Population Dynamics of Waterbirds in Wastewater Lagoons in Njoro, Kenya

Abstract

Waterbirds were studied in a series of three wastewater lagoons located at Egerton University, Njoro Division, Kenya, to elucidate their spatio-temporal population dynamics. The sacred ibis and the yellow-billed ducks were most abundant in the lagoon with raw unprocessed sewage (WLI). The population of yellow-billed ducks in WLI differed significantly from the populations in the middle lagoon (WLII) and in the last lagoon (WLIII) before the ‘processed water’ was discharged into the Njoro River. Post-hoc analysis using Bonferroni multiple range test ( = 0.05) indicated that WLI had the highest populations of yellow-billed ducks, sacred ibis and the Egyptian geese. The group sizes of the Egyptian geese in WLI were between 6-10 individuals. The diurnal distribution of the bird populations in the three lagoons did not define distinct patterns although the population of red-billed teals in WLI showed a peak increase at around 1200 h. The observed fluctuations of the populations within very short temporal periods indicated that the birds were not flying far from their habitat. They were returning to the lagoons within 2-4 hours. The highest number of birds was counted between 1200 and 1400 h, 0800 and 1000 h and, 1200 and 1400 h, in WLI, II and III, respectively. Chironomus gr. plumosus (Chironomidae) dominated the macroinvertebrate assemblages in the wastewater and could be food for the waterbirds. Total body lengths of the chironomids in all the three lagoons were similar. The size structures portrayed by the invertebrates in the three lagoons raised questions on food selectivity by the birds and/or survival strategies by the invertebrates themselves. The current data indicate that bird populations could be used to determine roughly the health of wastewater lagoons. The value of wastewater lagoons as small-sized bird sites needs full recognition.