Oikos 119: 277Á285, 2010 doi: 10.1111/j.1600-0706.2009.17892.x, # 2009 The Authors. Journal compilation # 2009 Oikos Subject Editor: Kenneth Schmidt. Accepted 1 September 2009
Beyond the information centre hypothesis: communal roosting for information on food, predators, travel companions and mates?
Allert I. Bijleveld, Martijn Egas, Jan A. van Gils and Theunis Piersma
A. I. Bijleveld ([email protected]), J. A. van Gils and T. Piersma, Dept of Marine Ecology, Royal Netherlands Inst. for Sea Research (NIOZ), PO Box 59, NLÁ1790 AB Den Burg, Texel, the Netherlands. TP also at: Centre for Ecological and Evolutionary Studies, Univ. of Groningen, PO Box 14, NLÁ9750 AA Haren, the Netherlands. Á M. Egas, Inst. for Biodiversity and Ecosystem Dynamics, Univ. of Amsterdam, PO Box 94248, NLÁ1090 GE Amsterdam, the Netherlands.
Communal roosting Á the grouping of more than two individuals resting together Á is common among animals, notably birds. The main functions of this complicated social behaviour are thought to be reduced costs of predation and thermoregulation, and increased foraging efficiency. One specific hypothesis is the information centre hypothesis (ICH) which states that roosts act as information centres where individuals actively advertise and share foraging information such as the location of patchily distributed foods. Empirical studies in corvids have demonstrated behaviours consistent with the predictions of the ICH, but some of the assumptions in its original formulation have made its wide acceptance problematic. Here we propose to generalise the ICH in two ways: (1) dropping the assumption that information transfer must be active, and (2) adding the possibilities of information exchange on, for example, predation risk, travel companions and potential mates. A conceptual model, inspired by shorebirds arriving at roosts after foraging on cryptic prey, is proposed to illustrate how testable predictions can be generated. The conceptual model illustrates how roost arrival timing may convey inadvertent information on intake rate, prey density, forager state (i.e. digestive processing capacity) and food quality. Such information could be used by naı¨ve or unsuccessful foragers to select with whom to leave the roost at the subsequent foraging opportunity and thus increase foraging success. We suggest that inadvertent information transfer, rather than active information exchange, predominates in communal roosts.
Communal roosting occurs in taxa such as mammals (Lewis A classical way to view the evolution of communal 1995), insects (Yackel Adams 1999) and arachnids (Grether roosting is by costÁbenefit analyses. Costs associated with and Donaldson 2007), but is best known from birds (Eiserer communal roosting can consist of increased exploitative and 1984). In this paper we focus on communally roosting birds interference competition (Grover 1997, Keddy 2001), and define a communal roost as a group of more than two increased likelihood of detection by predators (Page and individuals that come together to rest (Beauchamp 1999). Whitacre 1975, Eiserer 1984), and transmission of patho- The size of roosting groups varies from a few individuals as gens and parasites (Moore et al. 1988, Kulkarni and Heeb seen in house finches Carpodacus mexicanus (Dhondt et al. 2007, Buehler and Piersma 2008). Benefits include reduc- 2007), to roosts of several hundred-thousand individuals as tions in thermoregulation costs (Brenner 1965, Wiersma in some songbirds and shorebirds (Black 1932, van de Kam and Piersma 1994, Hatchwell et al. 2009), safety in et al. 2004, Winkler 2006). Within roosts, individuals can numbers from predation, as well as increased predator be highly site-faithful and consistently use the same resting detection (Lack 1968, Gadgil 1972, Krebs and Davies spot (Eiserer 1984). In shorebirds, Charadrii, communal 1993, Krause and Ruxton 2002), and increased information roosting sites can offer a degree of safety if they are located in on foraging opportunities (Ward and Zahavi 1973). open areas with unobstructed views of their surroundings Contradicting the main viewpoint at that time Á that the (Piersma et al. 1993b, Rogers et al. 2006, Rosa et al. 2006). evolutionary origin of communal roosting was only related In contrast, forest birds such as owls prefer concealed roosts to safety from predators Á Ward and Zahavi (1973) argued in trees (Hayward and Garton 1984, Wijnandts 1984). (1) that roosts and breeding colonies could act as information Some communal roost locations can be used for many years. centres where individuals actively advertise and share Some roosts of starlings Sturnus vulgaris have for instance information on the location of patchily distributed foods, existed for more than 180 years (Davis 1955). Despite and (2) that this advantage was the primary evolutionary extensive study and debate, the evolutionary origin of origin of communal roosting. This became known as the communal roosting remains unresolved (Danchin et al. information centre hypothesis (ICH). However inspiring, 2008). the ICH has been surrounded by controversy and objection,
277 in particular that information benefits were the primary Empirical support origin of communal roosting (Richner and Danchin 2001). To date, several studies have shown that the use of social Since Ward and Zahavi (1973), many studies have information can promote group living (Danchin and examined the possibility of communal roosts acting as Wagner 1997, Danchin et al. 1998, 2008, Brown et al. information centres, but usually fail to provide convincing 2000, Wagner et al. 2000, Dall 2002, Wagner and Danchin evidence (Ydenberg and Prins 1984, Mock et al. 1988, 2003). Nonetheless, the evolutionary origin of communal Richner and Heeb 1995). Generally, local enhancement roosting remains obscure, probably because there is no single (e.g. foragers cueing in on individuals that are already benefit which led to the evolution of communal roosting foraging and visible from the roost) cannot be ruled out (Crook 1965, Beauchamp 1999). The debate on the (Mock et al. 1988, Richner and Heeb 1995). Until recently, evolutionary origin of communal roosting has overshadowed only two studies on breeding colonies found evidence the debate that communal roosts could serve as information consistent with information centre mechanisms (Brown centres. In this contribution we will focus on the information 1986, Waltz 1987), and just one such study is available for centre mechanism of communal roosting and the informa- communal roosts (Rabenold 1987). Nonetheless, several tion benefits individuals could gain. Information is defined elegant and more recent studies now provide observations as ‘‘anything that reduces uncertainty and changes the state that are consistent with the idea that communal roosts act as of the receiver in a potentially functional manner’’ (from information centres. Marzluff et al. (1996) experimentally Jablonka 2002, Danchin et al. 2004, Dall et al. 2005). made some common ravens Corvus corax knowledgeable by Empirical evidence for communal roosts as information releasing them at newly created food sites, and kept others centres is mostly lacking (Mock et al. 1988, Richner and na¨ıve by holding them captive for 2Á30 days. After visiting Heeb 1995). Some argue that the ICH should be a communal roost, dominant knowledgeable birds led roost abandoned all together, because more parsimonious hy- mates to the food sources on several occasions. In another potheses are available (Richner and Heeb 1995, Danchin study on common ravens, Wright et al. (2003) observed and Richner 2001) and the ICH does not describe an that common ravens that discovered carcasses engaged in evolutionary stable strategy (ESS). The evolutionary main- pre-roost display flights and initiated early morning tenance of advertising foraging success to unrelated in- departures. Successive observations suggest that information dividuals can only be explained by reciprocal altruism, on the location of carcasses was obtained by na¨ıve birds that whereby individuals gain and lose in turn (Mock et al. roosted close to the knowledgeable birds. Communal roosts 1988, Richner and Heeb 1995). This, however, is thought of hooded crows Corvus corone cornix also appear to act as an unlikely condition for roosting birds, because roost information centres. Sonerud et al. (2001) created an composition is very dynamic (Conklin and Colwell 2008) unpredictable and ephemeral food distribution and radio- and cheaters would be hard to identify and punish (Trivers tracked 34 hooded crows. They report that na¨ıve birds that 1971). There are several reasons, however, why communal roosted with knowledgeable birds (i.e. birds that had roosts could function as information centres. Modelling discovered food sites on their own) were more likely to studies have provided evidence that even in the presence of find food sites than when no knowledgeable roost mates cheaters some individuals will always keep searching for new were present. Buckley (1997) reports that black vultures food patches as long as a ‘finder’s fee’ exists (Barta and Coragyps atratus benefit from communal roosting because Giraldeau 2001). Additionally, Lachmann et al. (2000) they were able to locate food by following knowledgeable showed that individuals living in groups have more conspecifics to carcasses. information available and at lower costs, and that commu- nal roosting can be an ESS if information cannot be hidden from roost mates. If information is inadvertent, aggrega- Generalising the information centre hypothesis tions could be maintained through information-sharing mechanisms and be evolutionarily stable. On the other The previous section shows that empirical evidence for the hand, there are cases in which information is exclusive and ICH is available only for birds that face very patchy and where the sharing of information has a cost, e.g., active temporary food conditions (i.e. carrion feeding corvids and information transfer by displaying. In such cases, mechan- vultures). Communal roosts as information centres might isms must operate that enable the information provider to also be advantageous under less extreme foraging condi- benefit from providing information (Danchin et al. 2008), tions. Moreover, the empirical evidence mostly focuses on as in the two strategy hypothesis (Weatherhead 1983), or advertent information transfer communicated through the recruitment centre hypothesis (Richner and Heeb 1995) signals. For example, the aerial displays at common ravens or through kin selection (Danchin et al. 2008). roosts. Even if they represent advertisements of foraging More than thirty years after publication of the ICH, its opportunity, such signals might advertise individual quality validation remains unresolved and controversial. Now, and thus increase the chance of finding a high-quality mate advancements in the ‘ecology of information’ and the (Marzluff et al. 1996, Wright et al. 2003). In agreement emergence of an information framework (Danchin et al. with Evans (1982) and Waltz (1982, 1987) information 2004), allow for a reconsideration of the ICH. In the does not need to be actively transferred. Indeed, inadvertent present attempt, we will first review recent empirical information (Valone 1989, Danchin et al. 2004) might be a evidence for the ICH. Second, we will generalise the more general and likely source of information at communal ICH. Finally, we will present a framework to guide future roosts. Inadvertent information can be available as food studies of information use at communal roosts, i.e. a carried in the beak or claws, distended crops, foraging conceptual model that allows for testable predictions. frequency, and body condition (Mock et al. 1988).
278 By its very nature, inadvertent information cannot be withheld by the providers and therefore its use at communal no no roosts can be an ESS (Lachmann et al. 2000). As long as the no yes pay-off for roosting communally outweighs that of solitarily roosting, individuals will return to communal roosts and thereby reveal inadvertent information to the potential benefit of roost mates. Moreover, under various conditions birds may even be forced to return to communal roosts. In coastal areas, for example, shorebirds are forced to retreat to areas not submerged during high tide (van de Kam et al. Ward and Zahavi (1973) maybe Weatherhead (1983) no Danchin et al. (2008) no Bayer (1982), Mock et al. (1988)Richner and and Heeb (1995) Waltz (1982), Waltz (1987) and Mock et al. (1988) 2004, van Gils et al. 2006). Because information at the Weatherhead (1983), Rabenold (1987) communal roost is inadvertent, cheating by withholding rs. information is not possible and punishing cheaters becomes ion ation al. (1988). irrelevant which increases the likelihood of reciprocal behaviour. The principle holds even for large communal s and roosts with variable membership. Given these conditions, we suggest that the ICH should be generalised such that: (1) besides advertent information, communal roosts are information centres for inadvertent information, and (2) communal roosts can be seen as information centres on foraging opportunities, but also on other types of information (e.g. on predators, timing of migration, potential mates, etc.). Mock et al. (1988) suggested a seven-link chain of events to falsify the ICH. Here, we argue this falsification chain should be abandoned, because most of the components do not suffice for falsifying the general concept that communal roosts act as information centres (Table 1). Instead, we propose two essential components which can be used for its falsification: (1) differential success and (2) adjusted beha- viour. First, differential success establishes that successful individuals have information and unsuccessful individuals are in need of this information. Second, information becomes meaningful if it results in behavioural change. Therefore, unsuccessful individuals should adjust their behaviour according to the gathered information. Informa- tion transfer can be falsified if it can be shown that members of a communal roost do not comply with one of these two components. Note that the long-term pay-off (component 7, Table 1) for individuals in communal roosts does not need to be positive for communal roosting to be an ESS as long as the pay-off for roosting solitarily is lower.
Inadvertent social information on foraging There are many ways in which inadvertent social informa- tion may be used by individuals to improve their foraging success. For instance, unsuccessful individuals could benefit from inadvertent social information by following the fattest roost mates to their foraging sites. Besides fatness, any other cue can be used when it is a reliable indicator of foraging success (below, we provide examples based on arrival time at the roost). Unsuccessful foragers can even follow randomly on foraging success. Following should be seen more generally as adjusted behaviour. suppliers could be displaced bybecause the roosting competitively more solitarily able is followers. Such more inferior costly. individuals Toleration could at keepdepends food returning entirely on to sites the communal is total roosts, could costs thus and be not benefits negative, an of but communal essentialexist as roosting component long versus and those of as could of the the possibly solitary pay-off ICH. roosting. function of For as instance, roosting the information solitarily information centres. is pay-off more negative than that of communal roosting, communal roosts can Synchronous departures can be mitigatedetc. by Therefore, weather, it tidal regime, is precise not language, an limited compass essential directions, component variable for predation support risks, nor falsification of the ICH. Even though differential behaviourdifferences between are successful not and a unsuccessfulforaging necessary success. individuals condition. can In An facilitate addition unsuccessful information to individual sharing, active could behavioural information randomly transfer, select unsuccessful a roost roost mates mate could to use follow inadvertent and information. thus increase Á selected roost mates to increase foraging success, assuming juveniles). Unsuccessful individuals can then followan skilled information individuals to centre increase mechanism. foraging success, and thus site fidelity is not essential for that they have an estimate of their relative foraging success. When unsuccessful foragers decide to search for a new food patch, they face the problem that such a patch could be located in every direction from the roost. By following a random roost mate they are more likely to arrive at a food patch, while saving the costs of searching for one. More- departure over, previously unsuccessful birds will have lower site- ICH component Critique References Essential 5 Following As opposed to strict following to the exact foraging site, the general direction that a (successful) roost mate takes can provide informat 6 Toleration7 Pay-off Competitively better foragers could use competitively inferior roost mates to locate food sites. At the The foraging (long-term) site pay-off the of latter information inform use does not need to be positive for communal roosts to function as information centres. This 4 Synchronous 2 Differential3 success Detection Ward and Zahavi (1973) argue that successful foragers actively advertise their foraging success to aid detection by unsuccessful forage Table 1. The power of observational components to falsifying the generalised Information Centre Hypothesis based on the taxonomy proposed by Mock et faithfulness than successful birds and departure directions 1 Site fidelity Innate differences in foraging abilities, such as the efficiency in locating food sites, could exist between individuals (e.g. adult
279 would be skewed towards more rewarding food sites (Waltz Selective learning may help to increase reproductive 1982, 1987). Therefore, the chances of finding a better food success for communally roosting birds when different types site are larger than finding one that is worse in case an of song are available (Catchpole 1986, Hasselquist et al. unsuccessful individual (i.e. a bird with lower than median 1996). Songs of male brown-headed cowbirds Molothrus foraging success) follows a random roost mate to its ater differ between populations, and females are more foraging area. An individual needs only a measure of responsive to male song from their own population than median foraging success in the surrounding environment from other populations. Similarly, female cowbirds devel- and of one’s own success, which might be as simple as body oped a preference for song types typical of their cage mates condition or intake rate at the previously visited patch. It (Galef and Laland 2005). has been shown that animals are able to assess foraging Information on potential predators might be available in success and adjust foraging decisions accordingly (Valone communal roosts as well. This can be especially beneficial 1989, 2006, Galef and Giraldeau 2001, Coolen et al. 2003, when a novel predator enters the habitat. Through fright van Gils et al. 2003a). behaviour from roost mates, such a predator can be identified without direct exposure (Griffin 2004).
Other kinds of social information Supply and demand of information In communal roosts other kinds of social information than Within a communal roost certain members might have foraging information can be available. Here we give some information that others are in need of. The supply and examples of social information that could be available to the demand of information can vary with individual, species, benefit of roost members. time and location. During winter, information on food may As proposed for breeding colonies, communal roosts be more important than information on potential mates. At might function as ‘hidden leks’ where extra-pair copulations the beginning of a breeding season priorities would change. could occur (Wagner 1993), and public information on the Moreover, the higher the benefit of information Á or the quality of mates and the distribution of potential mates can higher the costs of acquiring this information through trial be obtained. This could increase the likelihood of choosing and error Á the higher the demand will be, and the higher the best mate and thereby increase fitness (White 2004). For the adaptive significance of communal roosts as information instance, there is evidence that communal roosting reduces centres. Given the diversity in temporally fluctuating costs of mating and territory acquisition in red-billed information, its sum can provide year-round information choughs Pyrrhocorax pyrrhocorax (Blanco and Tella 1999). benefits in communal roosts. The available information, Indeed, finding a mate at a communal roost might be easier either advertent or inadvertent, can be used without direct than away from it (Møller 1985). After the breeding season, benefit to the information provider. Alternatively, both the the proportion of juveniles within a communal roost might supplier and the receiver(s) of information can directly also provide public information on breeding habitat quality benefit from the exchange of information. In such a case surrounding the roost. Black-legged kittiwakes Rissa tridac- communal roosts could be seen as information markets tyla were able to estimate breeding habitat quality by (Noe¨ and Hammerstein 1994, Seppa¨nen et al. 2007). For observing breeding success of neighbours within a breeding instance, successful foragers might negotiate foraging colony (Boulinier et al. 2008). Members of a communal information with unsuccessful individuals for premium roost could also assess potential opponent quality (Valone roosting positions (e.g. safety from predators sensu Weath- and Templeton 2002). Contests might occur over food, erhead 1983). Or successful common ravens could negoti- territories, nest sites or mates. These contests can be time- ate a share of discovered carcasses with unsuccessful and energy-consuming, and might entail a risk of injury individuals in return for displacement of competitors at (Krebs and Davies 1993). An individual can benefit by the carcass through group size, i.e. gang foraging (Wright obtaining information on fighting ability of possible et al. 2003, Dall and Wright 2009). opponents by observing contests between other individuals (i.e. eavesdropping, Johnstone 2001). Roosting communally can also provide information on Conceptual model: using roost arrival timing as the timing of migration to allow synchronised departures information on intake rate, forager state, and patch (Helm et al. 2006). Carefully timed migrations may decrease and diet choice predation risk (Leyrer et al. 2009), and synchronous In this section we present a framework to guide future Á departures enabling flight in structured flocks (Piersma empirical studies on information use at communal roosts by et al. 1990) Á may help reduce flight costs (Cutts and illustrating how roost arrival timing may convey informa- Speakman 1994, Weimerskirch et al. 2001). The advantages tion on where, with whom and on what to forage. We of synchronized departure and group flight during migration demonstrate the framework by presenting a conceptual can also apply to commuting between roosts and food sites. model based on what we already know about red knots Information on the timing of moult might also provide Calidris canutus in the Dutch Wadden Sea, but it is fitness benefits through its synchronization. Mallards Anas applicable to a wide range of communally roosting species. platyrhynchos are stimulated to moult by the presence of moulting flock mates (Leafloor et al. 1996). Moulting reduces flight ability and thus increases the risk of predation. Where to forage? By moulting synchronously, an individual reduces the risk of The time necessary for individuals to achieve their predation compared to when moulting alone. daily required energy intake depends on their average
280 instantaneous energy intake rate. If we assume that red With whom to forage? knots are time-minimisers (sensu Schoener 1971, for which Currently, it is increasingly acknowledged that Holling’s there is evidence as shown in van Gils et al. 2003b, 2005), type II functional response model ignores an important aiming to collect no more than the amount of energy which constraint that many foragers may face. Most species are they are going to spend, and that the rate of expenditure constrained by their digestive processing capacity before prey does not vary between individuals, then their daily foraging encounter rate or handling time become limiting (Jeschke et time is a good predictor of their energy intake rate. al. 2002). The red knot typically forages on molluscs buried Furthermore assuming that all birds leave the roost together in the sediment of intertidal mudflats (Zwarts and Blomert after high tide such that variation in foraging time is 1992, Piersma et al. 1993b). Once discovered, molluscs are swallowed whole and crushed in their muscular gizzards reflected in roost arrival times, roost arrival time becomes a (Piersma et al. 1993a). The size of the gizzard constrains direct function of intake rate (Fig. 1A). Individuals with digestive processing rates (van Gils et al. 2003b, 2005), high intake rates will return to the roost earlier, because they therefore, intake rate is not only a function of prey density, need less time to suffice their requirement. If a forager faces but also of gizzard size (Fig. 2A). At intermediate and high no other constraints on its intake rate than the rate of prey densities, the variation in intake rate is determined by finding and the rate of handling food, then Holling’s type II gizzard size and not by prey density. In such cases, arrival functional response (Holling 1959) couples the ‘expected time can give an estimate on gizzard size (Fig. 2B). A intake rate’ to an ‘expected prey density’ (Fig. 1B), and thus prediction that follows is that arrival times will be negatively roost arrival times can be indicators of prey density dependent on gizzard sizes. Data from a radio-tracking study (Fig. 1). A prediction resulting from this model would be confirmed the presence of this negative relationship (Fig. that at the subsequent low tide period, unsuccessful foragers 2C, based on van Gils et al. 2005). Additionally, the birds would follow individuals that had arrived at the roost early. leave the high-tide roost together, which is consistent with the assumption that red knots minimise their time at the return to the roost rather than at their departure (Fig. 2C). For species that face a digestive constraint, information on individual state (i.e. digestive capacity) could be useful, for 12 arival time departure time instance, because foraging with individuals in similar state 10 could provide synchronization of behaviour. Given similar prey types and densities, individuals with large gizzards have 8 higher intake rates than individuals with small gizzards. Therefore, individuals with larger gizzards satisfy their 6 required intake quicker than individuals with smaller 4 gizzards, and will return to the communal roost earlier (Fig. 2B). The consequence for an individual with a 2 relatively small gizzard joining birds with large gizzards
Tidal time (hours after high tide) A would be to remain on the food site alone with possible 0 increased predation risk, or return to the roost unsatisfied. 1000 Of course, the question remains, if gizzard size and not food density determines intake rate, why different individuals go ) 800 –2 to different patches. All patches would anyhow yield the
e same intake rate, as determined by a bird’s gizzard size. In v r 600 u case of the knot, the answer to this question lies in the c
c s variation in food quality. i 400 d 's g
Prey density (m n lli 200 Ho On what food type to forage? B Prey can differ in quality (i.e. the amount of energy per 0 gram indigestible shell mass) and patches of low and high 0155 10 2025 30 quality can be found (Fig. 3A). Whenever high-quality prey Energy intake rate (W) is collected at a slower rate than low-quality prey (e.g. Figure 1. (A) conceptual model showing how arrival time of red because densities of high quality prey are lower), this leads knot at communal roosts can give an estimate of past intake rate. to gizzard-size-dependent optimal patch choice (van Gils Arrival time (h after high tide) can be predicted, given that red et al. 2005). Red knots with small gizzards would maximize knots are time-minimisers, leave the roost simultaneously 3 h after their energy intake rate in patches containing slowly high tide, have equal energy requirements of 3.5 W (Piersma et al. collected high-quality prey, while birds with large gizzards 2003), and that 1.93 tidal cycles fit in 24 h. (B) via Holling’s disc would maximize their energy intake rate in patches contain- equation (Holling 1959), linking intake rates to prey densities, ing rapidly collected low-quality prey (Fig. 3B). In such average prey densities encountered by roost mates can be estimated using arrival times at communal roosts (see two such estimates cases, gizzard-size-dependent patch choice would be ex- shown as arrows going from panel A to B). The following pected. Indeed, published data reveal that red knots with parameters were used for Holling’s functional response model: small gizzards forage on patches with high-quality prey, and energy contents equals 300 J per prey, handling time equals 10 s, red knots with large gizzards forage on patches with low- and searching efficiency equals 0.001 m2 s 1. quality prey (Fig. 3C, based on van Gils et al. 2005). Arrival
281 Ho lli 30 ng 's d is A West-Terschelling c c u r N v 20 e Stortemelk
10
Oost-Vlieland Energy intake rate (W) rate intake Energy p ee t M I Wes RICHEL N O CO U 000 M TG H 1 IN OIN G G T OING TIDE Pre 800 TI IDE UTG 15 DE O y density600 (m OU 10 TG 400 OI L e (g) NG 5 TID 200 E E m I D -2 o IN T 0 0 Gizzard siz COMING BALLAST ) o GRIEND PLAAT tr B es la high-tide roosts i uw e Ou Slenk Vl de In schot A 15 5 02km
6 Eat high-quality prey Eat low-quality prey 5 10 15 7 8 9 5 10 11 time Roost arrival 10 6 (hours after high tide) Energy intake rate (W) rate Energy intake B 0 7
8 time Roost arrival 5 9 (hours after high tide) 12 10 11 Energy intake rate (W) 10 B 0 8
6 H low tide Mix 4 L Patch quality Patch C 2 Tidal time (hours after high tide) Tidal 0 3 6 912 C Gizzard size (g) 0 0 5 10 15 Gizzard size (g) Figure 3. (A) summary of typical red knot foraging itineraries in the western Dutch Wadden Sea, the Netherlands. The figure is Figure 2. (A) the functional response taking digestive processing taken from Piersma et al. (1993b). The arrows show red knot tidal capacity into account. At low prey densities red knot intake rate is movements during incoming and outgoing tides. The intertidal constrained by Holling’s disc equation, but at higher prey densities area is indicated by shading and bordered by the mean low-water by digestive processing capacity (i.e. gizzard size). The black line mark at spring low tide. The two high-tide roosts on Richel and indicates an example intake rate as a function of gizzard mass (g) at Griend are indicated in black. A patch containing low quality food a fixed prey density of 100 m 2. (B) the black line in (A) is (L) and a patch containing high quality food (H) are indicated plotted here in a 2-D perspective. Given that individuals are based on van Gils et al. (2005). (B) arrival times as indicators of digestively constrained, gizzard size determines intake rate and can food quality. The predicted intake rates are given for the high be estimated from arrival times (arrows). The digestive constraint quality food patch (dashed line; prey quality 9 J mg 1) and the is empirically derived as: 0.05 q gizzard mass2 (van Gils et al. low quality food patch (solid line; prey quality of 3 J mg 1). 2003b), where q denotes prey quality which equals prey energy When high-quality prey occurs in lower densities than low-quality content divided by shell content, the latter here set at 100 mg per prey, rate-maximizing patch choice depends on gizzard size. In this prey, yielding a value for q of 3 J mg 1. (C) field data of red knot case, red knots with small gizzards (grey surface on the left) should departure and arrival timing from their high-tide roost at Richel. feed at patches with low densities of high-quality prey, while red Independent of gizzard mass, red knots leave Richel about 3 h after knots with large gizzards (white surface on the right) should feed at high tide (open circles9SE). Arrival times back at the roost are as patches containing high densities of low-quality prey. Arrival times predicted qualitatively (panel B): a declining function of gizzard can give an estimate of gizzard size and with that an estimate of mass (filled circles9SE). Quantitatively there is still some food quality. (C) field data of patch choice as a function of gizzard mismatch between the predictions and field data, where birds mass. The high quality prey patch (H) was only visited by red with large gizzards are arriving somewhat later than predicted. knots with small gizzards, and the low quality prey patch (L) was Possibly this is because large gizzards incur higher metabolic costs only visited by red knots with large gizzards. Red knots with which are not included in the simplified predictions. Data taken intermediate gizzards visited both patch L and H (mix). Data were from van Gils et al. (2005). taken from van Gils et al. (2005).
282 timing is a function of gizzard size, and could thus give an then benefit in two main ways. First, knowledgeable estimate of the quality of food that roost mates encountered individuals in a similar state (i.e. body weight) are more (Fig. 3B, also between years it has been found that food likely to have found the optimal food patch given the food- quality determines daily foraging times, van Gils et al. safety tradeoff. Thus, na¨ıve birds following knowledgeable 2007). Given that individuals aim to maximise intake rate, birds can reduce the costs of finding such an optimal patch. thereby minimizing daily required foraging time, na¨ıve or Second, in case of a predator attack individuals with a unsuccessful red knots could find the optimal prey type that relatively large risk of predation (e.g. relatively large body matches their digestive capacity by following informed roost mass, advanced state of primary moult) could avoid being mates in similar state (i.e. gizzard size) during outgoing tide. the least manoeuvrable in a group with increased mortality A prediction would then be that red knots with small risk by foraging with individuals in similar state. gizzards follow individuals arriving at the roost late, but that In this contribution, we have argued to move beyond the individuals with large gizzards would follow individuals original ICH to study the ecological implications of arriving early. information transfer on food, predators, travel companions and mates in communal roosts. We hope our conceptual model stimulates further development of theory (including, Discussion e.g. stochasticity of the environment), generating more quantitatively testable predictions. Many studies have attempted to show support for the ICH by investigating roost-departure timing (Mock et al. 1988, Richner and Heeb 1995). The results, however, remain Acknowledgements Á We like to thank Dick Visser for preparing the equivocal because firm predictions were lacking (Mock figures, and Sjoerd Duijns, Matthijs van der Geest and Ken et al. 1988, Danchin and Richner 2001). Our conceptual Schmidt for helpful and constructive comments. We are also model is oversimplified, but meets our purpose of providing grateful to Ken Schmidt and Sasha Dall for enabling us to a framework to study information use at communal roosts contribute to the Ecology of information feature. and allow for testable predictions. 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