The importance of Indus valley as one of the major flyway of the world is well documented (Roberts, 1991). This is mainly due to the availability of variety of coastal habitats and wetland system.
Birds depending on wetlands are popularly defined as waterbirds/shorebirds or waterfowl. Birds from north usually use Asia/Pacific region as wintering ground mainly because of the enrichment of the area. Many threatened to survive bird species are mostly dependent on wetlands habitats.
Waterbird species are also considered as useful indicators of the health of their habitat, changes in their population trend during the study period was regularly monitored on the wetland sites along the Karachi coast. These important coastal wetlands arc under severe pressure of the developmental activities and pollution.
The present study deal with the migratory pattern and aspects of population dynamics of waterbird species of four localities of Karachi coast viz Hawksbay, Korangi creek, Clifton and Cape Monze. The Study was carried out for a period of four years from 1993 to 1996.
More emphasis was given on the "Guild" structure in order to gather information on foraging habits, diurnal rhythm and diet. Waterfowl community of the study sites was studied based on guild formation.
Detailed ecological characteristics of the four study sites were given to provide the effect of composition of sediment and water quality on the diet of the birds. The Karachi coast was inhabited by more then 93 waterfowl species. However, 53 of them were observed in sufficient numbers and, therefore, selected for detailed investigation in the present study.
The guilds were defined on the basis of food and feeding style, degree of social foraging, diurnal habits and habitat characteristics. All 53 species of birds are divided do 6 guilds: 1, Herbivorous birds; 2, Stalking birds; 3, Fishing birds; 4, Visual Surface Foraging birds; 5, Tactile Surface Foraging birds; 6, Pelagic Foraging bird.
Population of 53 species of waterbirds from study area are described. A total of 163,404 birds were counted at Hawksbay, maximum number was noted in 1993 and they were gradually reduced to 18276 in 1996. Decline in number of various species of birds indicate that habitat did not favour the population at Hawksbay. The whole population at Hawksbay exhibits a regular decline from 63386 (1993) to 52396 (1994), 29346 (1995) and 18276 (1996). A comparative assessment of waterbirds of Hawksbay showed that this coastal habitat is becoming unsuitable for the herbivorous (gui1d-l) birds. However, birds of guild-3 immerged as dominating group in terms of their numbers.
A total of 225492 birds were counted at Clifton. The number of birds showed decrease in 1995 and 1996 after a sudden increase in 1994. Birds of guild-5 represented 63 % of the total birds' population. Reduction of population of some species (dunlin, for example) may be related to the loss of feeding zone. Some species showed irregular annual changes in their population at Clifton. It was also noticed that species of the same guild may behave differently. For instant, Venellus malabaricus declined from 418 in 1993 to 130 in 1996, whereas population of V indicus remained almost constant.
In case of birds from Korangi creek, a total of 153357 were observed. The number of birds increased from 35011 in 1993 to 14995 in 1996. Noticeable fact is that herbivorous birds (guild-I) were totally absent. Two species of guild-5 i.e. Calidris minula and C. alpine are present in such great number that they represented 75 % of the total birds in guild-5. Habitat of Korangi creek was found more suitable for small size birds of guild-4. Birds of guild -2 were represented here in very small Humber ranging from 8 to 10) only. The major contribution of the birds fauna of Korangi creek resulted from gui!d-5 comprising of 7 species.
Cape Monze provides an ideal place for roosting and feeding due to the presence of a variety of habitat forms. The area is mostly rocky with patches of muddy, sandy and sandy cum rocky substratum. A total of 63542 birds were counted during the study period (1993-1996). Maximum number of birds were noted in guild-3. Changes in number of birds varied widely in birds of different guild. Five categories were defined in the present study to describe the seasonal year wise changes in the number of birds. The population of birds of guild-l at cape Monze is also affected drastically due to unavailability of fresh water as these are dependent on hydrophytes.
The changes of birds population was also studied with reference to food availability. The birds concentrate at places of relatively high productivity, Food availability was discussed ill the light of bio ecological factors such as Salinity, Temperature, Water depth, Tide, Estuarine water Mangroves, Mud, Land Based Pollution, Oil Pollution and Pesticides.
Abundance of birds of different guilds was related to the availability of food, selection of foraging site is discussed with reference to birds of different guilds. Impact of water depth and tidal level was also found important in determining the site for foraging.
Mangrove areas were found effective for migratory as well as resident population of birds, Similarly backwaters and mudtlats area is ideal foraging ground for guild-1 preference of different type of habitat was taken into account while discussing the relationship between population abundance and habitat characteristics. Food preference of 53 species of birds was analyzed. Seeds and weeds were found as the predominant food of all 5 species of guild-1. Most of the species of guild-2 prefer fish and crustaceans as food in addition to) fish are also preferred by guild-3 species. Birds of guild-4 showed their likeness for crustaceans or insects. Mollusks including oysters, mussels, gastropods are the favourable food for guild-5 birds. Birds of guild- 6 showed varied choice but mostly amphipods and copepods.
Role. of bill length was given special attention. Modification in the bill shape is described in detail. Bills of 17 species of birds were analyzed. Bill of female was found longer then the male. Food preference was studied with reference to type of bills. Whimbrels were found more common around Hawksbay compared to curlews which prefer soft and thick layer of mud.
Different patterns of flocking and roosting were mentioned and discussed in the light of food type. Nocturnal feeders were found strong flyers as they cover more distance during each flight as compared to waders who take rest even after short distance. Diurnal birds were found to feed early in the morning.