Significance and extent of secondary seed dispersal by predatory birds on oceanic islands: the case of the Canary archipelago

1. Secondary seed dispersal is a multistep process with two or more phases, which involve different dispersers that usually extend the distance from the seeds parent plant. This ecological process has been recorded in some subtropical oceanic islands, where predatory birds commonly consume frugivorous lizards and disperse seeds already consumed by the lizards. 2. We evaluated the overall importance of this type of secondary dispersal in the Canary Islands, the only place worldwide where it has been studied in depth. From an examination of all the islands and their suitable habitats, we found seeds from 78 plant species inside 2098 shrike pellets and 5304 kestrel pellets. A greater number of species were secondarily dispersed by kestrels (76; 97%) than by shrikes (26; 34%). 3. Forty-four (56%) of the total species detected in pellets were identified at the species level, comprising 73% native and 27% introduced species. Seventy per cent of these identified species were fleshy fruit-bearing plants and 84% of the interactions took place in open habitats, close to coastal areas. 4. Germination experiments showed that seeds of at least 32 plant species were viable after being removed from the bird pellets. A similar pattern of seed germination was detected for seeds from the droppings of lizards and pellets of shrikes, showing both to be effective dispersers. However, the seeds dispersed by kestrels had different levels of success depending on the number of gut passages experienced. Seeds that had undergone double gut treatment (lizard and secondary ingestion by kestrel) had reduced germination rates of many small-and medium-sized seeds compared with seeds ingested by lizards and discarded inside the lizard guts by kestrels. 5. We also studied the relationship between body length and gape width of lizards in order to assess limitations on the sizes and quantities of seeds available for secondary dispersal. Kestrels can disperse a greater number and variety of seeds because they predate larger lizards that potentially carry greater seed loads. 6. Synthesis. The current results show how these non-standard long-distance dispersal events produced by predatory birds can be considered as a regular and generalized process on all islands of the Canary archipelago.