Living in Groups

Groups:

• Incidental groups - birds are together simply because individuals are using a common resource such as food, water, or a roost site.
• Social groups - reflect intrinsic gregariousness and are very common among birds.

Are there adantages to being in such groups? If so, what are they??

Seminar - Ultimate and proximate benefits of social behavior in Superb Starlings by Sarah Guindre-Parker

Foraging groups:

1. May influence rate at which patches of food are discovered in temporally & spatially uncertain environments (Krebs et al. 1972):

• Great Tits (Parus major) were kept in aviaries with artificial 'trees'. At the end of each 'branch' was a container (N = 60) where birds could search for concealed food
• During the experiment, the amount of food found by each bird was recorded in a fixed time period & birds were tested alone, in pairs, & in flocks of 4.
• Results: % of birds finding food/15 min period =
  • single bird = 25%
  • two birds = 40%
  • four birds = 75%
• Conclusion ----> Local enhancement - process by which an individual's attention is directed to the location of food by other group members, i.e., learning what to eat or where to eat by observing others
   

2. Can reduce variance in an individual's feeding rate (Baker et al. 1981)

• Dark-eyed Juncos were placed in aviary with 20 ‘patches' (each patch with 4 covered cups). Seeds were randomly placed in 1 of 20 patches. Juncos were tested alone & in flocks of 4 (30 min sessions); flocks kept together so dominance hierarchy were established.
• Results:
  • Foraging alone - In 52 tests with 12 birds, food was found in 22 tests (42%) & no food found in 30 tests (58%).
  • Flocks:
    • 1) Dominant birds usually obtained more food than when alone; degree of advantage increased with rank (dominant birds supplanted lower-ranking birds that found food)
    • 2) Lowest ranking birds obtained about same or slightly less food than when alone
    • 3) Probability of flock finding no food = 20%
• Conclusion: Why join a flock?
  • 1) Dominant individuals benefit by obtaining more food
  • 2) Subordinates benefit by a reduced probability of getting no food at all (risk-minimization

 

Larger groups are more efficient problem solvers -- Group living commonly helps organisms face challenging environmental conditions. Although a known phenomenon in humans, recent findings suggest that a benefit of group living in animals generally might be increased innovative problem-solving efficiency. This benefit has never been demonstrated in a natural context, however, and the mechanisms underlying improved efficiency are largely unknown. Morand-Ferron and Quinn (2011) examined the problem-solving performance of Great and Blue tits at automated devices and found that efficiency increased with flock size. This relationship held when restricting the analysis to naive individuals, demonstrating that larger groups increased innovation efficiency. In addition to this effect of naive flock size, the presence of at least one experienced bird increased the frequency of solving, and larger flocks were more likely to contain experienced birds. These findings provide empirical evidence for the “pool of competence” hypothesis in nonhuman animals. The probability of success also differed consistently between individuals, a necessary condition for the pool of competence hypothesis. Solvers had a higher probability of success when foraging with a larger number of companions and when using devices located near rather than further from protective tree cover, suggesting a role for reduced predation risk on problem-solving efficiency. In contrast to traditional group living theory, individuals joining larger flocks benefited from a higher seed intake, suggesting that group living facilitated exploitation of a novel food source through improved problem-solving efficiency. Together these results suggest that both ecological and social factors, through reduced predation risk and increased pool of competence, mediate innovation in natural populations.

Links:

Birds of diverse feathers, problem-solving together

Feathered friends help wild birds innovate

Better problem-solving in groups

Social status and physiological costs -- For group-living animals, the maintenance of a position in the social hierarchy may be associated with physiological costs such as increased stress and energy expenditure or suppressed immune functions. Lindstrom et al. (2005) experimentally manipulated the social status of House Sparrows so that each bird experienced two social environments in random sequence: being dominant and subordinate. For 14 males, Lindstrom et al. examined how corticosterone concentrations, energy expenditure and immune functions were affected by these changes in social status position. The cost of maintaining a social status position differed between individuals and were related to individual body size. Birds with small body size had increased costs in terms of increased stress responses and reduced cell-mediated immune responses while being experimentally kept as dominants, while birds with large body size had increased costs while they were subordinates. Lindstrom et al. also found that birds with increased energetic and immunological costs as dominants obtained a low status position in the large group, while birds with increased costs as subordinates obtained a high status position in the large group. In summary, the costs associated with the maintenance of social status position differed between individuals and was related to the individuals’ body size. Furthermore, in a large group, individuals maintained a social status position that minimized energetic and immunological costs.

3 - Information center hypothesis. Groups or colonies may be advantageous if individuals share information about the location of ephemeral patches of food (e.g., Krebs 1974)

• Great Blue Herons:

Rate of food intake of adult Great Blue Herons as a function of flock size. The regression
equation describing the relationship was arrived at by step-wise multiple regression. The
data are grouped into blocks according to flock size purely to make the figure look neater;
the analysis was done on the original points. The vertical bars represent standard errors
of the means of grouped data (From: Krebs 1974).

• Red-billed Quelea, Quelea quelea (DeGroot 1980):
  • Six birds were trained to know which compartment had food & 6 were trained to know which had water; birds entering compartments 1 - 4 could not go back into chamber X. When all had moved into compartments 1 - 4, they were placed back into chamber X.

Red-billed Quelea

• Test: All 12 birds were placed in chamber X with either food (water in 1 compartment) or water (food in 1 compartment). Five trials were run.
  • Ho: Naive birds (the 6 not trained to know where a particular resource was located) would enter compartments 1 - 4 with equal probability.
• Results:
• Naive birds found resource significantly more often than expected by chance indicating that they were obtaining information about the location of the resource
• How was information transferred?
  • 1) Naive birds responded to 'intention' movements of knowledgeable birds
  • 2) Birds moving with 'confidence' and 'purposiveness' know location of a resource

4 - Groups may flush prey from hiding places & reduce time between successive prey captures

5 - Group hunting can influence the availability of different food resources & the efficiency of capture:

• Increased chance of catching prey
• Increased chance of capturing larger prey
• Increased success when competing with other species
  

Social dynamics of core members in mixed-species bird flocks change across a gradient of foraging habitat quality - Social associations within mixed-species bird flocks can promote information flow about food availability and provide predator avoidance benefits. The relationship between flocking propensity, foraging habitat quality, and interspecific competition can be altered by human-induced habitat degradation. Richardson et al. (2022) examined sociality of two ecologically important flock-leader (core) species, Carolina Chickadees (Poecile carolinensis) and Tufted Titmice (Baeolophus bicolor), to better understand how degradation of foraging habitat quality affects mixed-species flocking dynamics. The authors compared interactions of free ranging wild birds across a gradient of foraging habitat quality in three managed forest remnants. Specifically, they examined aspects of the social network at each site, including network density, modularity, and species assortativity. Differences in the social networks between each end of our habitat gradient suggest that elevated levels of interspecific association were more valuable in the habitat with low quality foraging conditions. This conclusion was supported by two additional findings: First, foraging height for subordinate Carolina Chickadees relative to Tufted Titmice decreased with an increase in the number of satellite species in the most disturbed site, but not at the other two sites. Second, the chickadee gargle call rate, an acoustic signal given during agonistic encounters between conspecifics, was relatively higher at the high-quality site. Collectively, these results suggest an increase in heterospecific associations increased the value of cross-species information flow in degraded habitats - From: Richardson et al. 2022 Tufted Titmouse Carolina Chickadee

Foraging & Group Size

• Search time - benefit of reduced search time reaches asymptote quickly as group size increases. In large groups, individuals may interfere with each other, individual search areas may overlap, & more individuals may increase the evasive action of prey.

• Energy needs - energy (i.e., food) available is limited but energy needs continue to increase with increasing group size (so, at some point, as group size increases, energy needs will exceed availability)

Bird groups and predation

1. Groups may detect predators sooner than solitary individuals:

• Ostriches (Bertram 1980)

• Wood-pigeons (Kenward 1978):

The percentage of hawk attacks which were successful at single pigeons and at flocks. Number of attacks is given above histograms (from Kenward 1978).

• An explanation for the 'group-size effect' is the 'Many eyes hypothesis':
• Requires 'collective detection' = all members of a group are alerted to threat IF at least one member detects it
• Requires (or assumes) that individuals monitor the vigilance behavior of others in the group.
• Individuals in groups use a 'conditional vigilance strategy' where individuals are cooperatively vigilant as along as all other group members remain vigilant (tit-for-tat); this implies that individuals monitor the behavior of others (Pulliam et al. 1982).

Dark-eyed Junco

• Tests of these assumptions (Lima 1995):
• Monitoring vigilance:  Lima introduced food-deprived Dark-eyed Juncos into flocks of non-deprived birds (food-deprived birds are non-vigilant because they spend all their time eating). Birds associating with these 'non-scanners' should scan more.
• Result: Vigilance of non-deprived juncos was no different in presence or absence of non-vigilant, food-deprived flock mates
• Collective detection:  Lima rolled a rubber ball down a ramp designed so only one flock-member could see the 'threat'
• Result:
• Only 20 (4.1%) of 485 prospective non-target responders actually did so (usually those closest to 'target' bird)
• Assumption of collective detection was not strongly supported
• More tests needed on more species, but 'many-eyes hypothesis' might remain viable if collective detection is based on detection by multiple individuals.

European Sparrowhawk
Photo source: BTO

So, individuals in groups may be safer from predators. However, there may be limits to this benefit. If groups get too large, individuals may be more vulnerable to predators.

• Page and Whitacre (1975) - Merlins attacking different-sized flocks of wintering shorebirds

In some groups, certain individuals or species (in mixed-species groups) may act as 'lookouts', e.g., scrub jays, and mixed-species flocks of birds in the tropics. For example, Munn (1986) found that, among mixed species flocks of birds in the Amazon forest, White-winged Shrike-tanagers act as sentinels in canopy flocks & Bluish-slate Antshrikes as sentinels in understory flocks. These species are usually the first to give alarm when bird-eating hawks approach; other species respond by freezing or seeking cover. Both species occasionally utter 'false' alarm calls to distract other birds & increase their chances of capturing prey. The 'penalty' for individuals of other species that ignore calls may be death.

2. Prey may form groups in an attempt to place conspecifics between themselves & an attacking predator.

• Individuals on periphery of group may experience increased predation
• 'Selfish-herding' may interact with other anti-predator benefits or foraging benefits (if not, why should 'peripheral' individuals remain in the group?)

3. Tightly aggregated prey may confuse or inhibit predator

• A Peregrine Falcon, for example, stooping at a dense flock of starlings could be injured (see video below).

• Increased group size = decreased chance any individual will be the 'victim'

 
Peregrine Falcon attacking large flock

Simulation of a predator attacking a bird flock (a, upper left, through h, lower right). The predator follows a simple rule: move toward
the highest perceived density of individuals. Flock members detect and move away from the predator and exhibit typical patterns, e.g.,
'flash expansion' as individuals move rapidly from the predator as it strikes, 'vacuolation', where expansion results in a cavity
forming around the predator, and the 'split effect', where the group fragments (Couzin et al. 2002).

A murmuration of European Startlings

What is the optimum group size?

• Caraco (1979):
• Model of optimal group size based on time budgets
• Survival = dependent on 2 risks: starvation & predation
• Time budget = 3 behaviors: scanning, feeding, & fighting (for food)
• Optimal group size - may vary from one habitat to another & from individual to another. The group size that maximizes one bird's fitness may not optimize another's (e.g., dominants & subordinates may differ in optimal group size). And, even when there is no dominance & all individuals have same apparent optimal group size, the realized group size may be suboptimal.


Optimal flock size model. (a) As flock size increases, birds spend more time fighting & less time scanning.
(b) At higher temperatures (or when food is more abundant), dominant birds can spend more time attacking
subordinates. The optimal flock size then decreases. (c) When predation risk increases, scanning should increase
and the optimal flock size then increases. Based on Pulliam (1976) and Caraco et al. (1980). From: Krebs and Davies (1987).

 

• Fretwell (1972):

• Because joining or leaving a group implies a change in location, habitat selection & group size selection can be considered simultaneously.
• Individuals always choose habitats & group sizes on the basis of fitness maximization.
• Assumptions:
• Quality of patches is highly predictable
• Cost of moving between patches is negligible
• Patch quality (& individual fitness) declines as numbers increase

Once individual fitness declines so much (because of increased group size) that a bird could do better in a lower-quality patch
(represented by the bottom triangle on the vertical axis), then some flock members should leave the higher-quality patch and move to the lower-quality patch.
With fewer individuals present, birds in the lower-quality patch may have greater fitness than those in the higher-quality patch.

• Optimal group size in dominance-structured groups:

For each group size, the fitnesses of all individuals are shown as separate points. Habitat 2 (triangle) will first be used when the most subordinate individual in Habitat 1 would be better off using the poorer habitat.

Individuals should join flocks based on their ability to dominate others & the current size of groups.

• Dominant & subordinate individuals may have different optimal group sizes.
• Equilibrium group size is not determined by these optima but by a comparison of the fitness of the lowest-ranking member of the group to the fitness the same individual could achieve as a solitary individual in a habitat of poorer quality.

CONCLUSIONS:

What is the optimal group size?

• Differs among individuals, e.g., dominants vs. subordinates, adults vs. juveniles, & males vs. females
• Differs with patch quality
• Differs with variables like predation pressure & time of year

• Temporary feeding aggregations may be optimal or close to optimal
• 'Permanent' or long-lasting groups may not be of optimal size because reproduction can increase group size and accidental death or predation can reduce group size

The group-size paradox: effects of learning and patch departure rules -- Joining groups is thought to enhance foraging success in many species. However, groups are often predicted to be larger than the size that represents the best option for individuals. This is because other solitary foragers in the habitat are expected to continue joining a group until remaining alone or joining the group becomes equivalent alternatives for these foragers. If such a mechanism were pervasive in nature, there would be paradoxically little incentive for group foraging to evolve as a successful strategy. Beauchamp and Fernández-Juricic (2005) proposed a solution to the group-size paradox by allowing foragers to learn about habitat quality. Foragers can learn what to expect in terms of food intake rate through repeated encounters with food patches within their habitat. Foragers are therefore predicted to leave groups when their current food intake rate falls below that expected for the whole habitat. Using a foraging simulation in a virtual habitat, they showed that under a wide range of population sizes, foragers using the above rules abandon under- and overcrowded patches ensuring that group size remains close to the preferred value. Beauchamp and Fernández-Juricic concluded that groups need not become too large when the best outcome for individuals occurs in groups of intermediate sizes. Therefore, there is no need to invoke relatedness between joiners and established group members, group defense against joiners, or other mechanisms that were proposed earlier to prevent groups from becoming too large.

Intake rate as a function of the number of foragers in a food patch. In this example, solitary foragers in a food patch obtain one food item per time step. The addition of foragers to the patch first increases and then decreases food intake rate. The optimal and equilibrium group sizes are shown.

Communal roosting in birds -- Three main benefits are thought to underlie communal roosting in birds: a reduction in thermoregulation demands, a decrease in predation risk, and an increase in foraging efficiency. Beauchamp (1999) investigated interspecific variation in communal roosting tendencies across categories of several ecological factors to examine the relevance of each functional hypothesis in the evolutionary transition to communal roosting and the secondary reversal to solitary roosting habits. The study phylogenetic tree included 30 families and 437 species. Evolutionary transitions to communal roosting occurred more often on branches with flocking species and with larger species but were not associated with diet, territoriality, geographical area, or time of day. The association with flocking activities suggests that increased foraging efficiency, a factor thought to operate through the formation of flocks, may have been a key factor in the origin of avian communal roosting. However, several transitions to communal roosting occurred on branches with nonflocking species, indicating that foraging efficiency may not be the only factor involved in the evolution of communal roosting. Secondary losses of communal roosting habits occurred on several branches, with a concomitant loss of flocking behavior and a tendency to exhibit territorial behavior and nocturnal foraging. Secondary losses suggest that communal roosting is costly to perform and maintain and may be lost when an asocial selection regime operates. Two lines of evidence suggest that communal roosting was costly to maintain and that under asocial conditions the behavior was selected against in favor of solitary roosting. First, most reversals occurred at the species level, which suggests that an asocial selection regime operated only recently, leaving little time for random drift mutations to act so consistently across so many species. Second, roosting behavior is often polymorphic in birds. In polymorphic species, some members of the population roost solitarily, while other members roost communally.

Territoriality and nocturnal foraging could promote an asocial selection regime and be involved in the transition to solitary roosting. Several species that reverted to solitary roosting are specialized foragers holding year-round territories. Joining a distant communal roost to increase foraging efficiency makes little sense in a territorial species and increases the risk that competitors take over the abandoned territory. Similarly, foraging at night can reduce the efficiency of visual recruitment and render group cohesion more difficult to maintain. Joining distant communal roosts can increase commuting costs, and, in the absence of benefits, communal roosting may make little sense for nocturnal foragers. In general, conditions that favor solitary foraging can create an asocial selection regime that could lead to the disappearance of costly social functions.

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