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f) Effect of Birds on Productivity
Of the animals listed in trophic level IV, only the wild birds could influence the quantity of economically valuable produce becoming available. Birds have been seen fishing in the kei wai and, on several occasions, startled birds have dropped fish of economic size. Estimates were made for the amount of produce removed by 8 species of birds that have been observed in the kei wais, and the results are given in Table 6.
Species Body wt. (kg.) Population size Period of occurrence (months) Annual Metabolic equivalent Daily rate/bird (kcal day-1) Population intake per bird (kcal year-1) Population intake (kcal) Yellow bittern 0.15 150 7.5 18.11 90.57 2,479,278 Little green heron 0.185 50 2.5 21.17 105.86 969,980 Grey heron 1.500 175 7.3 100.46 502.31 13,384,019 Purple heron 0.900 20 69.70 343.49 752,245 White-breasted waterhen 0.500 150 12 44.36 221.82 12,144,423 Moorhen 0.400 75 12 37.58 187.88 5,143,342 Spotted redshank 0.145 150 6 17.66 88.31 2,417,527 Greenshank 0.180 20 7 20.74 103.72 454,314Thus, the estimated energy intake by wild birds is approximately 3.77 × 107 kcal. per year. This can be converted to body weight of fish taken by using a value for energy per biomass; in the absence of reliable results for local fish we used a figure of 129 kcal per 100 gm of fish, which is the mean of values for herring, collared herring, haddock, sole and tuna. The corresponding biomass of fish taken by birds at Mai Po is therefore 30,800 kg. per year.
The total area of kei wai No. 7 is about 0.6% of the total area around Hau Hoi Wan (estimated at 16.7 km2) within which the birds might take fish; proportional removal of fish from kei wai No. 7 would be about 18.9 kg. of fishes per annum, which amounts to some 4.5% of the annual produce. This estimate takes no account of the fact that the birds' diets may include significant quantities of worms, amphipods and insects, and must therefore be considered as a maximum figure. Obviously the birds do not compete significantly with man for economic produce.