Abstract
Seed dispersal seems to be extremely important in agrocoenoses where suitable habitats (patches) are surrounded by an unfavourable environment (matrix). The role of the rook Corvus frugilegus, an omnivorous bird, in seed dispersal was studied in the agricultural landscape of Eastern Poland. We analyzed 739 pellets produced by regurgitation, which were collected under breeding colonies in April, May and June. Our goal was to i) assess the structure of the seed pool in pellets; ii) evaluate the temporal variation in the pellet seed pool on two different time scales; iii) compare the species composition of seeds in pellets and vegetation under the rook nests. Seeds were present in 18 % of pellets; 571 seeds were found, half of them belonging to dry-fruited species, without any obvious adaptations to endozoochory. These seeds could be an additional source of food, or they could have been accidentally swallowed during foraging for other food items. Taking into consideration the abundance of the rook population, we assessed the mean number of seeds transferred by one bird to be from 4 seeds per month in April and up to 160 seeds in June. The most important factor responsible for qualitative and quantitative structure of seed pool in pellets is the time when pellets were regurgitated. The type and availability of food determines the number and species structure of dispersed seeds. The comparison of the species structure of the seed pool in pellets and of the herb layer under the breeding colonies showed that the rook could effectively disperse seeds of weeds, meadow and ruderal species, that could germinate under the dense canopy of trees at the studied sites.
Highlights
Species composition is a function of the availability of seeds and safe sites suitable for germination
The situation changed in June when the diet became more varied: fragments of cereal grains were present in 95 % of pellets from April and only in 39 % pellets from June
We observed a huge increase in the number of seeds in June (362 seeds found in 80 analyzed pellets in comparison to 40 seeds in 384 pellets in April, and 169 in 275 pellets in May; mean values in Table 2), we must emphasize that the distribution of seeds in pellets was extremely
Summary
Species composition is a function of the availability of seeds and safe sites suitable for germination. Dispersal limitation is often crucial for local species composition and vegetation structure (Eriksson and Ehrlén 1992; Wang and Smith 2002; Ozinga et al 2005). Large herbivores can contribute to the dispersal of some seeds while eating the foliage of parent plants (Janzen 1984). All these plant-animal interactions have been occurring for millions of years and some special traits of seeds and fruits were selected to attract animals, enhance survival in gut and increase edibility of the plant vegetation at the time of seed and fruit ripening (Janzen 1984; Pakeman et al 2002)
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