Food niche segregation between the Malachite Kingfisher,Alcedo cristata,and the Pied Kingfisher,Ceryle rudis,at Lake Nokoué, Bénin

Several species of kingfisher occur on Lake Nokoué, southern Bénin, including Malachite (Alcedo cristata) and Pied Kingfishers (Ceryle rudis). Here, we compare their diet and estimate the degree of overlap in food niche by analysing contents of regurgitated pellets collected near nesting sites of Pied Kingfishers or inside the nest chambers of Malachite Kingfishers. Characteristic fish skull bones were identified using a reference collection of local fish skeletons. Malachite Kingfishers feed most frequently on fish that occur around floating vegetation, mainly Kribia sp. (56%), Hemichromis fasciatus (28%) and Sarotherodon melanotheron (8%). Important differences were found between different pairs, and between adults and nestlings, the latter being fed almost exclusively on Kribia sp. Larger fish are fed to nestlings than are eaten by the adults. Pied Kingfishers prey upon 14 different fish species, some of them being caught in the pelagic region of the lake, particularly clupeids taken by hovering. By comparison with Malachite Kingfishers, Pied Kingfishers feed on a wider diversity of prey, and take larger fish, so that the dietary overlap between the species is relatively low (O = 0.181).     

THE PROBLEM OF NOMENCLATURE

Whitfield, A. K. &; Blaber, S. J. M. 1978. Feeding ecology of piscivorous birds at Lake St Lucia, Part 3: Swimming birds. Ostrich 50:10-20. The diets, foraging periodicities and feeding behaviour of the Reed Cormorant Phalacrocorax africanus, Whitebreasted Cormorant Phalacrocorax carbo and White Pelican Pelecanus onocrotalus were studied at Lake St Lucia, Natal, South Africa, during 1975 and 1976. The Reed Cormorant fishes in shallow water within 100 m of the shore and mainly caught Sarotherodon mossambicus and Solea bleekeri, while the Whitebreasted Cormorant caught Mugilidae, Rhabdosargus sarba and Thryssa vitrirostris in deeper water. The diet of the White Pelican followed three distinct phases: a pre-incubation phase when the birds followed and preyed heavily on migrating Mugil cephalus shoals; an incubation and post-incubation phase when the adults flew a round trip of 200 km to the north to obtain freshwater fish, mainly cichlids, from the Pongolo pans where fishes were concentrated and densities high; and a post-fledgling phase when both adults and juveniles fed on a variety of marine species of fish in Lake St Lucia for about a month before dispersing to other areas. The feeding and breeding of Reed Cormorants and White-breasted Cormorants is discussed in relation to wind speeds, water turbidity and flooding of backwaters. The diet and long-range foraging behaviour of White Pelicans at St Lucia are compared with data from other African lakes. The breeding season of White Pelicans at St Lucia is related to availability of fish and inaccessibility of the breeding site to predators. The latter is determined by lake levels.     

CORMORANT AND DARTER PREY SIZE SELECTION UNDER EXPERIMENTAL CONDITIONS

Hustler, 1995. Cormorant and Darter prey size selection under experimental conditions. Ostrich 66:109-113.

The most profitable sized cichlid fish for Reed Phalacrocorax africanus and Whitebreasted Cormorants P. carbo lucidus and Darter Anhinga melanogaster were determined experimentally. A range of sizes was fed to all three species in captivity to determine whether, given a choice, they would choose the most profitable fish. All species chose certain sizes of fish preferentially, but not in accordance with their profitability. The hunger status of the birds at the time of the experiment and the way in which the experiments were designed and carried out probably influenced the size of fish chosen by the birds.     

SOME NOTES ON THE BIRDS OF THE KAOKOVELD

Whitfield, A. K. &; Blaber, S. J. M. 1978. Feeding ecology of piscivorous birds at Lake St Lucia, Part 1: Diving birds. Ostrich 49:185-198.

The diets of three species of diving piscivorous birds at Lake St Lucia, Natal, South Africa during 1975 and 1976 are described and related to availability and abundance of different prey species. Mugilidae and Clarias gariepinus were the chief prey of the Fish Eagle Haliaeetus vocifer, while Johnius belengcrii and Sarotherodon mossambicus were, respectively, the main food items of Caspian Terns Hydroprogne tschegruva, and Pied Kingfishers Ceryle rudis. The methods of prey capture used by the three species of birds are described.

Predation on particular species of fish was related primarily to their swimming depth, abundance, size and distribution in littoral areas. Fishing habits, densities and foraging periodicity of the birds are discussed with regard to physical parameters such as thermals, wind speeds and water turbidity. The timing of breeding seasons at Lake St Lucia is related to fish densities, lake levels, turbidity of the water and air temperatures.     

The geographic variation and potential risk of DDT in the blood of Pied Kingfishers from northern KwaZulu-Natal,South Africa

Evans, S.W. & Bouwman, H. 2000. The geographic variation and potential risk of DDT in the blood of Pied Kingfishers from northern KwaZulu-Natal, South Africa. Ostrich 71 (1 & 2): 351–354.

DDT has, since 1946 been used in the intradomicilliary control of malaria in northern KwaZulu-Natal. The Pied Kingfisher was selected as representative for organisms in relatively high trophic levels. Blood was obtained from Pied Kingfishers at Kosi Bay (n = 5), Pongolo Floodplain (n = 13), Mkuzi Nature Reserve (n = 4), Ndumu Nature Reserve (n = 4) and St Lucia (n = 3), extracted and analysed [SWEl] via gas chromatography. The highest blood DDE and σDDT concentrations were obtained for the birds from the Pongolo Floodplain (means of 95.92 μg 1^?1/107.01 μg μg 1^?1) and Kosi Bay Nature Reserve (means of 189.09 μg 1^?l/241.8 μg 1^?1). DDT was detected in the blood of Pied Kingfishers from Kosi Bay (mean 47.14 μg 1^?1) and Pongolo Floodplain (mean 44.34 pg 1^?1) only. This indicated their proximity to DDT application and the greater influx of DDT and its metabolites into the water component of these systems. The EDDT plasma concentrations in the Pied Kingfisher blood were calculated by multiplying the blood values of σDDT by 1.8. Using the regression, log₁₀Y = 0.7785 + 0.8593 (log₁₀X), relating the σDDT in eggs to σDDT in plasma of American Kestrel Falco sparverius it was possible to calculate the mean Pied Kingfisher egg σDDT concentration. The approximate mean Pied Kingfisher egg concentration of σDDT was calculated at 2.26 mg kg^?1 for Kosi Bay and 1.24 mg kg^?1 for the Pongolo Floodplain. Using the highest calculated plasma value of σDDT for Kosi Bay and the Pongolo Floodplain indicated that egg σDDT concentrations could be as high as 4.01 mg kg^?1 and 4.17 mg kg^?1 respectively. These calculated levels may be significant when compared to levels of DDE, known to have a detrimental effect, in the eggs of the Brown Pelican Pelecanus occidentalis, where a concentration of 2.5 to 3 mg kg^?1 was associated with substantially impaired reproductive success. The highest calculated egg concentration was nearing this level and it is therefore possible that the Pied Kingfisher population may be at risk.     

Sex‐specific prey partitioning in breeding piscivorous birds examined via a novel,noninvasive approach

Piscivorous birds frequently display sex‐specific differences in their hunting and feeding behavior, which lead to diverging impacts on prey populations. Cormorants (Phalacrocoracidae), for example, were previously studied to examine dietary differences between the sexes and males were found to consume larger fish in coastal areas during autumn and winter. However, information on prey partitioning during breeding and generally on sex‐specific foraging in inland waters is missing. Here, we assess sex‐specific prey choice of Great Cormorants (Phalacrocorax carbo) during two subsequent breeding seasons in the Central European Alpine foreland, an area characterized by numerous stagnant and flowing waters in close proximity to each other. We developed a unique, noninvasive approach and applied it to regurgitated pellets: molecular cormorant sexing combined with molecular fish identification and fish‐length regression analysis performed on prey hard parts. Altogether, 364 pellets delivered information on both, bird sex, and consumed prey. The sexes differed significantly in their overall prey composition, even though Perca fluviatilis, Rutilus rutilus, and Coregonus spp. represented the main food source for both. Albeit prey composition did not indicate the use of different water bodies by the sexes, male diet was characterized by higher prey diversity within a pellet and the consumption of larger fish. The current findings show that female and male cormorants to some extent target the available prey spectrum at different levels. Finally, the comprehensive and noninvasive approach has great potential for application in studies of other piscivorous bird species.     

Foraging strategies of Great Cormorants Phalacrocorax carbo carbo wintering north of the Arctic Circle

This study describes how 30 Great Cormorants Phalacrocorax carbo carbo managed to catch sufficient food for their daily energetic needs under conditions of reduced daylight and cold while wintering north of the Arctic Circle. Activity observations showed that the Great Cormorants' daily foraging pattern was generally bimodal, with morning and evening feeding peaks. They compensated for shorter daylengths in midwinter by starting to forage later and ending progressively earlier at lower light intensities. Fishing constituted only a minor part of their time–activity budget, and was one of the most efficient reported in marine birds. The Great Cormorants spent less than 60 minutes a day fishing in midwinter. Although subzero ambient temperatures and blizzards contributed to increased heat loss in midwinter, this potential energy loss did not seem to be compensated for by an increase in fish intake. Instead the Great Cormorants seemed to economize energy expenditure by halving the time spent at sea, and halving the number but doubling the mass of each fish taken.     

Aspects of population dynamics and feeding by piscivorous birds in the intermittently open Riet River estuary,Eastern Cape,South Africa

Aspects of the population dynamics and feeding activity of piscivorous birds in the small (c. 5 ha) intermittently open Riet River estuary, on the south-eastern coastline of South Africa, were investigated monthly from August 2005 to July 2006. A total of 188 birds of 13 species were recorded, of which six were wading piscivores, four aerial divers and three were pursuit swimmers. The Reed Cormorant (Phalacrocorax africanus) was the numerically dominant species, with a mean of 8.25 (SD ± 7.90) individuals per count. Mean numbers of the Pied Kingfisher (Ceryle rudis) and Giant Kingfisher (Megaceryle maximus) were 3.42 (SD ± 1.20) and 1.17 (SD ± 0.60) individuals per count, respectively. The remaining 10 species revealed mean values <0.5 individuals per count. Breaching events were associated with a change in feeding groups from waders to pursuit feeders, and a decrease in total bird numbers, most likely due to loss of potential littoral zone foraging habitat for waders resulting from reduced water levels. The highest bird numbers were recorded in winter reflecting the migration of large numbers of Reed Cormorant into the system. Monthly food consumption by all piscivorous birds showed large temporal variability, ranging from 26.35 to 140.58 kg per month.     

BREEDING NOMADISM IN SOUTHERN AFRICAN SEABIRDS-CONSTRAINTS,CAUSES AND CONSERVATION

Crawford, R.J.M., Dyer, B.M. & Brooke, R.K. 1994. Breeding nomadism in southern African seabirds-constraints, causes and conservation. Ostrich 65:231-246.

Fourteen species of seabirds representing three orders and five families breed in southern Africa. Nomadism in the breeding populations of 13 of these seabirds is examined. African Penguins Spheniscus demersus, Cape Gannets Morus capensis. Bank Cormorants Phalacrocorax neglectus, Whitebreasted Cormorants P. carbo and White Pelicans Pelecanus onocrotalus show strong fidelity to specific localities. Caspian Terns Sterna caspia change breeding localities at a low frequency; Cape Cormorants P. capensis, Hartlaub's Gulls Larus hartlaubii and Swift S. bergii and Roseate S. dougallii Terns at a higher frequency. At some localities, Bank Cormorants remain at the same breeding sites for extended periods, whereas Crowned Cormorants P. coronatus, Whitebreasted Cormorants, Kelp Gulls L. dominicanus, Hartlaub's Gulls, Damara Terns S. balaenarum, Swift Terns and Roseate Terns all alter breeding sites. Where space is not limiting, African Penguins may also change sites. Constraints on nomadism include strong attachment to traditional breeding localities, unwillingness to roost at non-breeding localities, and lack of suitable alternative nesting localities. Amongst causes of nomadism are excessive disturbance by humans and South African Fur Seals Arctocephalus pusillus and competition for breeding space, which have resulted in even the least nomadic of the species changing their breeding locations. Regularly nomadic species are likely to respond to environmental cues that enable them either to reduce cost of breeding or to increase reproductive output. Therefore, they are potentially useful indicators of ecological health. Nomadic breeders additionally have the ability to establish new breeding colonies. Some species have used artificial structures to expand their breeding range, and probably also to increase their overall populations. However, nomadic tendencies complicate the conservation of breeding activities. By contrast, breeding species with strong fidelity to traditional localities are more easily protected.     

FISHING TECHNIQUES AND FOODS OF THE PIED KINGFISHER ON LAKE VICTORIA IN UGANDA

Douthwaite, R. 3. 1976. Fishing techniques and foods of the Pied Kingfisher on Lake Victoria in Uganda. Ostrich 47: 153–160.

Pied Kingfishers fish by diving from hovering flight or from perches; the choice is influenced by location of the prey and the weather, especially wind speed. 40–50% of the dives into Lake Victoria were successful compared with 11% on the Kazinga Channel, near Lake Edward, but dives were made more frequently in the latter area.

Analysis of pellets from the northern shore of Lake Victoria showed the bulk of the adults' diet was composed of Haplochromis, 55–108 mm (median 80 mm) total length, but that Engraulicypris urgenteus. 40–64 mm (median 52 mm) total length was more important numerically. Flying termites, Macrotermes sp., were occasionally eaten in large numbers.

Size selection related to development determines the proportions of Huplochromis and Engraulicypris in the chick's diet. by comparison with the adult most bone consumed by the chick is digested. Two caged juveniles ate about 34.5 g of fresh fish daily, 45% of their post-absorptive weights. Wild birds may consume even more.     

EDITORIAL

Olver, M. D. &; Kuyper, M. A. 1978. Breeding biology of the Whitebreasted Cormorant in Natal. Ostrich 49:25-30.

From 1972–1975 Whitebreasted Cormorants Phalacrocorax carbo bred at the Cedara Dam, Natal, South Africa (29 32S, 30 17E), and from 1973 they fished for food at Midmar Dam, 5 km away and carried the food back to the nestlings. Breeding occurred from April to October and was preceded by a period of courtship. Nesting material was collected by the males and the nests built by the females. The mean clutch size for 1972–1973 was 3,1. Both parents incubated the eggs and guarded the nest and chicks. Growth of the chicks was studied in 1972–1973. The mean number of chicks reared was 1,6 per nest although seven nests contained three nestlings. At 28 days they left the nest when alarmed, but could not fly until 49 days old. The average flying age appeared to be about 53 days. The height of the nests above the ground seemed to determine the nest leaving age. Of the 186 eggs laid in the 60 nests observed over two years, 74% hatched. Fledging success was 52% of eggs laid and 69% of eggs hatched. Chick mortality seemed to be caused mainly by falling from the nests and dying of starvation.     

Great Cormorants Phalacrocorax carbo feed on larger fish in late winter

Capsule: Great Cormorants Phalacrocorax carbo prey on larger fish in late winter compared to early winter.

Aims: This study aimed to describe Cormorant diet and to discover whether prey size changed during winter.

Methods: The diet of Cormorants was studied using regurgitated pellets collected from roosting places at the upper Elbe River, Czech Republic, during winter (from December to March).

Results: The diet consisted of 24 fish species from 6 fish families. Roach Rutilus rutilus dominated in the diet (over 50% of biomass). Size of fish in the diet increased over time during the whole winter. Except for European Chub Squalius cephalus, the increased size applied for the majority of the most frequently consumed fish species: Roach, European Perch Perca fluviatilis and White Bream Abramis brama. Cormorants consumed mostly fish species of lower commercial and angling value.

Conclusion: Fisheries management should reflect on the fact that competition for larger-sized fish is the highest in late winter and in early spring. For that reason, stocking of potentially vulnerable fish could be delayed to the time when overwintering birds leave the area and recreational fishing could be restricted in late winter.     

Perch height predicts dominance rank in birds

Dominant individuals within animal groups will frequently place themselves in the most beneficial position for maximal protection against predation. Higher perches are generally associated with reduced predation risk in birds, so we predicted that dominant birds will preferentially place themselves on higher perches. We tested this by determining the dominance hierarchy in two populations of captive birds (Homing Pigeons Columba livia and Great Cormorants Phalacrocorax carbo), and relating rank within the dominance hierarchy to observed perch height preferences. We found that perch choice was significantly repeatable in pigeons, and that more dominant individuals of both species selected higher perches. As well as facilitating early detection of and escape from potential predators, higher perches are also likely to facilitate the display of aggression to other group members.     

The food and feeding ecology of Gerreidae, Bleeker 1859, in the estuaries of Natal

The food and feeding ecology of five species of Genes in the estuaries of Natal, were investigated from 1978 to 1980. The Kosi system, consisting of an estuary and three main lakes was selected as the main study area due to an abundance of Genes . Four other estuarine systems were sampled.
At Kosi polychaetes were important at the estuary, siphon tips (distal ±5 mm) of the bivalve Hiatula lunulata were most commonly taken in Lakes Makhawulani and Mpungwini, while chironomid larvae were important in Lake Nhlange. Ivlev's electivity test showed that Genes positively selected bivalve siphon tips when searching for food. The food taken by Genes from other estuaries was similar to that at Kosi, although the proportions of the different prey varied. At Kosi between three and five species occurred sympatri-cally. Resource segregation was through differences in diet and feeding periodicity and a superabundance of food ( H. lunulata siphon tips) during summer and autumn. During winter and spring when food may be limiting, most Genes leave shelf areas of the Kosi system, only G. acinaces remains in large numbers. Little is known of the diet of other fish which feed on benthic invertebrates in Kosi but there is probably little direct competition with Gerreidae, although Acanthopagrus berda and Pomadasys commersonni have been recorded feeding on the siphons of the bivalve Solen corneus , the former in Durban Bay and the latter at Kosi. In areas where the bivalve H. lunulata occurs it is suggested that Gerreidae have developed optimal foraging techniques which enhance resource partitioning.     

PARTITIONING OF NESTING SPACE AMONG SEABIRDS OF THE BENGUELA UPWELLING REGION

Duffy, D. C. & La Cock, G. D. 1985. Partitioning of nesting space among seabirds of the Benguela upwelling region. Ostrich 56:186-201.

An examination of nesting habitats used by the four main species of seabirds nesting on southern African islands (Jackass Penguin Spheniscus demersus, Cape Cormorant Phalacrocorax capensis, Bank Cormorant P. neglectus and Cape Gannet Morus capensis) revealed relatively minor differences and extensive overlaps between species, primarily in subcolony size, steepness of nesting substratum, and proximity to cliffs. A weak dominance hierarchy existed; gannets could displace penguins, and penguins could displace cormorants. This hierarchy appeared to have little effect on partitioning of nesting space. Species successfully defended occupied sites in most cases of interspecific conflict, suggesting that site tenure by one species could prevent nesting by another. The creation of additional nesting space on Namibian nesting platforms did not increase guano harvests, suggesting that nesting space had not previously limited the total nesting population of Cape Cormorants, the most abundant of the breeding species, in Namibia.

While local shortages of nesting space may occur, populations of the four principal species of nesting seabirds in the Benguela upwelling region do not seem to have been limited by the availability of nesting space on islands.     

Environmental determinants of Great Cormorant (Phalacrocorax carbo) distribution in small man-made waterbodies – a case study of gravel pits in southwest France

In the Midi-Pyrénées region (southwest France), the increasing number of gravel pits has allowed the wintering of numerous species of waterbirds such as Great Cormorants (Phalacrocorax carbo). The debate about cormorant predation on fish stock has been sufficiently strong to have resulted in reductions in cormorant numbers by control shooting. In this context, cormorants were studied during winters 1996/1997 and 1997/1998 at two gravel pit sites in the Garonne floodplain. Human disturbances and fish densities were found to be the main parameters determining the abundance of fishing cormorants. This work will help to prompt further research and the development of a management strategy for this species.     

Cormorant (Phalacrocorax carbo [L]) populations and patterns of abundance at breeding and feeding sites in Northern Ireland,with particular reference to Lough Neagh

Breeding cormorants, Phalacrocorax carbo, in Northern Ireland remain seasonally dependent on a coastal environment where they can be censused with accuracy, but numbers in other habitats and at other times of the year are less certain. This study establishes the long term and regional patterns of abundance at breeding and feeding localities, which might in turn be related to diet.Birds were regularly observed and counted at a variety of feeding sites, and some aspects of their breeding success and fledgling diet evaluated at the largest N.Ireland breeding colony.Numbers of breeding birds increased dramatically over a period of eight years but recently show signs of declining. There is likely to be a dynamic relationship between populations of a tapeworm (Ligula intestinalis L.), the numbers of roach (Rutilus rutilus [L.]) and cormorant populations feeding at Lough Neagh, the largest lake in the British Isles. While roach are not a major component in the diet of fledglings, the large numbers of cormorants feeding at the perimeter of L. Neagh suggest that changes in roach populations will affect birds most acutely during or following the winter. We suggest that this might result in a reduction in the proportion of the population volunteering to breed in the subsequent season, but require further data. The longer term effects of one fish species on cormorant populations are unlikely to be critical, since these birds are highly opportunist.In other habitats cormorant numbers are either very stable (estuary) or variable (river) depending on the seasonal and annual availability of their prey. There is no evidence for a systematic seasonal shift in habitat, as suggested by other studies.     

The influence of meal size on prey DNA detectability in piscivorous birds

Molecular methods allow noninvasive assessment of vertebrate predator–prey systems at high taxonomic resolution by examining dietary samples such as faeces and pellets. To facilitate the interpretation of field‐derived data, feeding trials, investigating the impacts of biological, methodological and environmental factors on prey DNA detection, have been conducted. The effect of meal size, however, has not yet been explicitly considered for vertebrate consumers. Moreover, different noninvasively obtained sample types remain to be compared in such experiments. Here, we present a feeding trial on abundant piscivorous birds, Great Cormorants (Phalacrocorax carbo), to assess meal size effects on postfeeding prey DNA detection success. Faeces and pellets were sampled twice a day after the feed of large (350–540 g), medium (190–345 g) and small (15–170 g) fish meals contributing either a large (>79%) or small (<38%) share to the daily consumption. Samples were examined for prey DNA and fish hard parts. Molecular analysis of faeces revealed that both large meal size and share had a significantly positive effect on prey DNA detection rate postfeeding. Furthermore, large meals were detectable for a significantly longer time span with a detection limit at ~76 hr and a 50% detection probability at ~32 hr postfeeding. In pellets, molecular methods reliably identified the meal consumed the previous day, which was not possible via morphological analysis or when examining individual faeces. The less reliable prey DNA detection of small meals or meal shares in faeces signifies the importance of large numbers of dietary samples to obtain reliable trophic data.     

PRELIMINARY DATA ON THE IMPORTANCE OF ISRAEL FOR THE CONSERVATION OF THE WHITE PELICAN PELECANUS ONOCROTALUS L.

Izhaki, I. 1994. Preliminary data on the importance of Israel for the conservation of the White Pelican Pelecanus onocrotalus L. Ostrich 65:213-217.

The majority of the western Palaearctic populations of the Great White Pelican Pelecanus onocrotalus stop over in Israel during their autumn migration. The annual average number of pelicans observed over Israel in autumn was 71,421±5,027. Although most of these birds were transient over Israel, increasing numbers of individuals (1130±303, between 1990 and 1993) stay and winter in the northern part of Israel. The natural feeding sites for pelicans were diminished during recent decades due to human activities such as the drainage of the Hula swamp in northern Israel 40 years ago. However, intensive fishery and aqua-culture sites became their favourite feeding laces either during migration or in winter. The fish industry complained that the birds seriously affect fish yield. The pelicans not only cause direct damage to the fix industry by predation, but they also transfer nematodes to fish (Oreochromis sp.). As et. there is no solution to the problem of these parasitic nematodes. This preliminary study revealed that the average daily fish consumption per individual was 0,52kg and the small sample size did not indicate any selection for certain fish species or size. Flight range estimation indicated that the pelicans, especially the weak individuals, nee xo feed during their stopover in Israel or in the relatively rare appropriate feeding sites along their southward migratory route over the desert in order to reach their wintering areas in eastern central Africa.     

Population changes in sympatric Great and Long-tailed Cormorants (Phalacrocorax carbo and P. africanus): the effects of niche overlap or environmental change?

Between January 1993 and January 1995, the number of Great Cormorants (Phalacrocorax carbo) using Lake Naivasha, Kenya (00° 45 S, 36° 20 E) for foraging and resting increased 56%, while the number of sympatric Long-tailed Cormorants (Phalacrocorax africanus) decreased 64%. In 1995 and 1996, we documented habitat changes and conducted monthly population and resource-use surveys of the two species in an attempt to discover the most likely reasons for these changes. The increase in Great Cormorants was probably the result of immigration from nearby Lake Nakuru due to extreme water level reductions there. Lake Naivasha also experienced falling water levels and transparency during this period, but these changes were not as severe and are not considered likely reasons for the decline in Long-tailed Cormorant numbers. Despite some probable dietary overlap, the two species were well separated in terms of foraging locations, foraging methods, resting habitats and breeding timing. The decline in Long-tailed Cormorant numbers may be connected with increased disturbance by fishermen along the lake littoral, this species' primary feeding location.