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Habitat use pattern of three species of egrets in a small coastal lagoon in Ghana a
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Francis Gbogbo , Gideon Kwarteng Acheampong , John-Mark Yaw Atiemo & Quist Elorm Crepindale
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Department of Animal Biology and Conservation Science, University of Ghana, PO Box LG 67, Legon, Accra, Ghana Published online: 17 Dec 2013.
To cite this article: Francis Gbogbo, Gideon Kwarteng Acheampong, John-Mark Yaw Atiemo & Quist Elorm Crepindale (2013) Habitat use pattern of three species of egrets in a small coastal lagoon in Ghana, Ostrich: Journal of African Ornithology, 84:3, 213-217 To link to this article: http://dx.doi.org/10.2989/00306525.2013.867549
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OSTRICH 2013, 84(3): 213–217 Printed in South Africa — All rights reserved
OSTRICH ISSN 0030–6525 EISSN 1727-947X http://dx.doi.org/10.2989/00306525.2013.867549
Habitat use pattern of three species of egrets in a small coastal lagoon in Ghana Francis Gbogbo*, Gideon Kwarteng Acheampong, John-Mark Yaw Atiemo and Quist Elorm Crepindale
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Department of Animal Biology and Conservation Science, University of Ghana, PO Box LG 67, Legon, Accra, Ghana * Corresponding author, e-mail:
[email protected] The foraging and pattern of habitat use of Western Reef Heron Egretta gularis, Little Egret Egretta garzetta and Great Egret Egretta alba were studied in Sakumo II Lagoon in Ghana to determine the extent of utilisation of non-fish resources by these species, which compete with humans for fisheries resources. The species were counted in grassland and marginal water areas of the lagoon on the assumption that individuals found in the grassland area feed on non-fish resources, whereas those in marginal water forage on fish and possibly aquatic invertebrates. Average numbers of E. gularis and E. garzetta making use of the grassland were higher than those utilising the water, indicating preference for the grassland area. Egretta alba did not show a preference for either habitat. Catch per unit effort and swallow rates of each species were similar in grassland and water habitats, with the exception of E. alba, which recorded a higher swallow rate in grassland. It was concluded that the grassland habitats of Sakumo II Lagoon are important for ardeids, and that non-fish resources constitute an important food for ardeids in coastal Ghana, contrary to previous observations. Keywords: arthropods, Egretta, fish, human-wildlife conflict, niche segregation, waterbirds
Introduction The importance of Ghana’s coastal wetlands for resident waterbirds, as well as migrant waterbirds from the Palearctic using African–Eurasian flyways, is well established (Ntiamoa-Baidu and Grieve 1987, NtiamoaBaidu and Hepburn 1988, Reneerkens et al. 2009, Gbogbo and Attuquayefio 2010). Coastal wetlands in Ghana are therefore important foraging areas for many waterbirds, both resident and migratory, during the boreal winter. On arrival at wintering grounds, many migratory waterbirds feed voraciously in order to build up energy reserves, which are crucial for their return migratory flights, survival and breeding on their nesting grounds (Battley et al. 2000, Berthold 2001, Piersma and Jukema 2002, LandysCiannelli et al. 2003, Morrison et al. 2007, Kober and Bairlein 2009). However, the fattening rate of waterbirds in West African wetlands is low because many of the available organisms on which waterbirds feed have comparatively low biomass (van de Kam et al. 2004). Consequently, many waterbirds spend most of their available time foraging (van de Kam et al. 2004, Suapim et al. 2007). The Black-chinned Tilapia Sarotherodon melanotheron constitutes about 90% of fish harvested from coastal lagoons in Ghana (Blay and Asabere-Ameyaw 1993, Abban et al. 2000). According to Gbogbo et al. (2008), about 99.6% of the fish harvested from coastal lagoons in Ghana were between 3 and 17 cm in length, which substantially overlapped the sizes consumed by the ardeids. Similarly, extensive overlaps exist in the sizes of crabs (Callinectes amnicola and Cardiosoma armatum) utilised by humans and birds in coastal lagoons in Ghana (Willoughby et al.
2001, Gbogbo et al. 2008). This competition for resources by humans and birds in coastal lagoons in Ghana adds to the difficulties of low fattening and long foraging periods that birds encounter during their wintering period in Ghana. Several species of ardeids occur in coastal Ghana, many of which are Palearctic and intra-African migrants. The different species of ardeids are known generally to feed on similar food items, including fish, amphibians, reptiles, crustaceans and insects (Piersma and Ntiamoa-Baidu 1995, Ntiamoa-Baidu et al. 1998, Kushlan and Hancock 2005, Liordos 2010). Hutchinson (1959) reported that the boundaries of realised niches are set by competition for limited resources. Thus in the light of the challenges associated with building up energy reserves against the background of human–bird competition on West African coastal wetlands, competition among species may lead to niche segregations. Understanding the nature and basis of such niche segregations is important for the development of successful conservation strategies (Hanski and Gilpin 1997, Dagobert et al. 2008, Principe 2008, Liordos 2010). In this paper, the habitat use pattern of Western Reef Heron Egretta gularis, Little Egret Egretta garzetta and Great Egret Egretta alba were examined to determine the extent of their utilisation of non-fish resources. The study was carried out in a lagoon that has a grassy island adjoining a shallow water area, on the assumption that individuals found in the grassland areas of the wetland feed exclusively on non-fish resources while those in marginal waters forage on fish and possibly some aquatic invertebrates.
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Materials and methods Study area Sakumo II lagoon (5°37′ N, 0°02′ W) is located in the Greater Accra region of Ghana. The site is 3 km west of Tema and has a total area of 1 364 ha (Nartey et al. 2011). The brackish water lagoon generally covers about 3.5 km2 of the total wetland area and is separated from the sea by a narrow sand dune on which the Accra–Tema coastal road is built. Prior to road construction, the lagoon was a typical closed system. During construction, pipes were laid under the sand bar, so that the lagoon is now permanently connected to the sea, representing a transitory stage between a closed and an open lagoon (Kwei 1974). The average surface area of the lagoon during the dry season (September/October to March/April) was estimated to be 1 km2 and its average depth to be 50 cm (Pauly 1975). The site is the third-most important of Ghana’s Ramsar sites (Anku 2006) and has recorded up to 30 000 shorebirds comprising 70 species (BirdLife International 2012). Approximately 98.8% of the fish in Sakumo II consists of the Black-chinned Tilapia, for which the modal length of individuals decreased from 10–11 cm (Pauly 1976) to 6–8 cm (Gbogbo et al. 2008). The shallow waters of the lagoon and its grassy island make a suitable place to investigate microhabitat preference and foraging patterns of waterbirds. Methods Data were collected between 07:00 and 16:00 GMT for 2 d in each of November and December 2012 and January and February 2013, giving 18 h observation per month and 72 h altogether. Observations were carried out simultaneously in (1) the marginal shallow-water area in the north-western bank of the lagoon and (2) the stretch of grassy island adjoining to the shallow water. These zones constitute one of the core areas utilised by waterbirds in Sakumo II and the presence of a grassland island habitat adjoining to marginal waters made the site particularly suitable for this study. The size of both the grassland and marginal water used for the study were unstable and varied from day to day due to fluctuating water levels. The numbers of E. gularis, E. garzetta and E. alba utilising each of the shallow water and grassy island habitats were counted on every hour – thus a total of 10 data points were collected for each species over 9 h of observation on every sampling day, giving a total of 80 data points per species for the study period. Following the method used by Piersma and Ntiamoa-Baidu (1995), Ntiamoa-Baidu et al. (1998) and Suapim et al. (2007), the number of each of the three species observed to be foraging (walking around and actively searching for prey), roosting (standing still without actively searching for prey) and involved in comfort activities (preening or bathing) in each of the habitats was noted on every hour. Individuals of the three species of egrets that were actively foraging in each habitat were randomly selected from time to time and observed continuously for 5 min, during which period the number of swallows, identified by a characteristic head and neck jerk (Heatwole 1965, Grubb 1976, Scott 1984, McKilligan 2005, Clancy 2011), and strides were recorded. Observations of the three species were carried out
Gbogbo, Acheampong, Atiemo and Crepindale
simultaneously by three individuals, each of which collected data on only one of the species. Data analysis The total numbers of each species involved in each of the three activities were separately divided by the number of data points (80) to obtain the average number of birds involved in a particular activity. To determine the dominant activity in a particular habitat (grassland or water), the average numbers of each species involved in the different activities were compared. Comparative foraging success of each species in the grassland and water areas was determined by comparing the mean number of swallows in the grassland to that in the water. For each species, number of swallows per unit effort was calculated by dividing number of swallows by the corresponding number of strides. Results Comparative occurrence in the grassland and water areas Over the 72 h of data collection, 1 575 individual birds consisting of 613 (38.9%) E. gularis, 523 (33.2 %) E. garzetta and 439 (27.9 %) E. alba were recorded. The average number of birds counted per day was 38 22, 32 21 and 27 12 for E. gularis, E. garzetta and E. alba, respectively. Of the 613 E. gularis counted, 61% occurred in the grassland area compared to 39% in the water. For E. gazetta 59% occurred in grassland compared to 41% in water, whereas 56% and 44% of E. alba occurred in grassland and water areas, respectively. Average numbers of each of the three species of egrets recorded utilising the water and grassland areas of Sakumo II Lagoon are shown in Figure 1. The number of E. gularis present in the grassland (4.9 3.2) was significantly higher than that in water (3.0 2.7, Mann–Whitney U-test, z 3.361, p 0.05, df 158). Similarly, the average number of E. garzetta present in the grassland area (3.8 3.4) was significantly higher than that in the water (2.7 2.4, Mann– Whitney U-test, z 2.208, p 0.05, df 158). In contrast, numbers of E. alba present in the grassland (3.1 2.6) were not significantly different from those in the water (2.0 2.3, Mann–Whitney U-test, z 1.703, p 0.05, df 158). Disparities in the use of grassland and water areas for foraging, roosting and comfort activities Figure 2 shows the average number of each of the three species of egrets utilising water and grassland areas in Sakumo II Lagoon for foraging, roosting and comfort activities. A significant difference was observed among the average numbers of E. gularis roosting (3.1 2.8), involved in comfort activities (1.2 1.6) and foraging (0.4 0.8) in the grassland area (Kruskal–Wallis H-test with post hoc Tukey’s test, H2,0.05 55.233, p 0.05). Similarly, the average number of E. gularis roosting (1.7 1.9) in the water was significantly higher than the number involved in comfort activities (0.7 1.2) and foraging (0.5 1.0, Kruskal–Wallis H-test with post hoc Tukey’s test, H2,0.05 22.028, p 0.05). Among E. garzetta utilising the grassland, the average numbers roosting (1.7 1.9) and involved in comfort activities (1.4 2.2) were significantly higher
215
Grassland area
8
Water area
AVERAGE NUMBER OF SWALLOWS PER 5 MIN
AVERAGE NUMBER OF BIRDS PRESENT
Ostrich 2013, 84(3): 213–217
7 6 5 4 3 2 1 Egretta gularis
Egretta garzetta
Egretta alba
Grassland area
14 12 10 8 6 4 2
Egretta gularis
Roosting
Comfort activities
3 2.5 2 1.5 1 0.5 Grassland
Water
Egretta gularis
Grassland
Water
Egretta garzetta
Egretta alba
SPECIES
Figure 1: Average numbers of three species of egret utilising grassland and water areas of Sakumo II Lagoon
AVERAGE NUMBER OF BIRDS PRESENT
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SPECIES
Foraging
Water area
Grassland
Egretta garzetta
Water
Egretta alba
SPECIES/HABITAT Figure 2: Average numbers of birds of three species of egret that displayed different activities in two habitats in Sakumo II Lagoon
than the number foraging (0.3 0.7, Kruskal–Wallis H-test with post hoc Tukey’s test, H2,0.05 47.369, p 0.05). The average number of E. garzetta roosting (1.9 1.2) was significantly higher than numbers involved in comfort activities (0.8 1.7) and foraging (0.6 1.0) in the water area (Kruskal–Wallis H-test with post hoc Tukey’s test, H2,0.05 15.079, p 0.05). With regard to E. alba, the average numbers foraging (1.2 1.3) and involved in comfort activities (1.2 1.5) in the grassland were higher than those roosting (0.6 0.8, Kruskal–Wallis H-test, H2,0.05 7.227, p 0.05), while the average number of E. alba foraging (1.0 1.2) in the water was higher than those involved in comfort activities (0.5 1.2) and roosting (0.5 0.7). Comparative foraging success and catching effort in the grassland and water The foraging success of each of the three species of egret in the two micro habitats in Sakumo II Lagoon is shown in Figure 3. The swallow rate (number of swallows per 5 min) of E. gularis in the grassland (6.7 3.4) was similar to the
Figure 3: Foraging success of three species of egret in two habitats in Sakumo II Lagoon
swallow rate in the water (7.9 4.5, Mann–Whitney U-test, z 0.837, p 0.05, df 42). Similarly, swallow rates of E. garzetta in grassland (4.7 2.5) and water (4.4 5.2) showed no significant difference (Mann–Whitney U-test, z 1.252, p 0.05, df 49). Egretta alba recorded a swallow rate of 9.8 4.8 in the grassland, which was significantly higher than that in water (5.4 3.3, Mann–Whitney U-test, z 0.837, p 0.05, df 42). In relation to the number of swallows per unit effort (stride), there was no significant difference for E. gularis feeding in grassland (0.3 0.2) and water (0.3 0.2, Mann– Whitney U-test, z 1.438, p 0.05, df 42), for E. garzetta in grassland (0.8 2.3) and water (0.6 1.2, Mann– Whitney U-test, z 1.114, p 0.05, df 49), or for E. alba in grassland (0.5 0.2) and water (0.5 0.2, Mann–Whitney U-test, z 0.2171, p 0.05, df 49). Discussion This study indicated that the average numbers of E. gularis and E. garzetta making use of the grassland islands of Sakumo II were higher than the numbers utilising the water and thus the two species preferred the grassland area compared to the marginal waters, whereas E alba did not show any particular preference. Ghana’s coastal wetlands have a rich diversity and high abundance of arthropod taxa (Gbogbo et al. 2012, Gordon and Cobblah 2000) and it could be that the preference shown by E. gularis and E. garzetta for grasslands relates to the availability of arthropods in the grassland as forage. However, if these species depend on grasslands for foraging, then foraging might be expected to be their dominant activity in grassland habitat, but this was not the case. Numbers of E. gularis and E. garzetta that foraged in grassland were lower than those involved in roosting and comfort activities. Thus, although both E. gularis and E. garzetta showed preference for the grassland habitat compared to the water, the preference cannot necessarily be attributed to choice of the grassland habitat over water for foraging. This observation is reinforced by the fact that the swallow rates of E. gularis and E. garzetta were similar
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in grassland and water habitats. Furthermore, the numbers of swallows per stride of both species were similar in the two habitat types. The concept of giving-up time (GUT) predicts that a forager will leave a feeding patch when its search time for the next prey item takes longer than a critical time interval, and that the reciprocal of GUT is a measure of encounter rates (Jan et al. 2003). Since this did not vary for either E. gularis or E. garzetta between grassland and water habitats, GUT appears to be similar in each habitat type. Thus E. gularis and E. garzetta seem not preferred either of the two habitats over the other as foraging grounds. Unlike results for E. gularis and E. garzetta, numbers of E. alba found in the grassland and water habitats were similar and foraging emerged as the most pronounced activity engaged in by E. alba in both habitats. Although the swallow rate of E. alba was higher in the grassland than water, the number of swallows per unit stride was similar in both habitats. Therefore, it is also uncertain that E. alba preferred either of the two habitats over the other as a foraging ground. Egrets and herons stand and wait for their prey, which they often ambush with well-directed jabs from their beaks (Piersma and Ntiamoa-Baidu 1995, Liordos 2010). The results from this study suggest that E. alba walks and chases its prey more often in grassland than in water, whereas in water its feeding strategy involves more waiting and surprise attacks. The higher swallow rates in the grassland indicated that the chasing of prey increased the number of prey encounters. The observations in this study that significantly higher numbers of E. gularis and E. garzetta occurred in the grassland areas compared to the numbers utilising the water, but that there was no significant difference between the numbers of E. alba using the two habitat types, is helpful in understanding habitat use of ardeids on the coast of Ghana. Piersma and Ntiamoa-Baidu (1995), Ntiamoa-Baidu et al. (1998) and Suapim et al. (2007) considered these three species to feed exclusively on fish in marginal waters not more than 15 cm deep in coastal Ghana. However, utilisation of arthropods by ardeids in many parts of the world is well established (Hancock and Kushlan 1984, Kushlan and Hancock 2005). Assuming ardeids are exclusively piscivorous on the coast of Ghana, as previously reported, one would not have expected the three species to have foraged in grassland habitat that is devoid of fish. Furthermore, the observation that swallow rates of E. gularis and E. garzetta are similar in both the grassland and water habitats and higher in the grassland for E. alba indicates that non-fish resources in the grassland area constitute an important source of food for ardeids in coastal Ghana. Prey items in the water might not necessarily be fish and may include some non-fish resources. Thus an important component of the foraging ecology of ardeids might either have been overlooked by Piersma and Ntiamoa-Baidu (1995), Ntiamoa-Baidu et al. (1998) and Suapim et al. (2007), or a niche shift of ardeids might have occurred as a result of human–wildlife competition (Willoughby et al. 2001, Gbogbo et al. 2008). This study identified roosting as the most dominant activity engaged in by E. gularis and E. garzetta in both
Gbogbo, Acheampong, Atiemo and Crepindale
the grassland and water areas. The foraging strategy of ardeids often involves stalking, walking, waiting and jabbing (Piersma and Ntiamoa-Baidu 1995, Ntiamoa-Baidu et al. 1998, Liordos 2010). It could be that during counts, some individuals in both the grassland and water areas were standing still, so appearing not to be searching for prey, whereas they were in the ‘waiting phase’ of foraging. This would have exaggerated the proportion of birds considered to be roosting. Nevertheless, the foraging data obtained in this study strongly suggest that the grassland habitats of coastal wetlands are important for ardeids and that non-fish resources constitute an important food for ardeids in coastal Ghana. Conservation implications The wintering season is an important period for many waterbird species during which energy reserves are built for spring migration and breeding (Wiersma and Piersma 1994). In coastal Ghana, competition resulting from the overlap in the sizes of fish used by humans and birds may negatively affect birds’ ability to build energy reserves (Gbogbo et al. 2008). These conditions can lead to unsuccessful migration and poor breeding, with a resultant negative impact on bird populations. Given that arthropods abound in coastal lagoons in Ghana (Gbogbo et al. 2012, Gordon and Cobblah 2000) and competition exists between humans and birds for fisheries resources (Gbogbo et al. 2008), utilisation of non-fish resources, which may largely be terrestrial arthropods, by egrets would not only release fisheries resources for specialist piscivorous bird species but would also enhance survival of mixed-diet foragers such as egrets and herons. Many classes of arthropods, such as insects and arachnids, which apparently constitute important diet of ardeids are not utilised by humans in coastal Ghana. However, processes such as wildfires can impact on the abundance of arthropods in wetland catchments and these processes need to be studied to ensure continued availability of arthropods to waterbirds. Acknowledgements — The telescopes and binoculars used in this work were funded by the International Foundation for Science. We are grateful to the Wildlife Division of the Ghana Forestry Commission for their support.
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Received June 2013, accepted October 2013 Editor: RJM Crawford
