Public information is information about the quality of a resource patch or commodity that can be obtained from the activities of other individuals in that patch. Theoretical benefits of public information use involve faster and more accurate assessment of uncertain commodities and therefore can favor social aggregations. We review empirical evidence of public information use in two contexts: assessment of food patch and breeding habitat quality. Socially foraging Starlings Sturnus vulgaris and Red Crossbills Loxia curvirostra use information from other foragers to decide how long to exploit food patches. Experiments in both the laboratory and field demonstrate that individuals exploit food patches more efficiently if they use public information. A breeding site is another commodity whose quality often must be estimated. Recent studies demonstrate that colonial nesting birds may visit potential breeding sites to obtain information about the current reproductive success of individuals. Such public information is then used by these individuals to select breeding sites adaptively in the future. Public information for resource assessment, derived from conspecific activities, may provide important insight into the evolution and maintenance of many kinds of social aggregations.

The evolution and existence of many kinds of social aggregations has long intrigued evolutionary biologists. Many animals aggregate and thus suffer negative consequences such as increased competition for resources and higher rates of pathogen transmission than if they lived alone (Hoogland & Sherman 1976; Brockman & Barnard 1979; Sutherland & Koene 1982; Duffy 1983; Furness & Birkhead 1984; Barnard & Thompson 1985). Given the existence of many kinds of social aggregations, the benefits of sociality must outweigh these costs. Consequently, much work has been devoted to identifying the positive aspects of social aggregations (e.g., Hoogland & Sherman 1976; Pulliam & Caraco 1984; Barnard & Thompson 1985; Clark & Mangel 1986) because understanding the benefits of social aggregations can provide insight into the evolution and maintenance of sociality.

One potentially substantial benefit of social aggregations involves estimation of environmental resources or commodities (e.g., Clark & Mangel 1984; Valone 1989, 1996; Forbes & Kaiser 1994; Danchin & Wagner 1997). Because of spatial and temporal variation in habitat quality, animals must continually estimate many aspects of their environment (Mangel 1990; Yoerg 1991). For instance, individuals often may not have complete information about: (1) the number of food items in a resource patch (Stephens & Krebs 1987); (2) the fighting ability of a competitor (Parker & Rubenstien 1981); (3) the quality of potential mates (Real 1990); or (4) the quality of a habitat (Orians & Wittenberger 1991; Badyaev et al. 1996). In such cases, individuals can collect information about these resources to estimate their quality and the accuracy of such estimates can strongly affect fitness (e.g., McNamara & Houston 1985; Valone & Brown 1989; Mangel 1990; Nishimura 1994; Giraldeau 1997).

Sociality can provide individuals with access to a source of information about their environment not available to solitary individuals: other individuals. It has long been known that individuals can learn about the location of food resources by observing other individuals, a process called local enhancement (e.g., Thorpe 1963; Krebs et al. 1972; Krebs 1974). Similarly, specific foraging skills can be acquired by observational learning (Herbert & Harsh 1944; Galef 1995; Giraldeau 1997; Templeton 1998). Both local enhancement and the acquisition of novel skills can be observed readily and so it is rather straightforward to determine when one individual has obtained information from another. A subtler transfer of information, however, is also possible: individuals can obtain information from the activities of others to improve their estimates of the quality of a commodity. Such information is known as public information (Valone 1989).

Theoretical studies of public information use show that it can reduce resource estimation time and increase the accuracy of resource estimation, and thus make resource exploitation more efficient (Clark & Mangel 1986; Valone 1989, 1993; Ruxton 1995; Boulinier & Danchin 1997). Consequently, the existence of public information can provide important benefits and thus promote social aggregations. As such, public information use may help explain the evolution and maintenance of social aggregations when commodity assessment is important. Two situations in which commodity assessment, and thus public information use, can strongly influence fitness include food patch and breeding habitat quality estimation. Here we review empirical evidence for the use of public information in these two contexts.

Food patches commonly contain hidden, and therefore unknown, quantities of resources and thus their quality must be estimated. Solitary foragers can derive food patch estimates from both pre-harvest and harvest information (Valone 1991). Pre-harvest information may involve knowledge of the distribution of resources in the environment (e.g., Green 1980) or knowledge about the renewal rate of known resource patches (Valone 1991). Harvest (or sample) information is information obtained during patch exploitation. Such information derived from the activities of an individual is called personal information (Valone 1991). Personal (harvest) information typically involves various combinations of two basic factors: resources harvested and time spent searching within a patch (e.g., Ydenberg 1984; Lima 1985; Kamil et al. 1988; Valone 1992).

Social foragers can use both of the above sources of information to estimate patch quality but they can also use public information to improve their estimate of patch quality. Such public information includes the number of resources harvested by other group members or the unsuccessful search time of others in a patch (Valone 1989).

Two predictions derived from models of public information use have been the focus of empirical tests (Valone 1989). The first prediction involves how foraging success within a patch affects the order of patch departure by individuals. In environments in which resources have a clumped distribution, the more successful a forager within a patch, the higher its estimate of patch quality (Iwasa et al. 1981; McNamara 1982; Green 1988). When a group of foragers exploits a patch in such an environment and does not use public information, then the least successful foragers in a patch will have the lowest estimates of patch quality and therefore should depart the patch before more successful foragers (Valone 1989; Valone & Giraldeau 1993). If, on the other hand, all individuals in the group obtain public information from each other and weight this equally with personal information, then all individuals will possess the same information about patch quality and thus possess identical patch estimates. When public information is used, therefore, individual foraging success of group members within a patch is not related to an individual's patch estimate and hence to the order of patch departure (Valone 1989).

The second prediction of public information use is that a group of foragers using public information should abandon an empty patch faster than a solitary forager and faster than a group of individuals not using public information (Clark & Mangel 1986; Valone 1989). This prediction makes sense because two or more individuals unsuccessfully searching a patch provide more information about patch quality than a single unsuccessful forager. In fact, if individuals weight public and personal patch sample information equally, then in the case of complete transfer of public information among group members, a group of N unsuccessful foragers should depart an unproductive patch before a group of individuals not using public information and N times faster than a solitary forager (Clark & Mangel 1986; Valone 1989).

The earliest tests of public information use focused on the predicted relationship between foraging success and order of patch departure. The first study by Valone and Giraldeau (1993) did not find evidence of public information use. They found that foraging success strongly influenced the order of patch departure in pairs of Budgerigars Melopsittacus undulatus feeding on artificial food patches in the laboratory: unsuccessful foragers departed resources patches before successful foragers. Subsequently, Templeton & Giraldeau (1995) examined the same predictions for small flocks of starlings feeding in the wild. In addition, Templeton & Giraldeau directly manipulated the ability of birds to obtain public information by varying the location of food within cups: when food was deeply recessed, individuals could not observe the foraging success of others on the patch and so public information was not available; when food was not recessed, public information was available because foragers on the patch could observe when others found food. Templeton and Giraldeau found that when public information was not available, the least successful foragers departed first as expected, and similar to the findings for budgerigars. When public information was accessible, however, individual foraging success on the patch no longer predicted order of patch departure as predicted for the case of public information use. Thus the behaviour of the starlings was consistent with the notion that foragers were using public information about the foraging successes of others to assess patch quality when such information was available.

The most recent empirical tests have focused on the prediction that public information use can reduce time wasted on empty patches by speeding patch estimation. Templeton and Giraldeau (1996) observed captive starlings exploiting empty resource patches either alone or with another bird that was trained to sample a patch at either a low or high rate. Focal subjects abandoned empty patches faster when foraging in a group of two compared to foraging alone and departed fastest when paired with a bird that sampled at a high rate. Since there was no food in the patch the only public information available was the unsuccessful probes of the patch by the other forager and this strongly affected patch residence time as predicted.

Smith et al. (1999) have conducted a similar set of experiments using captive Red Crossbills Loxia curvirostra. They examined the sampling behaviour of focal crossbills exploiting empty patches (artificial trees containing pine cones with hidden seeds) paired with a non-foraging partner (i.e., ‘solitary’), or paired with one or two other foragers. They found that focal birds in trios spent significantly less time and sampled fewer empty cones than birds exploiting patches as solitaries or in pairs. In other words, birds in groups of three apparently used public information about the lack of foraging success to reduce time spent on unprofitable patches.

Smith et al. then altered the type of public information available to the focal bird. Specifically they manipulated the quality of the two halves of a patch so that the focal bird and the two partner birds could realise different levels of foraging success on the same patch. When the tree contained no food (information on the two sides of the patch was similar) focal birds departed the tree faster than when they fed alone. But when the partner birds found food on their half of the tree, the focal bird would visit more cones on its half of the tree even though it never found food, i.e., the fact that individuals feeding on the same patch were finding food led the focal bird to continue unsuccessfully sampling the tree. Further, when the focal bird was successfully finding food, but none of the partner birds was successful (because their half of the tree contained no food) the focal bird would leave the patch sooner than when its partners also found seeds. Thus, Smith et al. demonstrated a benefit of public information use when all birds were feeding on a similar patch (both sides of patch/tree identical) and also demonstrated a cost of public information use when the two sides of the patch/tree provided very different information about patch quality. The latter result might be most relevant in situations when group members differ in foraging ability (e.g., as a result of differences in bill size), or when group members forage on large patches whose quality is spatially heterogeneous (cf., Schmidt & Brown 1996). This latter situation suggests that in some cases public information might be important for both patch estimation and to guide intra-patch search behaviour in a manner analogous to how local enhancement is used for inter-patch movements.

The quality of a breeding site can strongly influence fitness (e.g., Fretwell 1972; Cody 1985) and therefore individuals can benefit by generating accurate estimates of the quality of a potential breeding site (Badyaev et al. 1996). The notion of using public information to assess breeding site quality has been modelled for migratory birds by Boulinier & Danchin (1997).

Boulinier & Danchin (1997) envision an environment that consists of breeding patches that differ in quality. Breeding sites within a breeding patch are unlimited and equivalent in quality so that individuals only need to assess breeding patch quality. Boulinier and Danchin explore two ways individuals can assess breeding patch quality. Individuals can use their own prior individual reproductive success (IRS [sensu Switzer 1997]) or they can assess patch quality by noting the reproductive success of other breeders in the patch (PRS [sensu Switzer 1997]). In other words, individuals can rely solely on personal information or they can also use public information to assess breeding patch quality. First time breeders obviously can not use their own prior IRS and so they must either rely on public information to assess breeding patch quality or randomly select a breeding patch. Boulinier & Danchin (1997) show that when breeding patches differ in quality but are somewhat stable from year to year, individuals realise higher lifetime reproductive success by using public information about PRS to estimate breeding patch quality for future nesting decisions than if they did not use such information.

Boulinier & Danchin's (1997) model makes three predictions for the case where individuals use public information to estimate breeding patch quality: (1) the decision to move to a new breeding patch or to re-nest in the same patch next year should be a function of the average PRS in different patches, i.e. the quality of all potential breeding patches should be assessed using public information; (2) individuals that change breeding patches from one year to the next should move to a patch that had higher PRS than the patch in which they bred this year; and (3) if patches differ in average PRS, those patches that had higher than average PRS should grow in size because of a net recruitment of new breeders.

Most empirical support for the idea that individuals use public information of PRS to assess breeding patch quality comes from colonial nesting birds (Danchin and Wagner 1997). Prospecting (or reconnaissance) behaviour occurs in many colonial nesting birds and prospecting individuals often visit many colonies (Reed & Oring 1992; Boulinier et al. 1996; Reed et al. in press). This behaviour involves travelling over a colony and even sitting on the nests of other individuals (Danchin et al. 1991; Cadiou et al. 1994). Prospecting tends to occur near the end of the breeding season when the most accurate information about PRS is available (Boulinier et al. 1996). As such, the function of prospecting behaviour may be to gain information to estimate PRS of different colonies to determine where to breed in the future (Danchin & Wagner 1997). Indeed, prospecting behaviour is related to future breeding behaviour. For example, Reed & Oring (1992) found that prospecting Spotted Sandpipers Actitis macularia were more likely to return to breed at sites where the most eggs were laid the previous year. Similarly, Boulinier (1996) used data of Donazar et al. (1994) to show a positive correlation between colony growth and colony PRS the previous year for Buff-necked Ibises Theristicus caudatus.

In the most recent examination of the use of public information for breeding site selection, Danchin et al. (1998) examined year-to-year nesting decisions of colonial nesting Kittiwakes Rissa tridactyla. To determine whether individuals used public information to assess subcolony quality, they examined cases where personal and public information provided different information about subcolony (breeding patch) quality. In particular, Danchin et al. examined the nesting decisions of birds that experienced a nest failure in one reproductive season. Typically individuals that experience a nest failure (i.e. low IRS) are more likely to abandon their breeding site and nest elsewhere in future breeding attempts (e.g., Marzluff 1988; Switzer 1993; Boulinier & Danchin 1997). A colony nester that experiences a nest failure obtains personal information that indicates a breeding patch is of low quality. If such an individual only uses personal information to assess patch quality, it should move to a new patch for future breeding. However, nest failures can also occur for individuals that breed in high quality patches. Such individuals should not move to a new breeding patch because of the likelihood that they will move to a patch of lower quality (Boulinier & Danchin 1997). Colony nesters that experience a nest failure but use public information about PRS to estimate patch quality should decide to re-nest in the same patch following a nest failure, however, when PRS is high. Danchin et al. evaluated this prediction and found that individuals that had low IRS showed the same tendency as successful breeders to re-nest in the same patch when the PRS in the patch was high. Such behaviour is consistent with the idea that individuals used public information about breeding patch quality, derived from PRS, to decide whether to remain at the same breeding site after an unsuccessful breeding attempt (Danchin et al. 1998).

Additional empirical support for the idea that individuals use public information about the reproductive success of others to make breeding decisions comes from a non-colonial species. For a non-colonial breeder, a breeding patch is a nest location or territory. But, nest locations or territories will differ in quality and this quality can be assessed by noting the reproductive success of breeders in different nesting sites. Zicus & Hennes (1989) showed that Common Goldeneyes Bucephala clangula investigate nest cavities of other individuals during the breeding season, and during the next breeding season preferentially used cavities from which young had fledged the previous year. Such behaviour is consistent with the idea that individuals prospect to obtain information about the quality of nest sites for future breeding (Danchin & Wagner 1997).

We have reviewed evidence from two very different ecological contexts from several different species of birds indicating that individuals acquire information from other individuals to estimate the quality of uncertain environmental resources. In each case, individuals obtain information about the successes and failures of others engaged in fitness enhancing behaviours (energy acquisition and reproduction) to improve their estimate of the quality of unknown commodities (food and breeding patches, respectively). These situations differ most dramatically in the temporal scale of public information use (Boulinier & Danchin 1997). In food patch assessment, individuals acquire and use information over a very small time scale (seconds to minutes) to decide how long to exploit a resource patch. In breeding patch assessment, individuals acquire information over a longer period (days to weeks) and remember such information for future breeding decisions, often many months later (cf. Orians & Wittenberger 1991). The use of public information in these very disparate situations suggests that public information use may be widespread and may occur in a variety of ecological contexts. Public information provides informational benefits for resource assessment and so its use is another benefit of sociality: its existence, therefore, can help to account for two kinds of social aggregations, social foraging and colonial nesting.

The existence of public information use has many exciting implications. For example, Forbes & Kaiser (1994) have explored how public information about breeding habitat can affect another important decision of colony nesters: the decision to initiate a new breeding colony. Many colonial nesting species present a paradox: many adults forego breeding because they can not obtain a breeding site within existing colonies despite the availability of unoccupied, suitable breeding habitat (Birkhead & Furness 1985; Forbes & Kaiser 1994). In other words, individuals appear reluctant to initiate new breeding colonies. Forbes & Kaiser present a theoretical model to account for this observation. A key assumption of the model is that individuals prospect to obtain public information about breeding site (colony) quality. They show how the absence of information regarding PRS from sites that lack breeders might prevent individuals from establishing new breeding colonies because the cost of breeding at a site of unknown quality is prohibitively high. They call this an ‘information barrier’ to the formation of new breeding colonies and such a barrier could explain the aforementioned paradox.

In addition, most work involving public information has assumed that public information is obtained visually (e.g., Templeton & Giraldeau 1995, 1996). But, this need not be the case. Valone (1996) proposed that food-associated calls, specific vocalisations produced after food is found, may in some cases represent auditory public information about food patch quality that can benefit individuals by maintaining the cohesion of a feeding group. In a similar vein, crossbills may emit vocalisations that inform other foragers in a group about the lack of foraging success in a patch [Benkman, personal observations]. This type of public information of food patch quality can benefit individuals by speeding the group's departure from the low quality patches and it could be especially useful when visibility of other group members is restricted (Valone 1996).

And, finally, public information may even play a role in speciation. Smith et al. (1999) have argued that public information will be most useful when foraging efficiencies of flock mates are similar, which in turn favours assortative flocking by foraging phenotype. This could promote reproductive isolation if flock members are preferentially chosen as mates. Smith et al. suggest that such a mechanism could have been critical in the evolution of the different ecological forms or call types of red crossbills.

Because environments are not constant, resource estimation is ubiquitous for many species. We have reviewed empirical evidence of public information use for only two kinds of resources, food and breeding patches. Many other kinds of environmental parameters require assessment: individuals might benefit by using public information to estimate parameters such as the level of predation risk, the quality of potential mates and the fighting ability of opponents (e.g., Franck & Ribowski 1989; Lima & Dill 1990; Dugatkin & Godin 1992; Keeley & Grant 1993; Boulinier & Danchin 1997; Nordell & Valone 1998). Sociality allows individuals the opportunity to acquire public information to improve their estimates of these and other unknown environmental parameters.

In conclusion, the benefits of public information may have played a key role in the evolution of many kinds of sociality and may favour the continued existence of many kinds of social aggregations. Empirical and theoretical work on information use in social species is still in its infancy but findings to date suggest that future work will prove rewarding.

We thank E. Danchin and J. Templeton for insightful comments on an earlier draft.

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