Higher Perches Research Articles

Are high perches in the blackcapSylvia atricapillasong or listening posts? A sound transmission study

Birds often sing from high perches referred to as song posts. However, birds also listen and keep a lookout from these perches. We used a sound transmission experiment to investigate the changes for receiving and sending conditions that a territorial songbird may experience by moving upwards in the vegetation. Representative song elements of the blackcap Sylvia atricapilla were transmitted in a forest habitat in spring using a complete factorial design with natural transmission distances and speaker and microphone heights. Four aspects of sound degradation were quantified: signal-to-noise ratio, excess attenuation, distortion within the sounds determined as a blur ratio, and prolongation of the sounds with "tails" of echoes determined as a tail-to-signal ratio. All four measures indicated that degradation decreased with speaker and microphone height. However, the decrease was considerably higher for the microphone than for the speaker. This suggests that choosing high perches in a forest at spring results in more benefits to blackcaps in terms of improved communication conditions when they act as receivers than as senders.

Individual diet specialization, niche width and population dynamics: implications for trophic polymorphisms

SummaryWe studied a perchPerca fluviatilisL. population that during a 9‐year period switched between a phase of dominance of adult perch and a phase dominated by juvenile perch driven by cannibalism and intercohort competition. We investigated the effects of these population fluctuations on individual diet specialization and the mechanisms behind this specialization.Due to cannibalism, the survival of young‐of‐the‐year (YOY) perch was much lower when adult perch density was high than when adult perch density was low.Both the individual niche breadth (if weighed for resource encounter) and the population niche breadth were highest when adult population density was high and, consequently, individual specialization was highest at high adult perch densities.When adult perch density was low, the abundances of benthic invertebrate and YOY perch were high and dominated the diet of adult perch, whereas the density of zooplankton was low due to predation from YOY perch. At high perch densities, benthic invertebrate abundance was lower and zooplankton level was higher and some perch switched to feed on zooplankton.Our results show that individual specialization may fluctuate with population density through feedback mechanisms via resource levels. Such fluctuations may have profound implications on the evolution of resource polymorphisms.

Seasonality in Diet, Perch Use, and Reproduction of the Gecko Gonatodes humeralis from Eastern Brazilian Amazon

We studied seasonal variation in intersexual dietary overlap, perch use, and reproductive status of female Gonatodes humeralis in Eastern Brazilian Amazon. The diet of G. humeralis consisted primarily of invertebrates, and differed significantly between sexes in the rainy season, when there was also a significant intersexual difference in perch use (males perched higher). Reproductive activity was also more intense during the rainy season. Intersexual diet divergence may be a result of differential perch use, which appears related to intense reproductive activity in the rainy season. By using higher perches, males should increase their chance to find mates as suggested in other lizard species.

Degradation of whitethroat vocalisations: implications for song flight and communication network activities

Abstract Transmission of acoustic signals through the habitat modifies the signals and may thus influence their use in communication. We investigated the transmission of five different types of whitethroat (Sylvia communis) vocalisations, three types of song and two calls. Typical examples were broadcast and re-recorded in a whitethroat habitat with hedgerows and open meadow. We used a complete factorial design with speaker and microphone placed in different natural sender and receiver positions including high perches and song flights. Sound degradation was quantified in terms of signal-to-noise ratio, excess attenuation, tail-to-signal ratio and blur ratio. The results suggest that sound degradation generally increased with distance along a hedgerow, which means that birds here potentially may use degradation in assessing the distance to a vocalising individual. This is unlike the open meadow where the change in degradation with distance was negligible. Surprisingly, song flight relative to perched singing seems not to facilitate transmission of own vocalisations or perception of vocalisations from other individuals, and song flight vocalisations do not transmit differently from other types of vocalisations during song flights. One purpose of song flights might therefore be visual location by potential receivers and surveillance by the territory owner. Source level and degradation differed between the five types of vocalisations in accordance with their functions. Motif song and song flight songs used in attraction of females and/or deterrence of males could transmit through neighbouring territories, whereas the calls and the courtship diving song where a specific individual within or near the territory is addressed had relatively short communication ranges.

Use of Cool Perches by Broiler Chickens

Broilers under commercial conditions might experience relatively high temperatures during summer and leg disorders year round that may be partially alleviated by providing them with access to cooled perches. It is unknown, however, how perch temperature and factors such as height and position of the perch affect perch use. Furthermore, little is known regarding gender effects. Eight thousand 1-d-old, mixed-sex broilers were exposed to three perch treatments to determine preferences for water-cooled perches over ambient temperature perches and preferences for height, location, and temperature section of the perch. The experimental treatments were as follows: 1) three cool perches 15 cm above the floor (Cool 15), 2) three ambient perches 7.5 cm off the floor (Ambient 7.5), 3) three ambient perches 15 cm high (Ambient 15), and 4) control chambers with no perches. Total number of birds perching, their positions, and temperature section within the perch were recorded. The results indicate a strong preference for high perches as birds grow (P < 0.0001). The cooler sections of the perch were utilized more than warmer sections within the cool treatments (P < 0.05). Females showed a stronger tendency to perch than males, particularly within the cool treatment (P < 0.0001). The higher perch use could be one of the reasons for the higher eviscerated body weight found in females with access to cool perches (P < 0.05). Differences in mean body weight were not significant (P = 0.07). Potential beneficial effects of perch access in final body weights needs to be further investigated.

Lentic Green Sunfish Populations in Nebraska Sandhill Lakes

The green sunfish (Lepomis cyanellus) is common throughout Midwestern streams and rivers. However, relatively little is known about lentic populations. Our objective was to relate population characteristics of green sunfish to the fish and invertebrate community as well as the physical and chemical habitat of natural lakes in the Nebraska Sandhills. Green sunfish were collected in 10 of 30 lakes sampled in 1998 and 1999 with trap nets. The number of stock (i.e., 8 cm) and longer green sunfish per trap net (catch-per-unit-effort; CPUE) was variable, ranging from 0.2 to 1,429, but was positively related to yellow perch (Perca flavescens) CPUE. However, bluegill (Lepomis macrochirus) and green sunfish CPUE were not related, likely because only five lakes contained both species. Green sunfish condition, indexed with relative weight [Wr, stock to quality length (8–15 cm)], was positively related to yellow perch Wr [stock to quality length (13–20 cm)]. Green sunfish CPUE was lower in lakes containing largemouth bass (Micropterus salmoides). Green sunfish Wr and size structure were inversely related to Bosmina sp. mean length. In addition, green sunfish condition and size structure were high in lakes with a low proportion of submersed vegetation. These relationships suggest that high quality green sunfish populations coexisted with high quality yellow perch populations and are most likely to occur in lakes with low submersed vegetation coverage.

Perch Height of the Arboreal LizardAnolis nebulosus(Sauria: Polychrotidae) from a Tropical Dry Forest of México: Effect of the Reproductive Season

We studied perch height in a population of the arboreal lizard Anolis nebulosus from Chamela, Jalisco, México. A total of 1697 lizards were captured during 1988 (June through December), 1989, 1990, and 1991 (June through July). Males used higher perches than females, particularly during the reproductive season. At the beginning of the reproductive season (March) males started to use higher perches, which were highest during the peak of reproductive activity. However, perch height use by females did not follow the same pattern. Females used high perches in March before beginning reproduction, lower perches during the reproductive season from June to August and higher perches again in October at the end of reproductive activity. These patterns suggest that reproductive activity is an important factor determining differential perch height use between sexes, with the priority for males being courtship displays and defense of their territory, whereas protection from predators is more important for females. Adults used significantly higher perches than juveniles and hatchlings, and overall perches were significantly higher during the wet than during the dry season. Perch height of females and males increased with body, environmental, and microenvironmental temperatures, suggesting that perch height might play an important role in thermoregulation.

Density Dependent Growth and Size Specific Competitive Interactions in Young Fish

Body size is a major determinant affecting an individual's performance. In this study, four factors affecting an organism's competitive ability as related to body size: the attack rate, the maximum growth capacity, the metabolic demands and the size spectrum of available resources, were investigated for small stages of Eurasian perch (Percu fluviatilis) and roach (Rutilus rutilus). The size dependence of the attack rate on a 0.5-mm cladoceran zooplankton of larval and juvenile perch was estimated and compared with the results from similar experiments for roach. At equal body sizes, roach always had a higher attack rate than perch. In contrast the reverse was the case for maximum growth capacity. The maintenance requirements at the same size were higher for perch than for roach. Based on the above data we were able to gain a mechanistic understanding of the outcome of two field enclosure competition experiments, one between perch larvae and I-yr-old roach and one between perch larvae and roach larvae. In the first experiment, we found strong intraspecific density dependent growth in perch larvae, while the presence of I-yr-old roach had no effect on perch larvae growth. High perch densities had strong negative effects on the zooplankton resource. Due to the larger size of I-yr-old roach and consequently higher metabolic demands, I-yr-old roach showed a stronger negative response in growth to high perch larvae densities than perch despite roach's higher attack rate on zooplankton. In the second experiment, perch larvae were negatively affected by high densities of roach larvae and had a reduced growth at high intraspecific densities. In contrast, the growth of roach larvae was not affected by perch larvae or high intraspecific densities. This difference between species could be related to the simultaneous lower attack rate and higher growth capacity of perch, leading to a higher sensitivity in growth of perch than of roach to decreasing resource levels. Temporal variation in competition intensity was present in both experiments. This variation could be related to the foraging efficiencies and different growth capacities of the competitors and the species composition and size structure of the zooplankton resource. Our study points to the potential for both intra- and interspecific competitive interactions in fish larvae in freshwater systems. Our results also suggest that species specific differences in how foraging, growth and food processing capacities relate to body size are of vital importance for interactions in ecological communities.

The effect of foraging material and perch height on feather pecking and feather damage in laying hens

Feather pecking in laying hens ( Gallus gallus domesticus) may not only cause welfare problems, but also result in economic losses due to feather damage and increased mortality. The present study aimed at testing whether foraging material and perch height have significant effects on feather pecking and feather damage. From week 19 to week 30 after hatching, groups of 14 hens (white `Lohman Selected Leghorn' hybrids) were kept in pens with or without access to polystyrene blocks as foraging material and with low or high perches (45 cm or 70 cm above floor level; 2×2 factorial design; four pens each per housing condition). Time since introduction of the hens into the experimental pens ( P<0.0001) and provision of foraging material ( P<0.002), but not perch height, had significant effects on the rate of feather pecking interactions. Feather pecking was less frequent in pens with than without polystyrene blocks. On the other hand, feather damage in week 30 was significantly more pronounced in pens with low than with high perches ( P<0.05), but not significantly influenced by the provision of foraging material. Hens sitting or standing on the floor were especially likely to elicit feather pecking. Individuals characterised by relatively high rates of feather pecking showed more severe forms of this behaviour. It is concluded that hens should be provided with foraging material and high perches during the laying period to reduce feather pecking and feather damage.

Dietary Overview of Hemidactylus turcicus with Possible Implications of Food Partitioning

A stomach content analysis was conducted on Hemidactylus turcicus, the Mediterranean gecko, from 19 April 1990 to 15 October 1990, on the campus of Stephen F. Austin State University, Nacogdoches, Texas. Geckos (N = 167) were placed into four size groups based on snout-vent-length ( 1.52 m to c 3.05 m, and > 3.05 m). Stomach contents were analyzed and sex, size, and microhabitat groups were compared using Schoener's percent overlap index. Volumet- rically, the most important prey items taken were Orthoptera, Lepidoptera, and Isopoda. Geckos of different size groups showed some differences in diet. The greatest difference occurred between the smallest and largest size groups. A significant positive correlation was found between gecko size and prey size (r = 0.24, P = 0.0008). Differences in the diets of geckos inhabiting different microhabitats were very evident. Geckos occupying high perch sites ate more flying prey while geckos at lower perch sites ate more ground- dwelling prey.

Health care on higher perches.

Health care on higher perches.

Habitat-induced degradation of sound signals: Quantifying the effects of communication sounds and bird location on blur ratio, excess attenuation, and signal-to-noise ratio in blackbird song

The habitat-induced degradation of the full song of the blackbird (Turdus merula) was quantified by measuring excess attenuation, reduction of the signal-to-noise ratio, and blur ratio, the latter measure representing the degree of blurring of amplitude and frequency patterns over time. All three measures were calculated from changes of the amplitude functions (i.e., envelopes) of the degraded songs using a new technique which allowed a compensation for the contribution of the background noise to the amplitude values. Representative songs were broadcast in a deciduous forest without leaves and rerecorded. Speakers and microphones were placed at typical blackbird emitter and receiver positions. Analyses showed that the three degradation measures were mutually correlated, and that they varied with log distance. Their variation suggests that the broadcast song could be detected across more than four, and discriminated across more than two territories. The song’s high-pitched twitter sounds were degraded more rapidly than its low-pitched motif sounds. Motif sounds with a constant frequency projected best. The effect of microphone height was pronounced, especially on motif sounds, whereas the effect of speaker height was negligible. Degradation was inversely proportional to microphone height. Changing the reception site from a low to a high position reduced the degradation by the same amount as by approaching the sound source across one-half or one-whole territory. This suggests that the main reason for a male to sing from a high perch is to improve the singer’s ability to hear responses to its songs, rather than to maximize the transmission distance. The difference in degradation between low and high microphone heights may explain why females, which tend to perch on low brush, disregard certain degradable components of the song.

Anti-predator vigilance in the red-winged blackbird, : do males act as sentinels?

During the nesting season, male red-winged blackbirds frequently scan from prominent perches near active nests. Males engaged in such behaviour appear to be acting as sentinels, guarding their nests against predators. The sentinel hypothesis was tested in three ways. (1) Male response to a simulated predator was observed. On average, males spent significantly more time within 10 m of their nests following predator presentations than at other times during the same day, and many males looked into their nests immediately after the simulated predator was removed. (2) The height and distance distributions of putative sentinel perches near nests that successfully fledged young were compared with those near nests that failed as a result of predation. Successful nests were associated with significantly closer and higher perches than were depredated nests. (3) The effect of artificial sentinel perches on female choice of nest location in a prairie habitat lacking tall perches was observed. Females were more likely to place their nests in areas with nearby artificial perches than in similar areas lacking such perches. These results support the sentinel hypothesis. There was no evidence, however, that more vigilant male red-winged blackbirds were more aggressive in nest defence, or experienced higher pairing and reproductive success than less vigilant males.

Anti-predator vigilance in the red-winged blackbird, Agelaius phoeniceus: do males act as sentinels?

During the nesting season, male red-winged blackbirds frequently scan from prominent perches near active nests. Males engaged in such behaviour appear to be acting as sentinels, guarding their nests against predators. The sentinel hypothesis was tested in three ways. (1) Male response to a simulated predator was observed. On average, males spent significantly more time within 10 m of their nests following predator presentations than at other times during the same day, and many males looked into their nests immediately after the simulated predator was removed. (2) The height and distance distributions of putative sentinel perches near nests that successfully fledged young were compared with those near nests that failed as a result of predation. Successful nests were associated with significantly closer and higher perches than were depredated nests. (3) The effect of artificial sentinel perches on female choice of nest location in a prairie habitat lacking tall perches was observed. Females were more likely to place their nests in areas with nearby artificial perches than in similar areas lacking such perches. These results support the sentinel hypothesis. There was no evidence, however, that more vigilant male red-winged blackbirds were more aggressive in nest defence, or experienced higher pairing and reproductive success than less vigilant males.

Search tactics of a pause-travel predator: adaptive adjustments of perching times and move distances by hawk owls (Surnia ulula)

After having erected artificial perches of different heights in a clear-cut, I studied the effect of perching height on the allocation of search and pursuit effort of 9 individuals of the hawk owl. Hawk owls are diurnal and locate their small mammal prey visually. Horizontal attack and move distances increased with perching height in most individual cases, as well as across cases. Prey detection and giving-up times increased with perching height in half of the individual cases, but not across cases. Attack and move distances increased with perching time in only a few individual cases, and not across cases. Perching height explained more of the variation in attack and move distances than did perching time. Move distance depended on both preceding and subsequent perching heights. When giving up a perch the owls on average moved slightly farther than the distance predicted if travel distance as well as overlap between successive search areas are to be minimized. When the ground was snow-free, distances of successful attacks tended to be shorter than those of unsuccessful attacks. Moreover, perches from which prey were captured, but not perches from which unsuccessful attacks were launched, tended to be lower than perches which were given up. This may suggest that the predator-prey distance is more critical to the probability of capture than to the probability of detection. Longer horizontal attack distances from higher perches suggest that the size of the area effectively searched around each perch increases with the height of the perch. Therefore, longer giving-up times on higher perches indicate allocation of more search time to larger search areas, and longer moves after giving up or before landing on higher perches indicate avoidance of overlap with the larger search areas of these perches. These adjustments of giving-up time and move distance allow hawk owls to forage efficiently.

Left in Space

On April 7, 1984, soaring 257 nautical miles above the Earth, astronaut Terry Hart used the space shuttle Challenger's manipulator arm to float an experiment-laden, 12-sided satellite into proper orbital orientation and then surrendered it to the invisible tethers of Earth's gravitational field. There the laboratory-called the LongDuration Exposure Facility (LDEF) began its silent, vital task of just staying in space. More than 200 investigators from 70 companies, universities and research centers worldwide had loaded this orbiting techno-patchwork with 57 experiments, carried mostly on the 86 experimental trays covering the ends and the sides of the 14-foot diameter, 40-foot-long facility And LDEF did stay in space, stranded there more than four years longer than originally planned. For more than five years now, some of its experiments have probed, measured and monitored cosmic-ray nuclei and other space particles originating at the sun and at more distant stars. Others have studied particles trapped in Earth's magnetic field or collected micrometeoroids and other debris from the solar system's younger days. Many researchers hope to learn what happens to various materials including reinforced epoxy composites, reflective coatings and spacecraft components such as optical fibers and solar cells after long tours in the exotic environment of space. health of human space travelers and the structural integrity of future spacecraft or space stations may depend on such knowledge. NASA's original plan called for LDEF to remain aloft for 12 to 18 months, after which shuttle astronauts would retrieve it. Lacking telemetry for radioing data Earthward, the voiceless laboratory would horde its ever-growing treasury of data until researchers on terra firma got LDEF's experiment-laden trays into their laboratories and started to determine how space conditions had affected their samples. As part of a tightly scheduled space program in which delays seem the norm, a postponement of LDEF's recovery until autumn of 1986 came as no surprise. But on Jan. 28, 1986, Challenger which not only carried LDEF to its high perch but was slated to recover it exploded over the Atlantic Ocean. Seven crew members died, and NASA postponed all future shuttle missions. Five and a half years after LDEF's launch, scientists still await its results. The only information from LDEF so far is its orbital decay, says LDEF program manager E. Burton Lightener of NASA's Langley Research Center in Hampton, Va. Today, as NASA gears up for a year-end retrieval mission with the shuttle Columbia, anxiety runs high. orbit of the powerless spacecraft has dropped so much that if the planned retrieval fails, scientists will lose all their experiments and the data they carry when LDEF instead returns to Earth the hard way by burning up as it tumbles through the atmosphere sometime in early February Such a loss would be especially wasteful now, researchers say, because the value of LDEF data has multiplied with each extra year in orbit.

Avian attendance and foraging at army-ant swarms in the tropical rain forest of Los Tuxtlas, Veracruz, Mexico

ABSTRACTForaging and attendance of birds at army-ant swarm raids were studied in the tropical rain forest of Los Tuxtlas, Mexico. Sixty-eight raiding swarms were intercepted over a four-year period of which 57% were Eciton burchelli and 43% Labidus praedator. A total of 461 birds (37 species/12 families) were recorded at swarms of E. burchelli and 208 birds (34 species/10 families) were recorded at L. praedator swarms. The mean number of bird species detected per swarm was 7.2 at E. burchelli raids and 5.6 at L. praedator raids. Red-throated ant-tanagers (Habia fuscicauda) were most frequently seen at the swarms of both army-ant species. Other important species were the golden-crowned warbler (Basileuterus culicivorus), the white-breasted wood wren (Henicorhina leucosticta) and the Kentucky warbler (Oporornis formosus). Swarms of both army-ant species were active year-round. Birds weighing between 20 and 40 g dominated perches closest to the ground and the central zone of the swarms, richest in animal prey. Birds weighing less than 20 g occupied higher perches and tended to forage in more peripheral zones. At Los Tuxtlas raiding swarms of army ants are important in the foraging ecology of 44 bird species, including 12 North American migrants.

Seasonal Micro-Distribution of Sympatric Anolis Lizards in Haiti

A new cluster census method was used to examine the intra- and interspecific micro-dis- tributions of two syntopic lizards, Anolis cybotes and Anolis caudalis, across seasons (November, June). Numbers of A. caudalis were very stable across seasons; males slightly outnumbered females (1.2 sex ratio). In contrast A. cybotes had a 1:1 sex ratio in November, then numbers of male A. cybotes dramatically declined by June, resulting in a 2:3 ratio. We speculate that this was related to pronounced territorial behavior. Female A. cybotes were more closely associated with males in June (breeding season) than in November, resulting in multiple females per male in the breeding season. There was some partitioning of the structural habitat by species and sex classes which, for the most part, was stable between seasons. In general, the smaller A. caudalis preferred high perches (>2 m) on large trees, while A. cybotes perched under 2 m. Male A. cybotes also had a preference for large trees. Female A. cybotes, on the other hand, used all available vegetative structures without preference, and tended to perch below conspecific males (C = 1 m). Quantification of habitat partitioning, however, indicated much overlap among species-sex classes. Most habitat overlap coefficients were high (.90-.99). Discriminant analysis produced few significant differences between lizard classes, with poor discrimi- nation of niche uniqueness. Anolis cybotes males had the least discrete habitat niche (10% discrimination). The expectation of male A. cybotes (large and aggressive) dominating the smaller A. caudalis did not materialize. Data did not indicate any obvious interspecific competition as niche overlap values did not vary much with season and population density. In June when male A. cybotes are territorial, the habitat niches of other classes of lizards did not change significantly from November values, or they shifted toward that of A. cybotes.

Perch Height Preferences of Male and Female Tree Lizards, Urosaurus ornatus: A Matter of Food Competition or Social Role?

--Many field studies of lizard behavior have reported that males perch higher than females. Some workers have suggested that this difference is a means of eliminating competition between the sexes for food items when food is scarce (food-competition avoidance hypothesis). Other workers have suggested that sexual differences in social behavior, e.g., male territoriality, are responsible (social-role hypothesis). To test these hypotheses, pairs of tree lizards, Urosaurus ornatus, were placed in outdoor enclosures and provided daily with excess food. Their heights on 2-m tall cinder-block posts were recorded throughout the morning hours every 30 min for nine days during and for nine days after the reproductive season. The males perched an average of 29 cm higher than the females during the reproductive season (a significant difference), but only 17 cm higher during the post-reproductive period (not significantly different). Most of the reduction in distance between the sexes was due to the increased perching height of the females. I interpret these data as supporting the social-role hypothesis, and provide several suggestions to explain the increase in perch height of the females after the breeding season. This study was designed to test a pair of hypotheses concerning sexual differences in perch height preference often found among iguanid lizards. In general, males are seen perching higher than are females (but see Simon and Middendorf, 1985). Intraspecific competition for limited food resources has been proposed by some investigators to explain the perch height and size differences seen between the sexes (Rand, 1967; Schoener, 1967). Other workers, however, saw sexual differences in social roles as the explanation (Andrews, 1971; Scott et al., 1976; Stamps, 1977). According to this scenario, males perch higher so that they may survey the area for territorial intruders and for potential mates; females perch lower to search efficiently for nourishment that is needed for egg production (Scott et al., 1976). To test these conflicting hypotheses, Pounds and Jackson (1983) recorded the height at which male and female fence lizards, Sceloporus undulatus, were seen perching in the field. They found that males shifted perch height with season: during the breeding season, males perched significantly higher than did females, but after the breeding season ended, males perched at the same level as the females. They interpreted their findings as supporting the social-role hypothesis. However, they cautioned that it was still possible that the lizards were more food limited during the breeding season (early summer) than after. This could result in males looking for different food items from higher perches during the breeding season. They suggested that further study would be necessary to tease these two hypotheses apart. Using the tree lizard, Urosaurus ornatus, I attempted to do just that. Previous studies concerning perch height preferences were based on observations of basically undisturbed natural populations. In order to control food supply and provide uniform perches, the present study was conducted in semi-natural outdoor enclosures. Thus, by providing excess food, I could eliminate the possibility of competition for limited This content downloaded from 157.55.39.251 on Fri, 13 May 2016 07:15:30 UTC All use subject to http://about.jstor.org/terms

Prey detection in the red-backed shrike ( Lanius collurio): an experimental study

Distance-dependent prey detection was experimentally studied in the red-backed shrike ( Lanius collurio L.). Maggots were presented at varying distances and the time a food-deprived bird scanned for prey before attacking it was measured. The probability of detecting a prey decreased with increasing distance. There was a tendency for less precision in the strike with increasing attack distance. There was a greater probability that the birds would detect a prey at a given distance from a high perch than from a low perch. Prey concealment reduced the detection probability. Detection time for a prey increased significantly with increasing predator-prey distance. Several of the findings are consistent with the predictions of an optimal predator search model (Andersson 1981).