Interactions between frugivorous birds and weeds in Queensland as determined from a survey of birders

Abstract

We used a questionnaire survey of experienced bird observers to identify the main bird-dispersal agents of introduced plants and weeds in Queensland. From the survey results we gathered information on the foraging habits of different bird groups (small and large generalist frugivores, fruit specialists and seed destroyers). We also examined the relationships between 1) fruit size and the number of bird species utilizing each weed species and 2) weed invasiveness and the number of bird species utilizing each weed species. The study yielded 230 observations of frugivory on introduced and weedy plants. Thirty eight bird species were observed feeding on the fruit of 28 weed species. Weed fruit ranged in size from 2.5 mm (Rubus fruticosus L.) to 50 mm (Opuntia stricta Haw.). Eighty nine percent of all fruit consumed by frugivores was below 15 mm in diameter. Most fruit was gulped, but birds resorted to pecking or mashing fruit that exceeded gape width. Birds with the most diverse weed fruit diet included the figbird (Sphecotheres viridis
Vieillot), silvereye (Zosterops lateralis Latham), olive-backed oriole (Oriolus sagittatus Latham), Lewin’s honeyeater (Meliphaga lewinii Swainson), pied currawong (Strepera graculina White) and regent bowerbird (Sericulus chrysocephalus Lewin). Those weed species fed on by the largest number of bird species were camphor laurel innamomum camphora L.), ochna (Ochna serrulata (Hoechst.) Walp.), wild tobacco (Solanum mauritianum Scop.), umbrella tree (Schefflera actinophylla (Endl.) Harms), glossy nightshade (Solanum americanum Mill.) and Chinese elm (Celtis sinensis
Pers.). Bird functional groups differed with regard to flock size, the size of fruit eaten, the habitat type where feeding was observed, the amount of time spent feeding in fruiting plants, the number of fruits consumed and flight distances after a feeding bout. Weed invasiveness was moderately correlated to the number of frugivorous birds observed feeding on each weed species.