The sample of the flocks varied from individual foragers to the wood duck groups comprising up to 20 individuals. The rate of scanning and the time taken in vigilance were noted respectively. The results were negatively correlated between the rate of scanning and the size of the flock. Sane correlation was found between the vigilance of the individual and the size of the flock. The time for foraging showed a mean value of 280±30 (N=150) which corresponded to the size of the flock. The geometry of the flock and the average distance in cm were also recorded. It was found that only 28% of the individuals shifted among the flocks whereas 68% of the individuals shifted to the corner from the edge and 32% shifted to the edge from the centre position. The time of scanning, the time expenditure in vigilating and time of foraging was significantly related in between the individuals in the peripheral and in the centre. More than 93% of observed time was due to foraging. Low aggression rate should no effect in between the group size and aggressive behaviour. It was found that as the group size increased, the aggression also increased.


Based on other relevant research studies, it was found in this study that the vigilance in individual reduced with the increased in size of the flock in Australian Wood Duck (Fern?ndez-Juricic 2012). The incorporation of the left out time in the activities concerning the foraging was noted due to the reduced time in individual vigilance.

The reduced distance recorded among the members of the flock can be linked with conspecifics vigilance. This action can improve the responding towards predators which is possible due to fast mode of information transmission among the flockmates. The vigilance of this type can be demonstrated by peripheral vision, as the birds with head-down are capable of detecting threats though in lower rates as compared to vigilant birds (Edwards et al. 2013). The distance can be another factor between the members of the flocks that affects the rate of vigilance.

In relating the risk of predation, the position of the flock plays an important role and has direct effect in the rate of vigilance. The amount of time spends by the individuals in vigilance purpose and foraging was related to their position among the flock. The individuals of Wood Duck birds at the edges functioned as more vigilant than the individuals in the middle (Beauchamp 2015). It was found that the individuals in the middle of the flock took more time during foraging as compared to the individuals in the edge of flock. Various studies revealed that the individuals preferred the central position as explained in the “selfish herd “hypothesis (Pincock 2012). This tendency can be due to the feeling of being protective in the middle of the flock by the individuals present in the peripheral region of the flock. This helps the centred individuals with the benefit of spending less amount of time in vigilance and gaining more time in foraging purpose. It can predict that due to a pre-established hierarchy in grouping, the frequency by which the positional shifting occurs is lowered. Taking this prediction into consideration, it can be said that social organizing occurs by gaining benefit being in the central position of flock and hence are occupied by the dominating individuals. Due to this differences in the behaviour can be expected in allocating the time during vigilance and other activities such as foraging.

The results have suggested that though there are low aggression rates, still there lies a relation between the size of the flock and the aggressive behaviour. This relationship indicates the interference effect that influences the rate of vigilance. Hence, it can be said that the individuals need to direct the vigilance towards the potential predators and also to the other members of the flock (Beauchamp 2015). However, more data are required to understand the effect of the size of group as the aggression was observed in rare case.

Thus, it can concluded that the size of the group significantly affect the vigilance and the main force for driving the effect of the group size and trade-off found in the case between the foraging and vigilance is the pressure created due to predation. Scramble competition is also found to play a crucial role in effecting the size of the groups (Clutton?Brock and Janson 2012). The increased scramble intensity with decreased vigilance is found to be related with increase in group size


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