all behaviour together now Why do birds form groups? At different times, many birds gather in groups, numbering from a handful of individuals to hundreds, thousands or even mil- lions. Some birds gather to breed in colonies but disperse when not breeding; others breed solitarily but come together in flocks at other times of year. Some join groups to feed, others to roost. Some aggregate with members of their own species, while others join forces with different species. To persist, these behaviours should confer advantages on the individuals involved. Based on a draft written by the late Phil Hockey, Peter Ryan explores the factors that drive birds to flock. >

ISAK PRETORIUS

32 african birdlife january/february 2014 group living 33 seabirds ... are colonies provides the additional locked into their benefit of group defence against evolutionary leg- predators that can access the col- ony – a flock of birds can see off a acy of breeding on maurauding crow whereas a single land, forcing them pair would stand little chance. to crowd onto the This explains some unusual nest few, tiny islands mates. Even though Caspian Terns to breed spend a fair amount of time and effort deterring gulls from trying Although colonial species typi- to steal their eggs or chicks, they cally return to the same breeding often breed in Kelp Gull colonies site year after year, some do not. because the more numerous gulls These tend to be species whose help to protect them from more food is unpredictable in space and dangerous predators such as water time. In Africa, such ‘mobile colo- mongooses. And even if a predator nialists’ include the sparrow-larks can’t be deterred by mobbing, there and Wattled Starlings. Conditions is safety in numbers: the predation favourable for breeding occur after risk to an individual nest is less in rain for these arid-zone species, a colony simply because there are One of the main ad- where it is the food that they need many other possible targets. vantages of being in a to provision their chicks rather However, colonial breeding is group is the enhanced than the breeding site that deter- not without its costs. Large groups vigilance against preda- mines where and when they breed. of birds tend to be more conspicu- tors. However, this only Most seabirds breeding along ous, and can attract the attention works if birds can assess continental margins are also colo- of predators. In the case of the vast how many individuals nial breeders, but for a rather dif- colonies of Red-billed Queleas, the in the flock are actually ferent reason from their oceanic birds rely on their sheer numbers keeping watch. As a re- cousins: they gather at sites that to swamp predator demand. The sult many birds, such as are inaccessible to terrestrial pred- close proximity of birds also in- ducks and these Caspian ators. This also explains why many creases the potential for the trans- Terns, have distinctively waterbirds breed together in tall mission of diseases and parasites. coloured eyelids to trees and large reedbeds as well as For example, in recent years there signal when they are on islands and cliffs. Breeding in have been several outbreaks of > asleep. peter ryan (2) Think of spectacular flocks of for birds forming groups have a likely you are to be forced to breed birds and images spring to mind of more biological basis. in groups. Flamingos’ mud nests vast seabird colonies, pink carpets need to last for at least one breed- of flamingos, locust-like swarms of any species aggregate to ing season, but not all types of mud Birds can elect to occur queleas, or spirals of raptors at key breed. For oceanic sea- are up to the task. And they have to singly or in groups, and migration points. In each case, the birds, coloniality is en- breed close to a reliable source of their decision often rests birds gather for different reasons. forcedM by the paucity of breeding food, in an area relatively inaccess­ on their activity. Among The aggregation of migrating rap- opportunities. Vast tracts of the ible to predators. As a result, Africa’s the many different tors is easy to understand because ocean are rich in food for seabirds vast flamingo population breeds flocks shown here, most they typically rely on thermals to but have few islands. Unlike whales, regularly at only a handful of sites. Pied Avocets are tightly gain height with little effort, glid- seabirds and seals are locked into Crab Plovers are another interest- clustered together to ing from one thermal to the next. their evolutionary legacy of breed- ing example. Their dependence on roost, but a few feeding But thermals only form over land, ing on land, forcing them to crowd crabs requires them to breed in sum- birds are away from so raptor migration routes cross onto the few, tiny islands to breed. mer when crabs are most active, but the group. The Cape water bodies at their narrowest Limited breeding habitat also con- the searing heat in the Middle East Shovelers are loosely ag- point. Palearctic-breeding rap- tributes to coloniality in terrestrial forces them to breed underground. gregated, and couldT be tors migrating into Africa avoid species with specific nest require- There are only a few areas where the considered to be a single the Mediterranean Sea by passing ments, such as bee-eaters and mar- sand is not too coarse or dry to pre- flock or several sub- through the Straits of Gibraltar, the tins that breed in sandbanks. vent their burrows collapsing, and groups with scattered Bosphorus and the Sinai Penin­ Indeed, the more specific your so they breed in colonies wherever birds in between. sula. Mostly, however, the reasons breeding requirements, the more there is suitable habitat.

34 african birdlife january/february 2014 group living 35 For many species, bird encounters a patch of high without diluting the advantage of Granivores, such as sharing the need prey density, other birds converge many eyes scanning for predators these Red-billed Queleas, on the area. This association has is to associate with other species get very little water in for vigilance benefits that extend beyond for- that have different food prefer- their food, so most are ALBERT FRONEMAN (2) against predators aging. When in mixed-species ences. This probably explains why compelled to come to The specialised nest avian cholera in Cape Cormorant nearshore fish supplies become aloes, fruiting fig trees, and even more than offsets flocks, Crowned Lapwings reduce many woodland and forest birds water to drink, exposing habitat of bank-nesting colonies, which sometimes spill increasingly scarce, they are forced at artificial food sources, such as the losses caused their vigilance levels (and can thus travel in bird ‘parties’ when not them to a host of avian birds, such as these over to other species breeding in to forage further away, in a phe- behind trawlers. But in some cases spend more time feeding), whereas breeding. In parts of Africa and predators. The risk to by competition with White-fronted Bee- the area. And on the guano islands nomenon known as ‘Ashmole’s foraging efficiency is increased Black-winged Lapwings do not. Asia, drongos act as specialist sen- an individual is reduced eaters, probably en- off Peru, the numbers of ticks be- Halo’. We now know that many when a group of birds works to- other birds This might seem to be a one-sided tries within these flocks, allowing by travelling in groups, courages group nesting, come so high in some years that seabird colonies have more or less gether, such as the synchronised relationship, yet Black-winged Lap- other birds to spend more time and the large numbers but they may also gain seabirds have to abandon their exclusive foraging areas, the size of herding of fish schools by Great their numbers have dwindled, their wings benefit when a predator ap- foraging. However, in the Kalahari, can also confuse their other benefits such as colonies, sometimes only return- which depends on the number of White Pelicans and cormorants. ability to form feeding groups has pears, because most of the effort Fork-tailed Drongos exact a fee for attackers. increased vigilance, ing years later once tick numbers birds in the colony. Little Egrets have a rather less el- been reduced – an example of an expended in driving the intruder their services. When another bird safety in numbers and have decreased. egant version of the same behav- ‘Allee effect’, whereby the very fact away is made by the more aggres- locates a particularly large prey even group mobbing of Finally, the very fact that many iven the competition for iour, using foot stamping and wing that a population is small causes it sive Crowned Lapwings. item, a drongo might cause it to predators. birds breed together places pres- food that occurs around slapping to drive shoals of small to struggle to survive. This further highlights one of the flee by giving a false alarm call, al- sure on food supplies around the breeding colonies, it might fish into the shallows. Groups of Working together can also help key advantages of being in a group: lowing the drongo to swoop down colony. This effect was first de- seemG odd that many birds flock to- African Penguins herd pelagic to locate patchy prey. In highveld reducing the risk of predation. For and steal the spoils. scribed by Philip Ashmole for gether when feeding. Such flocks fish by swimming rapidly around grasslands, Crowned and Black- many species, sharing the need for large colonies of Sooty Terns can form simply because there is schools, concentrating them into a winged lapwings feed together, vigilance against predators more any birds that are other­ breeding on tropical islands. Early a concentration of available food. tight bait ball to make them easier advancing line abreast while than offsets the losses caused by wise solitary come to- in the breeding season the terns For example, many birds aggregate to catch. This was fine when there monitoring the foraging success competition with other birds. And gether to roost. This hunt close to their colony, but as at termite emergences, flowering were plenty of penguins, but as of neighbouring birds. When one one way to reduce competition mayM be for one of three possible >

36 african birdlife january/february 2014 group living 37 reasons. In the same way that for- right Some birds aging flocks diminish the need for huddle together for individual vigilance, communal warmth, such as this roosts may fulfil the same function. trio of European Bee- For example, African Oystercatch- eaters. Their fluffed-out ers are territorial, but only forage plumage is another at low tide. When roosting at high indication that the tide, especially at night, they are at birds are trying to risk from terrestrial predators such retain as much heat as Cape foxes. By roosting together, as possible. they realise the benefits of group vigilance. They further decrease below Black-winged the risk of predation by roosting in Pratincoles feed on sites with extensive, all-round vis- insects flushed by ibility, such as at estuary mouths, ploughing activity. or on promontories that limit the In the past they oc- direction from which predators can curred in vast flocks approach. In the breeding season, associated with brown adults must remain with their eggs LIZ HART locust swarms, but or chicks, and it is at this time of the control of locusts year that most adult oystercatchers Wood-Hoopoes invariably roost birds simply because their relatively has caused their flock are killed by predators. communally in tree cavities, and large surface area causes them to size (and numbers) to Another benefit of roosting in studies show that energy expendi- lose heat much faster at night than diminish. groups is to help keep warm. Green ture at night is lower for members large birds. It is no coincidence that of large groups. Mousebirds also our smallest birds, the penduline- obtain a physiological benefit from tits, roost in groups. roosting together. Experiments on The third possible advantage of captive birds showed that they al- roosting together is that roosts may lowed their body temperature to fall act as ‘information centres’. This at night, going into torpor to save was first mooted by Peter Ward energy. However, this makes them and Amotz Zahavi, who suggested sluggish and easy prey for noctur- that birds like Red-billed Queleas nal predators. Subsequent experi- and Common Starlings that for- ments on wild birds found that their age in small groups during the day body temperatures don’t fall, even gather in large roosts at night in on cold nights, because they huddle part to assess the condition of their together in a ball. By reducing their neighbours. Birds that have done surface area, they greatly reduce the poorly can follow neighbours that rate of heat loss. Indeed, communal have had a better day back to their roosting is more common in small foraging grounds the next morn- WARWICK TARBOTON ing. It’s a nice idea, but despite be- Changing group size as an index of environmental change ing proposed 40 years ago, there is still no compelling evidence that ust as the decision of whether or not to flock depends on the balance between the costs and this behaviour occurs in nature. Jbenefits of group living, so too does the ideal group size. Monitoring changes in bird flocking To conclude, there are numer- behaviour has been neglected as a way to track environmental change. ous reasons why birds form flocks One species that has changed its flocking behaviour over the past century is the Black-winged to breed, forage or roost. In each Pratincole. Although this species probably has become less common, this trend cannot account for case there is a reason, telling a bio- the marked reduction in flock size. Whereas now a flock of more than a few hundred birds is note- logical story. And although many of worthy, accounts from the 19th century described pratincoles ‘darkening the air’ and ‘coming in their these stories are fairly well under- millions’. This change in behaviour presumably is linked to changes in the swarming behaviour of stood, some are still far from being locusts. A newspaper account in 1870 described how millions of pratincoles associated with a swarm resolved. One of the simplest ob- of locusts caused ‘a continuous shower of locusts’ wings falling on the ground’. Locust control meas- servations of bird behaviour is less ures have led to smaller, more frequent locust outbreaks, forcing the pratincoles into smaller groups. well understood than the Theory of We predict the opposite trend among granivorous birds, where flock sizes might increase thanks to Relativity. Maybe that’s what drives increasingly large areas of cereal crop monoculture. biologists to try to understand it. 

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