Foraging Ecology of Sanderling Calidris Alba on the Western Coast of Ghana

Abstract

Migration is an important process in the annual cycle of shorebirds that enables them to escape unfavourable conditions at certain times of the year and provides them opportunity to breed in one area and spend the non-breeding season in other latitudes. The selection of a suitable non-breeding habitat by migrants is dependent on a number of factors which define the quality of the habitat, namely: levels of disturbances, predatory risk, suitable environmental and climatic conditions such as depth and temperature of water, abundance and availability of food. The Esiama beach on the Ghana coast is important for sanderlings of the East Atlantic Flyway during the non-breeding season, supporting 40-70% of all sanderlings along the Ghana coast. The overall aim of this study was to investigate and document the foraging ecology of sanderlings on the western coast of Ghana; and using the species as a model, describe and assess the quality of Esiama beach as a suitable and preferred non-breeding site for sanderlings in Ghana. Specifically, this study sought to measure the variations in the distribution, availability and quality of benthic macroinvertebrates as prey for sanderlings and assess the spatio-temporal variation in the distribution of sanderlings in relation to food availability. The study also focused on describing foraging behaviour and strategies employed by sanderlings, determine prey preference and evaluate the impact of human disturbances and other pressures on the foraging behaviour of sanderlings. Data for this study were obtained through direct field observations of foraging birds, benthic macroinvertebrate sampling, mist netting, faecal collection; laboratory analysis of macroinvertebrates and estimation of biomass; as well as experiments to determine intake rates and prey preference of captive sanderlings. A total of 62,000 macroinvertebrates belonging to four Phyla: Mollusca (Donax pulchellus, D. rugosus, Hastula aciculina and Agaronia acuminata), Arthropoda/ Crustacea (Excirolana chiltoni, Emerita talpoida, Mysid shrimp), Annelida (Glycera spp., Nereis sp.) and Nemertea were recorded in benthic samples. Donax pulchellus, E. chiltoni and Glycera spp. constituted 91.66%, 6.45% and 1.40% respectively of all individuals in benthic samples. About 95.0% of all macroinvertebrates in benthic samples were present in the top 3 cm depth and therefore were considered available to sanderlings. About 99.18% of all D. pulchellus individuals were available to sanderlings. About 99.68% of all available macroinvertebrates were distributed within the low- and inter-tidal zones and was dominated by D. pulchellus. Excirolona chiltoni was the most abundant prey within the high tide zone. The diet of sanderlings from the analysis of faecal samples was made up of bivalves, gastropods, polychaetes, isopods and fish. Donax pulchellus was the most abundant prey found in droppings of sanderlings, occurring in about 78.0% of all faecal samples and constituted of 79.0% of the total number of prey items found in faecal samples. The mean length, biomass and quality of D. pulchellus were 6.9 ± 1.7 mm, 1.42 ± 0.04 mg AFDMflesh and 1.04 kJ/ g and for E. chiltoni 5.2 ± 1.4 mm, 1.44 ± 0.06 mg AFDMflesh and 7.75 kJ/g. There were two recruitment periods for D. pulchellus; between July – October and January – March. Peak densities of D. pulchellus occurred in August – October and January – March, which explained the occurrence of high numbers of sanderlings along the beach, coinciding with the autumn and spring migrations respectively. Spatially, densities of D. pulchellus declined towards the estuaries (p < 0.001) with mean densities up to 15,509 individuals/ m2 whiles densities of E. chiltoni increased towards the Ankobra estuary (p < 0.001) with mean densities up to 960 individuals/ m2. The spatial distribution of sanderlings was explained by the densities of D. pulchellus (p < 0.001), but not E. chiltoni (p = 0.91). Sanderlings spent 60.45%, 36.93%, 1.15% and 1.48% of the 12-hour day time foraging, resting, in locomotion and engaged in comfort activities respectively. The time-activity budgets of sanderlings could be explained by the tidal cycle, diurnal time, number of sanderlings on the beach and density of prey. For example, more sanderlings foraged between 0600 – 0900 GMT and 1500 – 1700 GMT which coincided with 2 hours before and after high tide. Such observation suggests that sanderlings take an optimal course of foraging action, maximizing foraging opportunities presented by tidal action. Three foraging methods were described for sanderlings in exploiting their prey: pecking, probing and sewing. Pecking was a visual foraging method whereas probing and sewing were tactile methods. Sanderlings in captivity were also observed to use sewing methods to search for the appropriate prey size. Foraging rates of sanderlings varied with prey densities and sanderling numbers. For example, sanderlings pecked more when density of prey was high (25.24 ± 19.29 pecks/min) than when prey densities was lower (15.19 ± 13.74 pecks/min; p < 0.0001), and also when sanderlings were abundant on the beach (25.04 ± 19.01 pecks/min) than when there were fewer sanderlings (17.66 ± 16.16 pecks/min; p < 0.0001). Sanderlings also spent more time probing when bird numbers were high (9.80 ± 12.74 s/min) than when they were low (6.23 ± 8.19 s/min). Foraging rates of sanderlings increased with flock size below 28 individuals (p < 0.0001), beyond this threshold, foraging rates declined (p = 0.04). Due to competition for limited resource. Foraging rates of sanderlings also increased with nearest neighbour distance for conspecifics foraging in flock size greater than 28 individuals. Highest foraging rates were observed for sanderlings immediately upon arrival on the beach and peak sanderling count periods. The variation in foraging rates could be explained by competition for food in line with actual or apparent decline of prey densities. There was no significant difference in foraging rates in the field (23.68 ± 18.54 pecks/min) and in captivity (25.13 ± 14.03 pecks/min; W = 3810, p = 0.37). The intake rate of D. pulchellus by sanderlings in captivity was 1.61 ± 0.73 individuals/ min but varied between small size (1.61 ± 1.03 individuals/min) and medium size (0.44 ± 0.36 individuals/min). The estimated biomass of prey consumed in a 12-hour period from faecal samples were: D. pulchellus (0.72 ± 0.62 g AFDM), E. chiltoni (0.49 ± 0.42 g AFDM) and Glycera spp. (0.36 ± 0.26 g AFDM) making a total of 1.57 ± 0.96 g AFDM. The biomass of E. chiltoni was likely to be underestimated due to the time of faecal collection. Therefore, University of Ghana http://ugspace.ug.edu.gh using estimates of intake rates of isopods from literature, a value of 7.34 g AFDM was estimated as the total biomass consumed in a 12-hour period. Sanderlings showed preference for E. chiltoni to D. pulchellus (p = 0.01). Between small and medium size D. pulchellus, sanderlings preferred the former (p < 0.001). Several disturbance activities were observed along the beach: human related activities (74.0%); predators such as yellow-billed kite Milvus migrans parasitus (13.0%) and dogs (6.0%); scavengers (5.0%) and engine-driven machines (2.0%). Out of the proportion of human-related disturbances, fishing activities, passers-by, shellfish harvesting and recreational activities accounted for 45.0%, 37.0%, 14.0% and 4% respectively of the observed activities. The mean encounter rate of fishing activity was 0.5±0.3 activity/km with an average human density of 21±11 individuals/activity. The spatial distribution of sanderlings was related to distribution of D. pulchellus (p < 0.05), human densities (p < 0.001) but not E. chiltoni (p = 0.6). The impact of the disturbances along the beach by sanderlings was determined using the Minimal Approach Distance (MAD) method. Sanderlings could tolerate approaching humans up to a distance of 25.51±10.03 m and kites up to a height of 11.93 ± 6.29 m beyond which they showed signs of being disturbed. Multiple linear regression of all possible factors indicated that the MAD for sanderlings to humans depended on flock size and width of the beach for foraging sanderlings (R2 = 0.27, df=5, p = 0.0002). The findings of this study provide additional information on the foraging ecology of sanderlings along the Esiama beach, describe the quality of the Esiama beach as a habitat for sanderlings and make a case for protection of the Esiama beach and associated Amansuri wetlands as a Ramsar site (i.e. an internationally important wetland). University of Ghana http://ugspace.ug.edu.gh