Seasonal changes in bird population densities with special emphasis on rice depredation and the side effects of quelea Control measures on non-target birds at mwea irrigation scheme, Kenya

Abstract

This study was carried out from October 1984 to September 1985 at Mwea Irrigation Scheme (M.I.S.), which covers an area of 12,145 hectars• This study aimed at:- (I) Determining whether significant damage was caused to the rice crop by birds, particularly Quelea quelea, when ln large populations. (2) Determining the side-effects of Fenthion (organophosphate pesticide) spray when used as a control measure on the rice pests, particularly on Q. quelea and other bird species.-- Irrigation-fed rice cultivation covered 6 I' "·0hee tares and 2,500 hectares in 1984 season and 1985 season, respectively , The rest of the area in'both seasons was under various land uses. This study covered two rainy seasons: October - November 1985 and March - May 1985 and two dry seasons: December 1984 - February 1985 and June - September 1985. Monthly population estimates of birds were obtained by transect counts. Birds' population sizes fluctuated with seasons, and was most spectacular in Q. quelea P. rubiginosus E. Qlbonotatu$~ and Dendrocygna bicolor. These population fluctuations were largely due to migrations into and out of M.P.S., presumably due to changing conditions of the feeding and breeding grounds. The rates of these population changes, particularly in the common grain-eating birds suggested that breeding and mortality were not significant factors that influenced the fluctuations. Diets of birds were investigated by crop contents analysis of the mist-netted birds. The principal food types of the grain-eating birds were rice, grass seeds and incidentally, animal matter. Availability of rice, grass seeds and cereal crops other than rice was influenced by rainfall and agricultural practices at M.l.S. Most of the grain-eating birds ate rice more frequently and at higher amounts than grass seeds even during periods of high grass seed abundance. This suggests that these birds preferred rice to other available seeds. Generally, the grain-eating birds profited physiologically while in M.l.S. Data on body fat deposit suggests that birds fattened while at M.l.S., but data on daily and monthly body weights were insufficient to conclude whether significant changes occurred while the birds were at M.l.S. Different methods of estimating damage to rice by birds gave different results. The shortcomings of each method are discussed. Overall damage to rice due to birds was higher in the 1984 season than in the 1985 season despite the significantly higher birdpressure Ln the 1985 season than in 1984 season. Damage to rice occurred from the "milky" phase to harvest time. Generally, the levels of bird damage to rice was not found to be different at varying distances from birds'- roosting nests mainly the reed marshes. Out of the 87 bird species o.b.served at M. I.S., only a few were found to be rice pests, and their importance in terms of the amounts of rice they ate is as follows:- E. aZbonotatus~ . Q. quel.ea, P. rubiqinoeue and P. cucul/latue, The other bird species were regarded to have caused no significant loss. The total loss of rice attributed to bird action in both seasons was 3,142 tons, which was about 10% of the total yields. The population of Q. quelea which exceeded 100,000 individuals per bird-day in September 1985 when rice was most vulnerable to these birds prompted attempts to reduce their number by spraying them with Fenthion. It was assumed that such action would reduce the estimated high losses of rice. The Fenthion spraying had important biological and economic implications. Q. quelea were sprayed at sites where they assembled in large roosts often with many other non-target bird species. Thus, non-target birds were killed when the quelea roosts were being destroyed. Other hazards that might have occurred to the birds and to the biosphere in general within the spraying sites and their proximal zones were not studied. This study brought to light the conflict that existed between the principles of rice protection from bird damage and that of conservation of wildlif e in using toxic Fenthion at N.1.S. It 1S suggested that in future, the ecological and economic implications of employing toxic pesticides such as Renthion as a bird control strategy should be fully evaluated be(ore and after they are implemented.