Relations écologiques des Avifaunes sédentaire et migratrice dans une savane sahélienne du bas Sénégal

Overview & Synopsis

Monthly censuses were made of an area of 25 hectares (±62 acres) in the sahelian savannah of the lower Senegal valley, from February 1960 to May 1962. All the birds seen were counted, palearctic and african mi grants as well as breeding pairs and nomadic individuals, to estimate the monthly biomasses of the avian consumers. The average adult weigt, the migratory status, the staple food, the layer at which the birds ordinarily live, and the seasons of reproduction of the 97 species recorded (66 sedentary species and 31 palearctic migrants) are given in table 1. Among the sedentary species, only very few live all the year round in the actual census area, most of them being nomadic. Table 2 gives, month by month for the three consecutive years, the number of individuals counted, the number of species and the biomasses in grams, both for sedentary birds and for migrants. Tables 3 to 5 give the same data for the three main trophic categories of our avian population : vegetarian, polyphagous and insectivorous. Eight months out of twelve, palearctic migrants were present in this area ; for most of that period they form an appreciable part of the total population. The highest densities and the greatest biomasses occur at the same time of the year in african species and in migrants. The maximum population density of african birds reached 4,2 birds per hectare in September 1960 and 8,2 birds per hectare in October 1961. The corresponding bio masses were 183 grams per hectare and 485 grams per hectare respectively. For migrants the highest densities were 1,8 bird per hectare (60 gr/ha) in October 1960 and 2,1 bird per hectare (58,3 gr/ha) in October 1961. The lowest densities of sedentary birds were reached during the second half of the dry season : 0,8 bird per hectare (52,5 gr/ha) in May 1960, 1,3 bird per hectare (106,1 gr/ha) in April 1961 and 1,4 bird per hectare (93,1 gr/ha) in May 1962. Therefore the carrying capa city of the environment for the critical periods of the annual cycle is probably of no more than 1 bird per hectare (± 100 gr/ha).- The majority of these birds were vegetarian. Even during the most difficult periods of the year all the trophic groups of the avifauna are still represented in our area. Vegetarians and polyphagous birds are the more numerous. Not only seeds are abundantly produced in such savannahs, but a number of trees bear fresh leaves, flowers and fruits during the dry season. There is therefore no inter-specific competition among vegetarian birds. Some seedeaters ( Quelea quelea and Passer luteus) form mixed troops at that time of the year and even roost together. The situation is different for insectivorous species. Among « sedentary » forms six of the main insect consumers leave the area during the dry season. On the other hand migrant and sedentary birds very seldom belong to the same genus or to a closely related one. They are but six exceptions, five of them being repre sented by species with very low populations. As for the hoopoes, the african form nests at a time of the year where the migrant species is absent. Migratory birds, as a whole, constitute a « floating population » of consumers taking advantage of the seasonal « energy surplus » existing in their breeding and wintering areas. The resident populations of birds living permanently in boreal climates as well as in tropical savannahs, are indeed unable to consume all the food locally produced, since the low carrying capacity of their environment during the critical stages of the yearly cycle keeps their populations down. Migrations (both short and long) are therefore considered as an adaptation for the efficient utilization of seasonal food surpluses that sedentary species are unable to exploit entirely.