Seed dispersal and movement patterns in two species of Ceratogymna hornbills in a West African tropical lowland forest

We studied two species of Ceratogymma hornbills, the black-casqued hornbill , C. atrata, and the white-thighed hornbill, C. cylindricus, in the tropica l forests of Cameroon, to understand their movement patterns and evaluate t heir effectiveness as seed dispersers. To estimate hornbill contribution to a particular tree species' seed shadow we combined data from movements, de termined by radio-tracking, with data from seed passage trials. For 13 indi viduals tracked over 12 months, home range varied between 925 and 4,472 ha, a much larger area than reported fur other African avian frugivores. Seed passage times ranged from 51 to 765 min, with C. atrata showing longer pass age times than C. cylindricus, and larger seeds having longer gut retention times than smaller seeds. Combining these data, we estimated that seed sha dows were extensive for the eight tree species examined, with approximately 80% of seeds moved more than 500 In from the parent plant. Maximum estimat ed dispersal distances fur larger seeds were 6,919 and 3,558 m for C. atrat a and C. cylindricus, respectively. The extent of hornbill seed shadows sug gests that their influence in determining forest structure will likely incr ease as other larger mammalian dispersers are exterminated.