Vigilance during food handling by Oystercatchers Haematopus ostralegus reduces the chances of losing prey to kleptoparasites

Oystercatchers Haematopus ostralegus periodically pause while handling mussels Mytilus edulis to make visual scans. This paper presents evidence that scanning is associated with the high incidence of intra-specific food stealing among mussel-eating Oystercatchers. Scanning increased in frequency as bird density - and the risk of being attacked for mussels increased and the duration of attacks decreased. Additionally, among a sample of individually marked adults, the aggressive dominant birds spent half as much time scanning as the less aggressive subdominants and were also less likely to be attacked. Whereas detecting an attack made no difference to the success with which the dominants defended their mussels, subdominants increased their chances of retaining the mussel if they detected and carried the mussel away from the approaching attacker. The extra time which the less aggressive birds spent in vigilance seems best understood as a tactic for reducing food loss to kleptoparasites.     

Individual and age differences in the feeding ecology of Oystercatchers Haematopus ostralegus wintering on the Exe Estuary, Devon

The habitats and foods used by a sample of marked individuals and by the whole population of Oystercatchers Haematopus ostralegus wintering on the Exe estuary, South Devon, are described. In the spring and early summer, only a few hundred immatures were present. Two thousand adults and several hundred juveniles arrived in late summer from the breeding grounds and remained until the following spring. The majority of the birds were then adults. At low water, most adults fed on the mussel Mytilus edulis beds and ate mussels. Most individuals specialised on this prey and ate little else. Some adults fed on mud-flats and sandflats within the estuary and along the coast, and specialised on a mixture of Nereis diversicolor and Scrobicularia plana or on Cerastoderma edule or Littorina spp. Though some juveniles ate mussels from the time they arrived, most did not. In winter, they took Scrobicularia on mudflats, earthworms Lumbricidae and leatherjackets Tipulidae larvae in fields and Spisula and mussels along the coast. At other times of year they mainly ate Nereis. However, as they grew older, more birds began to specialise on mussels, especially in their second, third and fourth summers when the adults were away. At high water, most birds roosted at the mouth of the estuary or along the coast. However, several hundred fed in the fields in winter with more doing so late in the winter on warm days on Neap tides when less time was available for feeding on the estuary in daylight. Birds feeding on Nereis and Scrobicularia at low water were most often seen in the fields, irrespective of age. Birds feeding on Mytilus and Littorina occurred there rarely, but no bird eating Cerastoderma was seen there. The findings are discussed in relation to age differences in feeding skills and ability to compete successfully with other birds.     

4. SOME BIRDS OBSERVED AT LAKE MENTZ

Randall, R. M. &; Randall, B. M. 1900. The hard-shell4 diet of African Black Oystercatcher chicks at St Croix Island, South Africa. Ostrich 53:157-163.

The hard-shelled diet of African Black Oystercatcher Haematopus moquini chicks was established by monitoring piles of food remains brought to the chicks. Mussels were the dominant prey species constituting about 90% numerically and about 95% by mass. Limpets were the next most abundant prey item, while false limpets, keyhole limpets and whelks were present in small numbers. Transects down the intertidal area showed that African Black Oystercatchers ignored some abundant species such as barnacles and small gastropods. They took most prey species in the same relative proportion that they were represented in transects, but there was evidence of selection for limpets and avoidance of false limpets and whelks. The modal size class of mussels in feeding piles was greater than in transects.     

Individual differences in aggressiveness and food stealing among wintering oystercatchers, Haematopus ostralegus L

Oystercatchers feeding on mussels foraged in small, overlapping areas and aggressive encounters were common amongst all birds irrespective of their method of opening mussels. Individuals varied consistently in aggressiveness. The more aggressive birds were more frequently present on the mussel bed, fed in those parts where food was most abundant, and were more successful in stealing mussels from other birds. Only the more aggressive birds stole more mussels than they had stolen from them, and stealing increased their rate of food intake.     

SEASONAL CHANGES IN BODY-WEIGHT OF OYSTERCATCHERS HAEMATOPUS OSTRALEGUS

The body-weights of Oystercatchers Haematopus ostralegus wintering in Morecambe Bay, north-west England, showed marked seasonal changes between late summer and late winter, with considerable differences apparent between adult and immature birds. An attempt is made to relate these changes to recorded seasonal variations in prey biomass and to the annual cycles of breeding, moult and migration of the Oystercatcher. The mean weight of females invariably exceeded the mean weight of males in samples collected on the same dates, regardless of age. Adults returned from northern breeding areas in very lean condition, with mean weights ranging from 526 g in males to 540 g in females. Mean weight then increased progressively, due mainly to fat deposition, to a peak in March (up to 662 g in males and 675 g in females) around the time of their main departures for breeding. Heaviest birds then exceeded 800 g. Birds migrating to Iceland in spring would need to be of above average weight in March to make the shortest crossing (850 km, 13 h), via Scotland, while Oystercatchers of 700 g and over could probably make a direct flight (1500 km, 25 h) from Morecambe Bay in favourable weather. Breeding weights of British Oystercatchers were similar to those of post-breeders returning to Morecambe Bay in late August. The mean weights of first-year Oystercatchers arriving in August were very low, 449 g in males and 478 g in females. Their weights, and those of second- and third-year immatures, then rose rapidly in autumn, with some fat deposition, and reached mean values ranging between 551 g (males) and 597 g (females) by November-December. Mean weight then fell by 10–17% from December to March returning close to or below the September levels, whereas adults gained a further 6% during these winter months. Summer and autumn weight gains, and the major moult of adults and older immatures, occurred when the biomass of their two staple mollusc preys, Mytilus edulis (mussel) and Cardium edule (cockle), was maximal. Winter loss in mean weight of immatures corresponded with declining prey biomass, suggesting either that they were less efficient than adults in coping with deteriorating winter food supplies, or that they had no need to accumulate further (premigratory) fat reserves. The autumnal increases in mean weight of immatures are interpreted as an adaptation for withstanding adverse feeding conditions in winter. The Oystercatcher appears to be the only wader species in Britain in which adults increase, rather than lose, weight during the winter. This may be a consequence of an early breeding season, but it may be regarded also as a measure of the success Oystercatchers have achieved by specializing on a difficult but plentiful prey source.     

Prey depletion and foraging strategy in the Oystercatcher Haematopus ostralegus

Summary The responses of a population of Oystercatchers to their own depletion of their prey, the edible cockle Cerastoderma edule, have been examined in Strangford Lough, Northern Ireland., Cockle stocks were severely depleted each winter as a result of predation by Oystercatchers and about half the birds present in October in the main study area had left by March: the reduction was greater when the initial population of Oystercatchers was high than when it was low. Oystercatchers were initially widespread around the Lough but tended to aggregate by January into a few good sites. Within each site annual differences in the location of feeding effort were correlated with year to year variation in the location of second winter cockles.Within the feeding areas in a bay the birds apparently hunted in the short-term on the basis of expectation, ceasing to crop, the densest cockle beds once an average yield for the area has been removed; this yield was apparently estimated from the local density of the anvils on which captured cockles were broken open. This effect led to marginal areas being depleted of cockles more rapidly than high density areas, so that the birds gradually concentrated their hunting onto an increasingly restricted area of the bay. The birds initially took only second winter and larger cockles but by late winter cockle densities had fallen so much that smaller cockles were accepted: areas previously abandoned or unused for hunting were then economically viable because of the reduced expectations of the birds and were incorporated into the feeding areas.These results are seen as consistent with Royama's profitability theory.     

Spatial consequences of aggressive behaviour in flocks of oystercatchers, Haematopus ostralegus L.

Oystercatchers were distributed non-randomly on mussel-beds in winter; the loose foraging flocks showed a ‘spaced-out gregariousness”. Aggressive defence of personal space within the flock was evidently not responsible for this spacing, as most attacks were not directed against birds unusually close to aggressors. Mutual avoidance, rather than overt aggression, apparently maintained the observed non-random spacing. Oystercatchers seemed to attack others in order to steal newly-opened mussels, but the likelihood of successfully gaining an illicit mussel in this way declined as the initial distance between attacker and victim increased. In this situation, the risk of falling victim to food theft may make avoidance of conspecifics advantageous.     

The influence of predation by herring gullsLarus argentatus and oystercatchersHaematopus ostralegus on a newly established musselMytilus edulis bed in autumn and winter

Predation by herring gullsLarus argentatus and oystercatchersHaematopus ostralegus was evaluated on a newly established musselMytilus edulis bed on tidal flats of the German Wadden Sea. The mussel bed covered an area of 2 ha and showed a decrease in biomass of 40% in the most densely covered parts from August to January. Synchronously, the extent of the mussel bed was reduced, resulting in a decrease of average biomass of 98% over the whole mussel bed. From the beginning of August 1994 to mid January 1995, the average size of mussels increased from 10.7 to 20.3 mm. The P/B-ratio was 0.68 in August and 0.18 between September and November. Herring gulls and oystercatchers were the most important mussel predators. On average, 266 herring gulls and 63 oystercatchers were present on the mussel bed during one low tide; 34% of the herring gulls and 78% of the oystercatchers were observed to be feeding. Herring gulls fed at a rate of 4.2 mussels per minute and oystercatchers at a rate of 1.3 mussels per minute. While herring gulls took the most common mussel sizes (mean: 20 mm), oystercatchers searched for the largest mussels available (mean: 25 mm). Herring gulls consumed 13 mussels/m² (0.3g AFDW) during one day and oystercatchers 1.7 mussels/m² (0.1 g AFDW). Predation by birds was compensated by 33% of the production. The proportion removed by bird predation amounted to 10% of abundance and to 16% of biomass (including production). Oystercatchers were responsible for 1% of the reduction in abundance and for 3% of biomass. Removal was highest in the most common size classes of mussels, mainly caused by herring gulls. However, the highest proportion of mussels was eaten in the largest size classes, mainly by oystercatchers. *** DIRECT SUPPORT *** A03B6035 00004     

Molluscan predation by Ovalipes punctatus (De Haan) (Crustacea: Brachyura: Portunidae)

Molluscan predation by the three-spot swimming crab was investigated. The dentition of the heteromorphic chelae allowed crushing, shearing, cutting and holding of prey. Laboratory investigations indicated that small mussels and gastropods were crushed, the larger mussels were prized open, and the foot of the larger gastropods shredded and bits removed. Stomach contents of freshly captured crabs indicated that the crabs are selective carnivores and preferred prey species which are not most abundant in situ (crabs from Kings Beach, Donax serra Röding; crabs from Maitlands River Beach, Bullia rhodostoma Reeve). Ovalipes punctatus (De Haan) foraged on a variety of prey and had no upper prey size limit, but the crabs did show preferences for certain prey sizes. Data indicate that the swimming crabs can effectively utilize the entire mollusc populations on the beaches as prey items.     

Prey selection and the impact of the starfish Marthasterias glacialis (L.) and other predators on the mussel Choromytilus Meridionalis (Krauss)

The diet of the starfish, Marthasterias glacialis (L.), consists of a variety of mollusc species, as well as ascidians and barnacles. Starfish densities are maximal where mussels, Choromytilus meridionalis (Krauss), are abundant and in such areas mussels form the bulk of the diet. Laboratory feeding experiments indicate that Marthasterias glacialis select mussels of particular sizes and that the length of prey taken is an increasing function of predator arm length. The time taken to consume each mussel is determined by the ratio of shell length to starfish size. The number of mussels consumed per day increases only slightly with starfish size, but because the prey taken increase in size, energy consumption is maintained at a relatively consistent 1% of predator body energy per day. Using prey selection and feeding rate data for different sized starfish, predictive three dimensional predation surfaces are developed for a natural starfish population feeding on either one or two cohort Choromytilus meridionalis populations. The models indicate that predatory effort should be concentrated on the smallest mussels when a single adult cohort is present, but on recruiting mussels just above the minimum prey size limit where two cohorts are present. Other major predators of mussels, the rock lobster, Jasus lalandii (Milne Edwards), and the whelk, Natica tecta Anton, appear to select similar size-ranges of prey to starfish, despite their differing body forms and feeding methods. Since the juveniles of all three predators can only take small mussels, predator recruitment may well depend upon the successful settlement of strong mussel cohorts. Evidence for such entrainment of predator cohorts to settlements of mussels is presented.     

DIET OF HARTLAUB'S GULL LARUS HARTLAUBU AND THE KELP GULL L. DOMINICANUS IN THE SOUTHWESTERN CAPE PROVINCE,SOUTH AFRICA

Steele, W.K. 1992. Diet of Hartlaub's Gull Larus hartlaubii and the Kelp Gull L. dominicanus in the southwestern Cape Province, South Africa. Ostrich 63:68-82.

Hartlaub's Larus hartlaubii and Kelp Gulls L. dominicanus in the southwestern Cape feed on a wide range of prey species. On average, Kelp Gulls feed on larger-sized prey than do the smaller Hartlaub's Gulls. In an undisturbed environment the preferred foraging habitats of Hartlaub's and Kelp Gulls are rocky shores and open, sandy beaches (where sand mussels Donax serra are common) respectively. However, as a result of man's activities, several new foraging habitats have become available including croplands, fishing harbours and refuse dumps. The provision of supplementary food at these new foraging habitats is likely to be the cause of a recent population increase. Kelp Gull chicks are fed predominantly “natural” prey which indicates that the population increase may not be due to enhanced reproductive success, but to improved post-fledging survival of juveniles, which are known to aggregate at sites where supplementary food is provided.     

THE BEHAVIOUR OF THE HOTTENTOT TEAL

Following recovery and successful rehabilitation, a young Steppe Eagle Aquila nipalensis was tagged with a 45 g GPS satellite transmitter to track its migration and identify potential wintering and summering areas of the species passing through the United Arab Emirates (UAE). The study is part of a larger study on understanding migration of important birds of prey species from the UAE. The satellite-tagged Steppe Eagle was released near the town of Al Ain, UAE on 5 January 2009 and was tracked until 6 November 2010. Two complete spring and autumn migrations were tracked in addition to its onward autumn migration from the UAE. The tagged eagle continued its autumn migration from its release site and reached Yemen after stopovers in Saudi Arabia. Unlike other Steppe Eagles, the bird did not cross the strait of Bab-al-Mandeb and wintered in the area before undertaking its first spring migration. In the second spring migration in 2010, the bird migrated along the Suez–Eilat route and demonstrated a loop migration. The bird spent the summer on the steppes in Kazakhstan, with marked differences in the home ranges between 2009 and 2010, whereas wintering areas used in 2009 and 2010 in Tanzania were overlapping.     

Prey selection and the functional response of sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin)) and blue mussels (Mytilus edulis Linnaeus)

Predators in nature include an array of prey types in their diet, and often select certain types over others. We examined (i) prey selection by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) when offered two prey types, juvenile sea scallops (Placopecten magellanicus) and blue mussels (Mytilus edulis), and (ii) the effect of prey density on predation, prey selection, and component behaviours. We quantified predation rates, behavioural components (proportion of time spent searching for prey, encounter probabilities) and various prey characteristics (shell strength, energy content per prey, handling time per prey) to identify mechanisms underlying predation patterns and to assess the contribution of active and passive prey selection to observed selection of prey. Sea stars strongly selected mussels over scallops, resulting from both active and passive selection. Active selection was associated with the probability of attack upon encounter; it was higher on mussels than on scallops. The probability of capture upon attack, associated with passive selection, was higher for mussels than for scallops, since mussels can not swim to escape predators. Sea stars consumed few scallops when mussels were present, and so did not have a functional response on scallops (the target prey). Rock crabs exhibited prey switching: they selected mussels when scallop density was very low, did not select a certain prey type when scallop density was intermediate, and selected scallops when scallop density was high relative to mussel density. The interplay between encounter rate (associated with passive selection) and probability of consumption upon capture (associated with both active and passive selection) explained observed selection by crabs. Scallops were encountered by crabs relatively more often and/or mussels less often than expected from random movements of animals at all scallop densities. However, the probability of consumption varied with scallop density: it was lower for scallops than mussels at low and intermediate scallop densities, but tended to be higher for scallops than mussels at high scallop densities. When mussels were absent, crabs did not have a functional response on scallops, but rather were at the plateau of the response. When mussels were present with scallops at relatively low density, crabs exhibited a type II functional response on scallops. Our results have implications for the provision of protective refuges for species of interest (i.e., scallops) released onto the sea bed, such as in population enhancement operations and bottom aquaculture.     

Prey selection within a size-class of mussels, Mytilus edulis, by oystercatchers, Haematopus ostralegus

By comparing the shells of those mussels Mytilus edulis that had been opened by oystercatchers Haematopus ostralegus with mussels of similar size that had not been opened, it was shown that oystercatchers that break into their prey by hammering a hole in the shell selected between prey within a size-class. Ventral hammerers selected mussels that were relatively thin on the ventral surface, were brown in colour and carried few barnacles. Dorsal hammerers selected eroded mussels with thin dorsal shells. Stabbing oystercatchers did not select for thin-shelled prey. In conjunction with the great individual variation in flesh content between mussels of the same length, prey size in this case can be only a poor predictor of prey profitability.     

Behavioural components of prey-selection by dogwhelks, Nucella lapillus (L.), feeding on barnacles, Semibalanusbalanoides (L.), in the laboratory

Adult dogwhelks, maintained on mussels for 60 days before experimentation to reduce prior effects of ingestive conditioning and handling skills appropriate to barnacles, and thence deemed “inexperienced”, preferred the largest barnacles presented to them. Juvenile and adult dogwhelks, maintained on barnacles and deemed experienced, preferred barnacles of intermediate sizes that were correlated with the sizes of the dogwhelks. “Inexperienced” dogwhelks penetrated barnacles significantly more often by drilling than by prising, whereas experienced dogwhelks did the reverse. A predominant tendency to prise open barnacles was developed by previously “inexperienced” dogwhelks after they had eaten six to eight consecutive prey. Larger dogwhelks prised open greater proportions of larger barnacles than did smaller dogwhelks. Experienced adult dogwhelks prised open all barnacles of ≈2-mm opercular diameter but only 20% of those ≈ 6 mm in diameter. The preferred barnacles, 4–5 mm in opercular diameter, were of a size that the experienced adult dogwhelks could prise open in 50% of attacks.Dogwhelks inspected barnacles by crawling over them for ≈20 min when the prey were subsequently rejected and ≈30 min when subsequently eaten. Penetration time and ingestion time were linear functions of barnacle diameter, total handling time ranging from 4–26 h for barnacles of 2.5–2.7 mm opercular diameter. Experienced dogwhelks handled barnacles faster than “inexperienced” dogwhelks, largely because they used the quicker method of penetration by prising whenever possible. The yield of flesh per unit handling time was an accelerating function of barnacle size and, with experience, could be increased by a factor of ≈1.2 for barnacles of 7 mm opercular diameter to ≈2.0 for barnacles of 2 mm opercular diameter. The preference of experienced dogwhelks for barnacles somewhat smaller than the largest available, may have reflected the greater frequency with which these prey could be penetrated by the quicker method of prising. Minimizing handling time could be important in nature if there is a risk to feeding dogwhelks of being displaced by competitors. In the laboratory, 12% of dogwhelks where thus displaced.Barnacles of ≈5 mm opercular diameter were estimated to be slightly more profitable than mussels of ≈17.5-mm shell length to a dogwhelk experienced with barnacles, but less profitable to a dogwhelk that has recently fed extensively on mussels. Dogwhelks, therefore, might switch between diets predominately of barnacles or of mussels if prey of comparable profitabilities change drastically in their relative abundance on the shore.     

Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II ('disc equation') formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m-2). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.A multivariate analysis of 468 'spot' estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases. We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested. As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.     

Habitat selection and web plasticity by the orb spider Argiope keyserlingi (Argiopidae): Do they compromise foraging success for predator avoidance?

Abstract Orb web spiders face a dilemma: forage in open habitats and risk predation or forage in closed habitats to minimize risk but at reduced foraging profitability. We tested whether Argiope keyserlingi opts for safer habitats at the expense of foraging success by (i) determining habitat selection indices in open and closed habitats; (ii) marking and releasing individual juvenile, subadult and adults over two 4‐week periods to determine if life‐history stage influences habitat selection; and (iii) determining the biotic and abiotic environmental parameters that relate to A. keyserlingi abundance. We found that A. keyserlingi selected closed habitats. Sedge and anthropogenic structures were selected and trees were avoided. Juveniles were never found in open habitats, most likely because of high postdispersal mortality. Subadults and adults may shift from closed to open habitats while juveniles never shifted habitat. Foliage density, plant height, potential prey abundance, and mantid and bird abundance were correlated with A. keyserlingi abundance, with only bird abundance explaining habitat selection. We measured web capture area, spiral distance (distance between spiral threads) and the number of decoration arms (0, 1, 2, 3 or 4) in the field and did laboratory experiments to test the influence of (i) space and vegetation; (ii) prey abundance; and (iii) web damage, on web architecture. Argiope keyserlingi webs exhibited geometric plasticity by having larger prey capture areas and spiral distances in open habitats. Decoration design did not differ between habitats however. Variation in space availability, air temperature, prey abundance and web damage explained the variations in web architecture. Potential prey size and diversity differed between habitats but prey abundance did not. As large prey may be important for spider survivorship, foraging success appears to be compromised by occupying closed habitats.     

白头鹞迁徙和越冬习性的初步观察

于1993－1995年每年的9月至翌年的4月,在贵州省六盘水市钟山区场坝一带进行猛禽迁徙和越冬习性的定位观察时,获得4号白头鹞Circusaeruginosus标本,为贵州省冬候鸟新纪录。同时,还对白头鹞的迁徙作了初步观察。     

THE INFLUENCE OF PREDATION ON THE COMPOSITION OF FRESH-WATER ANIMAL COMMUNITIES

1. The introduction of Salmo trutta into an artificial pond was followed by great reduction in the numbers of tadpoles, certain beetles and Notonecta, all species to be seen in the open water. Of the species that sheltered more securely in the plant cover, the effect on some was a curtailment of their range; on others there was little reduction of range or numbers, particularly the numbers at the end of a generation. This was attributed to self-regulation of numbers and the creation of a reserve from which losses due to predation by fish on larger specimens could be replenished. 2. Changes in the number of fish in stretches of a stony stream exerted little effect on the Ephemeroptera, but records indicate that Gammarus and fish are rarely numerous in the same stretch. 3. The most abundant invertebrate carnivores in the fishpond wait for prey to come to them; of two others, a leech swims well but has poor seizing organs, a caddis-larva the reverse. The amount consumed by such predators falls rapidly as the prey becomes scarce. Moreover the main source of prey for the common predators is from the small Crustacea which are abundant only in summer and reproduce quickly. This prey thus has properties that prevent much reduction of numbers by predation. 4. On the stony substrata of Lake District lakes, Asellus and Planaria are numerous where conditions are productive, Ephemeroptera and Plecoptera elsewhere. The absence of these insects from productive places is attributed to predation. 5. Planaria cannot move fast and have no efficient seizing organs, but compensate to some extent by laying trails of slime in which prey becomes entangled. As Asellus grows, its chances of being overpowered by Planaria decrease. Planaria, therefore, feed regularly only when prey is abundant. When it is scarce they rest, and they are able to withstand starvation for a long period. 6. Planaria are preyed upon extensively by Odonata, newts and Plecoptera, and the first two keep them out of weedy ponds. The last may keep their numbers low in streams and perhaps also on stony shores of unproductive lakes, though here scarcity of food is important too. In productive lakes predation on flatworms is slight. 7. Protozoa exhibit three relationships between predator and prey, two of which have been seen in larger organisms. Prey avoid predation in cover. Predators cease activity when prey becomes scarce. Prey occurs in isolated colonies which when found are destroyed by the predator, but generally not before some individuals have dispersed and founded new colonies. 8. Only small invertebrates can survive predation by fish in the open water. Many also reproduce rapidly as long as conditions are favourable and enter a resting stage when they are not. Intense predation may eliminate large species. In some ponds in the Colorado mountains salamanders eliminate a large carnivorous copepod, which enables small Cladocera to survive. Absence of the copepod and presence of Cladocera of suitable size for it to feed on enable a Chaoborus larva to co-exist with the salamander. 9. Small planktivorous fish occur in the open water of some African lakes. The great size of these lakes probably makes possible the co-existence of small fish and their predators, but also makes investigation of the relations between the two difficult. 10. When species not previously present have gained access to lakes, the numbers of the native species of fish have often been greatly reduced. The exact nature of the relationship between newcomer and native has, however, not been established because other factors have been varying, observations have been scanty, or records have not been made for long enough. 11. In temporary and very small bodies of water predation is mainly by invading individuals that were reared somewhere else. Characteristic organisms are phyllopods in impermanent pools and mosquito larvae in both types of water, two groups that feed in the open and away from cover, an activity possible only where predation is slight.     

THE LOCOMOTION OF THE LORISES,WITH SPECIAL REFERENCE TO THE POTTO

The stealthy locomotion of the lorises is seen to be directly related to catching prey such as insects and roosting birds. The method of stalking prey contrasts strongly with the quick leaping method of food-catching presented by the galagos. The lorises, by virtue of their food-catching habits, are restricted to true forest, whereas some of the galagos, by the same token, have colonized more open country. This throws some light on the nature of the post-Miocene link between Africa and Asia and is suggested as a reason why the galagos are restricted to Africa whereas the lorises are found in both Africa and Asia.