Foraging behaviour of non-breeding Western Sandpipers Calidris mauri as a function of sex, habitat and flocking

Individuals within a population may vary considerably in the way they exploit available food resources. If the sexes differ in the size of their feeding apparatus, there can be differences in foraging behaviour and habitat use, hence one sex may be more susceptible to competition. We examined relationships between sexual dimorphism in bill size and foraging behaviour, and habitat and microhabitat use of non-breeding Western Sandpipers Calidris mauri at Bahía Santa María, northwestern Mexico. Western Sandpipers are sexually dimorphic, with females about 15% longer-billed than males. Males used a higher proportion of pecks, had a lower probing–pecking rate, walked at higher rates, foraged at sites with lower water content, and had greater variation in foraging technique than females. Moreover, males decreased their proportion of pecks and foraged at a higher rate than females when they changed from feeding alone to feeding in flocks, suggesting a greater safety advantage or susceptibility to conspecific interference when flock feeding. We compared behaviour and microhabitat usage in three habitats: brackish flats, mangroves, and cattail marshes. Sex-related differences in foraging behaviour and microhabitat use were consistent among habitats. Birds in brackish flats and mangroves used a higher proportion of pecks, foraged at lower rates and walked at higher rates, and foraged at deeper sites, with a lower proportion of water cover, than those in cattail marshes. Sex-related differences in foraging behaviour and microhabitat should reduce the level of competition between sexes, and may account for sex differences in Western Sandpiper distribution observed between habitats in Bahía Santa María.     

Low Foraging Success of Semipalmated Sandpipers at the Edges of Groups

Variation in foraging success in relation to spatial position in a group is little known in species that feed on mobile prey that can hide or flee upon disturbance by foragers. I examined the foraging success of individuals located either at the edge or at the centre of flocks of semipalmated sandpipers (Calidris pusilla) feeding on a burrowing amphipod (Corophium volutator) during migration stopover in the Bay of Fundy, Canada. The rates of pecking, prey capture and success were lower for individuals foraging at the edge than at the centre of flocks. Edge birds spent more time running and more time flying than centre birds. Edge birds moved away from the centre of the flock and made frequent short flights towards the centre. In contrast, centre birds rarely moved in a specific direction and flew mostly to relocate elsewhere with the whole flock. Sandpiper flocks foraged over a large area in a relatively short amount of time. In addition, amphipod density is high in this habitat. It thus appears unlikely that prey depletion or low food availability at the edges of groups could explain the spatial variation in foraging success. Low foraging success at the edges of flocks thus arose mainly because of time costs related to flock expansion and retraction. The effect of mutual interference among foragers and of predation risk by falcons is discussed with respect to flock expansion and retraction.     

Experimental tail elongation in male Barn Swallows Hirundo rustica reduces provisioning of young, but only in second broods

At the beginning of the breeding season, the outermost tail feathers of 31 male Barn Swallows Hirundo rustica were either shortened by 20 mm, elongated by 20 mm or left unmanipulated. In first broods, the number of feeding bouts (per nestling per hour) by males and females did not differ significantly among experimental groups. However, in second broods, males with elongated tails fed their nestlings less often than males with shortened or unmanipulated tails. Male tail elongation may have been detrimental to flight, making capture of insects more difficult. Females paired to long-tailed males did not compensate for this reduction in feeding by males and their nestlings received less food. Neither feeding rates nor brood size differed significantly between first and second broods. Variation in abundance of large insects at different times of the day (high around noon and low in the morning and afternoon) matched variation in feeding rates, supporting the importance of large insects in the Barn Swallow diet. Different feeding rate patterns in first and second broods by males with elongated tails could have at least three explanations: (1) such males were able to adjust their parental effort to some extent, temporarily compensating for an imposed handicap; (2) they were more sensitive than males in the other experimental groups to late season food shortage (a decrease in numbers of large insects, the main Barn Swallow prey, was observed as the season progressed); and (3) they were more sensitive than males in the other experimental groups to the deterioration in their physical condition after first broods.     

Foraging behavior of migrant warblers in mixed‐species flocks in Venezuelan shade coffee: interspecific differences,tree species selection,and effects of drought

Shade coffee has been identified as an important habitat for Nearctic‐Neotropical migrants during the non‐breeding season, including species of conservation concern such as Cerulean Warblers (Setophaga cerulea). To better understand habitat features important for migrants in shade coffee, we studied the foraging behavior of migrants in mixed‐species flocks at six shade‐coffee farms in the Cordillera de Merida, Venezuela, in 2008–2009 and the El Niño drought year of 2009–2010. We examined interspecific differences in foraging behavior and tree species selection of three foliage‐gleaning migrants, Blackburnian (Setophaga fusca), Cerulean, and Tennessee (Oreothlypis peregrina) warblers, and aerial‐foraging American Redstarts (Setophaga ruticilla). For morphologically similar Blackburnian and Cerulean warblers, we also examined factors influencing foraging rates (attack and movement rates), capture of large prey, and maneuver/substrate type. We found that aerial‐foraging American Redstarts foraged lower, used more aerial maneuvers, showed no tree species selection, and were less likely to forage in flocks than foliage‐gleaners. Although foraging rates were similar for Blackburnian and Cerulean warblers, the three foliage‐gleaners differed in foraging height and use of maneuvers. Cerulean Warblers foraged lower than the other two species, whereas Blackburnian Warblers used the greatest proportion of woody gleans. All three foliage‐gleaners selected Inga spp. (a commonly planted shade tree in shade‐coffee farms) for foraging, and Blackburnian and Cerulean warblers captured a greater proportion of large prey in Inga spp. than in other tree species. During the drought year, Blackburnian and Cerulean warblers captured half as many large prey and used a greater proportion of woody‐gleans. We found that interactions among behavioral, floristic, and environmental drivers influenced the foraging behavior of migrants wintering in shade coffee. Our results support those of previous studies suggesting that migrants partition resources behaviorally during the non‐breeding season, that foliage‐gleaners may benefit from the presence of shade trees, especially Inga spp., in agroforestry systems, and that drought may influence the foraging behavior of foliage‐gleaning migrants, presumably due to reduced prey availability.     

Intrusion into neighboring home range by male Madagascar paradise flycatchers, Terpsiphone mutata: a circumstantial evidence for extra-pair copulation

The seasonal change in home range size and distribution of the male Madagascar paradise flycatcher, Terpsiphone mutata, was studied in deciduous dry forest of the Ampijoroa Forest Station, northwestern Madagascar. Male home ranges were spaced out in the prebreeding period but overlapped during the breeding period; this change resulted from male intrusion into neighboring home ranges. Male intrusion was often observed during the prelaying and laying stages of females in invaded home ranges. These stages are assumed to be in a fertile period of females. Intruder males tended to have longer tails as compared to resident males. Intruder males may be seeking extra-pair copulations. Females may prefer long-tailed males to short-tailed males as extra-pair partners, or long-tailed males may be dominant to short-tailed males. Received: February 17, 2000 / Accepted: April 25, 2000     

THE FORMATION, STRUCTURE AND FUNCTION OF MIXED-SPECIES INSECTIVOROUS BIRD FLOCKS IN WEST AFRICAN SAVANNA WOODLAND

Mixed-species flocks of birds were observed during the wet season (July to September 1975) in savanna woodland in Ghana. Thirty-four flocks contained birds of 56 species in 20 families, including insectivorous, granivorous, and nectarivorous species, using a wide range of foraging methods. Only two species occurred in more than half the flocks. There was no correlation between the number of flocks joined by a species and its abundance in the community. Among insectivores, but not granivores, the species which joined most flocks were those which habitually occurred in the largest single-species groups. All stages of breeding activity were represented by the various members. Some species joined flocks only while these were passing through their territories. Of the two species which were present most frequently, there were no differences between mixed and single-species flocks for Eremomela pusilla, but Parus leucomelas foraged and called on more occasions in mixed flocks than single-species flocks, though the rates of foraging and calling were related only to the number of P. leucomelas present. Groups of P. leucomelas appeared to initiate some flocks by attraction due to their conspicuous wing-bars, active movement, and loud calls. Black-and-white species joined them first, followed by birds of other plumage patterns. The advantages of mixed flocking are thought to be connected with finding patches of the food of bark- and foliage-searching insectivores, which were the only species regularly seen foraging in the flocks. Because of dry season burning which leaves small unburnt patches of savanna, these insect species may share a common, patchy distribution. Birds may also gain protection from predators, and some species probably gain no advantages. The species composition and behaviour of flocks previously recorded elsewhere in African savannas are similar to the Ghana flocks.     

Foraging Trade‐offs between Prey Size,Delivery Rate and Prey Type: How Does Niche Breadth and Early Learning of the Foraging Niche Affect Food Delivery?

Optimal foraging theory suggests that avian parents should prefer the most energetically efficient (largest) prey items when delivering food to offspring at a central place. However, during periods of high demand, selectivity of prey may decline, leading to the delivery of smaller and/or less nutritious items. We compared foraging trade‐offs between great tits (Parus major) which had a wider feeding niche than blue tits (Cyanistes caeruleus). We also compared the foraging efficiency of cross‐fostered young, which had learned the spatial foraging niche and prey size of the foreign species, to that of control conspecifics. Mean delivery rates did not differ between control and cross‐fostered parents of either species but as delivery rates increased, prey size declined for both species and both treatment groups. However, across the range of increasing delivery rates, parents were able to increase the total biomass of prey delivered. Cross‐fostering did not alter the proportion of different prey taxa in the diet, but cross‐fostered birds shifted the size of the prey taken to that of their foster species. Consistent with their broader feeding niche, great tits, but not blue tits, incorporated more unpalatable items (flies) as delivery rates increased. Although great tits foraged less efficiently in the blue tit niche, paradoxically, blue tits seem to deliver more prey biomass when foraging in the great tit niche.     

Disentangling interference competition from exploitative competition in a crab–bivalve system using a novel experimental approach

In predator–prey relationships such as those between crabs and their bivalve prey, interference competition is a topic of intense investigation as it can have profound consequences on the dynamics of both predator and prey populations. However in laboratory experiments – also those on crab–bivalve systems – workers never adequately disentangled interference competition from exploitative competition, as prey depletion was never compensated. Hitherto, experimental studies on crab–bivalve systems lack direct behavioural observations and have provided only indirect and thus inconclusive evidence of interference competition. We studied interference competition in adult male shore crabs Carcinus maenas that foraged on blue mussels Mytilus edulis. We developed a novel type of experimental tank to replenish each consumed mussel, and thus to keep prey levels constant. We conducted two experiments in which we varied number of crabs (1, 2, 4) and number of mussels (first experiment: 4, 8, 16, 32; second experiment: 8, 32, 128) and directly observed the foraging behaviour of crabs (foraging area=0.25 m²). In the first experiment, feeding rates decreased with increasing crab density only at mussel density 16 because both search time and time spent in agonistic interactions increased. At other mussel densities, variation in crab density did not affect feeding rates, possibly because of low statistical power and the narrow range of mussel densities offered. In the second experiment feeding rates decreased with increasing crab density because crabs spent more time in agonistic interactions and handling their prey. Feeding rates increased with increasing mussel density. Overall, crabs spent on average 14–18% of their foraging time in agonistic behaviours, while on three out of 64 occasions feeding rates decreased because mussels were stolen (kleptoparasitism). Concluding, we have shown that interference competition occurs in absence of prey depletion, while conducting direct behavioural observations aid to identify the behavioural processes that underlie interference competition.     

Foraging behaviour of Lesser Sheathbills Chionis minor exploiting invertebrates on a sub-Antarctic island

Summary During winter (May through October) many Lesser Sheathbills Chionis minor at Marion Island in the sub-Antarctic were obliged to leave their preferred foraging habitat in penguin colonies to forage for invertebrates on the island's coastal plain. The study describes factors affecting feeding success, time budgets and predation risk of the sheathbills which exploited these small, patchily dispersed prey. The birds appeared to select prey 1 mm in diameter, and ignore smaller, common invertebrates.Sheathbills were highly selective of foraging habitat. During 17 censuses made through the winter, 97% of the 1,504 birdsightings were at only eight of the 19 available vegetation types. Multiple regression analysis revealed that prey density was the most important criterion in habitat preference, followed by plant canopy height and distance of the habitat from the sea. These variables accounted for 78% of the variance of habitat use. Focal-animal observations in a sample of habitats showed that feeding success was correlated with prey density and distance from the sea. Tall vegetation impeded the locomotion and foraging of sheathbills. The sheathbills reduced predation risk from skuas Catharacta lonnbergi and travelling time by foraging near the shore. The spatial distribution of prey within vegetation types was apparently unimportant in habitat selection.During winter 83% of the sheathbills in the study foraged communally and 98% roosted communally. Flocks occurred only on good quality habitat and flocking probably facilitated habitat selection. Feeding success increased initially with increasing flock size but decreased in flocks greater than 15 birds, which was attributed to localized prey deletions. The sheathbills spent 88% of the daytime foraging; and feeding, looking around and walking comprised 99% of foraging time. Feeding time increased with increasing flock size, looking around decreased but walking was unaffected. Aggression was rare, was unaffected by flock size and did not significantly affect feeding. A probability model showed that sheathbills could greatly reduce predation risk by flocking but the benefits would not improve much in flocks greater than eight birds.The habitat selection, time budgets and feeding success of adults, subadults and juveniles were very similar.The exploitation of terrestrial invertebrates by sheathbills was interpreted as an expansion of the population's trophic niche to tap an underexploited resource on a species-poor island.     

Adaptive variation in the foraging behaviour of Grey Plover Pluvialis squatarola and Whimbrel Numenius phaeopus

Between-site variation in the diet and foraging behaviour of a fixed-method forager, the Grey Plover Pluvialis squatarola , and a versatile forager, the Whimbrel Numenius phaeopus , was examined and compared at nine tropical and south temperate sites. Grey Plover always foraged in a run-stop-search manner but changed the emphasis of components of this behaviour in response to prey type. Whimbrel foraged tactilely or visually and readily changed foraging speed and habitat. Both species ate a variety of prey species depending on their availability. The diet of Grey Plover was dominated by small prey, polychaetes and crabs, and Whimbrel ate mostly crabs, with Whimbrel being the more specialized in prey choice. The broader diversity of prey types consumed by Grey Plover was attributed to limitations imposed by obligate visual foraging, whereas the flexible foraging behaviour of Whimbrel allowed the latter to concentrate their efforts on the most profitable prey. Grey Plover appeared to have a density upper limit, determined by their stereotyped foraging behaviour. Whimbrel densities varied greatly in response to prey type and foraging method. Foraging effort could not be predicted from measurements of instantaneous daytime energy intake rates, and reasons for the lack of this relationship are discussed.     

Age-specific differences in the winter foraging strategies of rooks Corvus frugilegus

Summary Foraging efficiency and intraspecific competition were compared between wild adult and immature rooks Corvus frugilegus with respect to flock size. Behavioural time budgets, and observations of prey selection and prey energetic values revealed that adult rooks in large flocks (> 50 individuals) consumed smaller, less profitable prey, but allocated more time to feeding and fed at a faster rate and with greater success than adults in small flocks. By contrast, immature rooks in flocks of more than 30 individuals allocated proportionally less time to feeding, fed at a lower rate and fed with no increase in success rate than when foraging in smaller flocks. Agonistic encounters and the avoidance of adults by immature rooks appeared responsible for such inefficient foraging. Hence immature rooks showed a preference for smaller flocks (< 50 individuals) with low adult: immature ratios while adults preferred larger flocks (> 50 individuals). We discuss the possible influence of competitive disadvantages on immature rook distribution, flock composition and post-natal dispersal.     

Feeding sites for extractive foraging by the aye-aye,Daubentonia madagascariensis

The aye-aye, Daubentonia madagascariensis, uses its middle digit to tap on woody sources in search of subsurface cavities containing prey. The acoustical properties of these cavities are thought to be important to this percussive foraging, but the contributions of cavity size, configuration, and contents to efficient prey capture are not known. The purpose of this study was to characterize these cavities and their residents. An analysis of foraged trees at two sites in Madagascar revealed that many of the foraged cavities are mines bored by large cerambycid beetle larvae. Apparently cerambycids, as well as inquiline residents of their mines, are major targets of aye-aye foraging behavior. The larvae bore extended mines that course approximately parallel to the long axes of the trees in which they reside. The orientation and large size of the mines offer an acoustical trail that the aye-aye may follow to its prey. © 1995 Wiley-Liss, Inc.     

Diel activity of resident and immigrant waterbirds at Lake Turkana, Kenya

Of the 42 dominant species of waterbirds at Lake Turkana, Kenya, 14 foraged uniformly throughout the day and night, five foraged mostly during the night, five foraged during both the night and day but with diurnal peaks, 17 were exclusively diurnal and only one was exclusively nocturnal. Species with uniform feeding activity usually captured small prey, using tactile or visual plus tactile cues; most diurnal species captured large prey, using visual cues. However, some species which fed mostly at night, or uniformly, relied exclusively on visual cues. We found support from only one species that moonlight influenced foraging activities. Palaearctic immigrants spent significantly more time foraging than partial migrants and residents; they were also smaller and mainly microphagous. Only gulls and terns were restricted to diurnal feeding, presumably by their need to see and capture prey while flying. The other groups were formed by species which foraged uniformly over 24 h or partially by day or night. These patterns indicate that in most waterbirds feeding activities are not basically tied to any phase of the diel cycle. Since most waterbirds display some degree of nocturnal activity, time budget studies based only on diurnal observations are likely to be misleading.     

Fine-scale food hoarding decisions in New Zealand Robins ( Petroica australis ): is inter-sexual competition important?

Competition for cache retrieval is hypothesised to influence food hoarding intensity. Previous work has tested this hypothesis by evaluating food hoarding rates during foraging bouts when animals are exposed to different levels of competition for cache retrieval. Little is known about how competition might influence fine-scale food hoarding decisions within foraging bouts. I evaluated fine-scale food hoarding decisions of New Zealand Robins (Petroica australis) by offering mealworms to competitively dominant males and subordinate females, both when they were alone and when they foraged together. I then compared food hoarding rates of sequentially handled prey between sexes and social conditions by assessing how the total number of prey cached increased with the total number of prey handled. Relationships for solitary females, solitary males and paired males were non-linear, indicating that they were more likely to consume initially handled prey, and increasingly likely to cache subsequently handled prey items. Non-linear rates of food hoarding may result from declines in the energetic value of prey that are consumed and stored internally as birds become satiated. Somewhat differently, the relationship for paired females was linear, indicating that paired females make a single food hoarding decision based on bout-level foraging conditions, which results in constant fine-scale food hoarding rates. Constant food hoarding rates in paired females, which experience the strongest competitive effects of any treatment, suggest that food consumption is consistently more advantageous than food hoarding under these conditions, regardless of satiation level. Overall results from this study indicate that New Zealand Robins continuously update food hoarding decisions according to their competitive environment and satiation levels, resulting in scale-dependent patterns in food hoarding intensity.     

Foraging behavior of Cerulean Warblers during the breeding and non‐breeding seasons: evidence for the breeding currency hypothesis

Birds require additional resources for raising young, and the breeding currency hypothesis predicts that insectivorous species exploit large soft‐bodied prey during the breeding season, but shift to small, likely hard‐bodied, prey during the non‐breeding season. To test this hypothesis, we examined prey use by Cerulean Warblers (Setophaga cerulea), foliage‐gleaning Nearctic‐Neotropical migrants, during the breeding and non‐breeding seasons. We collected data on foraging behavior during the breeding season (including observations of prey items fed to young) in upland mixed‐oak forest in southeastern Ohio in 2009 and 2010 and, during the non‐breeding season, in shade coffee in the Cordillera de Merida, Venezuela, in 2008–2009. Cerulean Warblers captured 7% more large prey (visible prey extending beyond the bill) during the breeding than the non‐breeding season, but foraged at similar rates during both seasons. Large, soft‐bodied prey appeared to be especially important for feeding young. We found that adults delivered large prey on >50% of provisioning visits to nests and 69% of identifiable large prey fed to nestlings were greenish larvae (likely Lepidoptera or caterpillars) that camouflage against leaves where they would tend to be captured by foliage‐gleaning birds. Availability of specific taxa appeared to influence tree species foraging preferences. As reported by other researchers, we found that Cerulean Warblers selected trees in the genus Carya for foraging and our examination of caterpillar counts from the central Appalachian Mountains (Butler and Strazanac 2000 ) showed that caterpillars with greenish coloration, especially Baileya larvae, may be almost twice as abundant on Carya than Quercus. Our results provide evidence for the breeding currency hypothesis, and highlight the importance of caterpillars to a foliage‐gleaning migrant warbler of conservation concern.     

The feeding success of cattle egrets in flocks

The feeding rates of grouped (<1.5 m from conspecifics) and solo (>5 m from conspecifics) cattle egrets (Bubulcus ibis) in loose flocks away from cows were compared, to test the hypothesis that grouped cattle egrets benefit from feeding on prey flushed inadvertently by nearby conspecifics. The flock feeding rates were also compared to those of grouped and solo egrets near cows, to determine the effects of flock membership on feeding rates. Birds in flocks captured prey faster than those with cows, and tended to capture larger prey, but field observations and captive experiments failed to show that the feeding success of flock members was enhanced by the hypothesized ‘beater’ effect. Increases in prey density, however, always resulted in higher feeding rates, so some cattle egret groups may form in response to local concentrations of prey. Prey size may also play a role in group formation, because birds in the field tended to feed at greater distances from their neighbours when larger prey were captured, regardless of prey density. When small groups did form among cattle egrets feeding on relatively large prey, group members occasionally captured prey items that had been discovered by nearby conspecifics. This behaviour was not observed among birds in dense aggregations, which fed on small, highly abundant prey. These data indicate that there is a potential cost associated with feeding too near others unless the prey are relatively small and abundant.     

SEXUAL DIMORPHISM AND DIVERGENCE IN WINTER FORAGING BEHAVIOUR OF THREE-TOED WOODPECKERS PICOIDES TRIDACTYLUS

Measurements of 48 males and 45 females of Three-toed Woodpeckers shot in Norway revealed that the mean lengths of wing, tail, bill and tarsus of males were significantly greater than those of females. Sexual dimorphism was most marked for the bill and tarsus.
Feeding observations of the species from spruce-dominated mixed forests during the October-March period indicated an intersexual partitioning of the foraging niche. The males exhibited a stereotyped foraging pattern of bark scaling low down on the main trunks of dead spruce, whilst females used a more differentiated feeding technique and utilized a greater variety of trunk and branch sizes of dead, decaying and living trees of several different species. Significant intersexual differences were found in tree height preference and in the diameter of trunks and branches. The males foraged almost exclusively on the trunks of trees over 10 m high and over 15 cm in diameter, whilst females often frequented dead spruce, under 5 m high, and foraged on thinner trunks and branches. Foraging height was significantly lower for males than for females.
The relationship between the sexual dimorphism, the intersexual partitioning of the feeding niche and their biological significance, is briefly discussed.     

DIFFERENTIAL COSTS OF A SECONDARY SEXUAL CHARACTER: AN EXPERIMENTAL TEST OF THE HANDICAP PRINCIPLE

The evolution of reliable signaling can be explained by the handicap principle, which assumes that (1) the cost of a signal guarantees its reliability, and (2) cheating is prevented because the cost of a unit of display is greater for low-quality than for high-quality individuals. A test of these two assumptions was performed using manipulations of the length of the outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject to a directional female mate preference. We found that survival decreased with tail elongation and increased with tail shortening of males, supporting the assumption that the secondary sexual character is costly. Naturally long-tailed males were better able to survive with an elongated tail, whereas naturally short-tailed males improved their survival following tail shortening. This observation supports the second assumption of a differential cost of a signal. One mechanism imposing differential costs on sexually signaling barn swallows is foraging. Males with elongated tails captured smaller, less profitable Diptera, whereas males with shortened tails captured large, profitable prey items. The conditions for reliable sexual signaling by the tail ornament of male barn swallows are thus fulfilled.     

Limitation of similarity and feeding habits in three syntopic species of newts (Triturus, Amphibia)

Differences in body size between both species and sex were observed to be close to Hutchinsonian ratios in an assemblage of three species of newts (Triturus italicus, T alpestris and T carnifex) in southern Italy According to feeding, differences in body size were not related with a strict food niche segregation The diet of larger newt included indeed all the prey eaten by smaller newts but showed new prey types Differences between species were more marked than between sexes of the same species, despite a strong sexual dimorphism in body size Larger newts foraged in a wider range of microhabitats and exhibited a greater diversity of foraging tactics Triturus italicus fed essentially on plankton and chironomids in shallow water at the banks of the pond Triturus alpestris foraged also in deeper water, particularly on lsopods in vegetation Prey fallen on the water surface constituted the main part of T carnifex diet According to recent data on newt evolution, speciation consisted in the reduction of body size Our data show that this process has been related with a specialization of the diet and of habitat use     

The advantages of social foraging in downy woodpeckers

I investigated the advantages gained by downy woodpeckers (Picoides pubescens) which join mixed-species winter flocks. Woodpeckers foraging alone showed high levels of vigilance as measured by head-cocking rates, and low feeding rates. Woodpeckers foraging with one or two flock members showed intermediate rates of head-cocking and feeding, while woodpeckers foraging with flocks of three or more birds showed low head-cocking rates and high feeding rates. Although local enhancement and copying may contribute to the woodpeckers' increased foraging efficiency in a flock, these do not appear to be the main factors. As downy woodpeckers spend less time on vigilance, they devote more time to foraging, thereby increasing their foraging efficiency