Berries provide the means for some plants to disperse their seeds over longer distances than would otherwise be possible, without the help of some other creature. The plants often offer nutritious fleshy fruits to attract birds to take the seeds, hidden inside, and ingest them. The seeds have tough external coats that protect them from the digestive systems of birds, allowing them to be deposited elsewhere once they have passed through the bird’s gut.
There is plenty of evidence that some plants are particularly valuable to birds as a source of berries or seeds, and many people plant berry-producing plants and shrubs in their gardens for birds. We wanted to know when berries are available and how they are used by different species of bird.
The study had three key components which investigated how birds use berry plants throughout the autumn and winter months:
- Berry availability survey
- Berry depletion survey
- Birds feeding on berries
Our main focus was the use of berries by wintering thrushes: Blackbird, Song Thrush, Mistle Thrush, Redwing and Fieldfare.
However, participants could also record other birds feeding on berries, like Waxwing, Starling, Blackcap and Woodpigeon.
Methods
Berry availability survey
This component investigated which berries were available and when, and which thrushes fed on them. We wanted to know which plants produce berries at different times throughout the winter and for how long berries remain available.
Participants kept a simple weekly record of which plants in their gardens had unripe and ripe berries, and which winter thrushes they saw feeding on the berries.
Birds feeding on berries
This component investigated which birds fed on which berries and how efficient they were at handling the different types of berry.
David and Barbara Snow famously studied how birds fed on different berry-producing plants in the countryside around the Vale of Aylesbury. We wanted to expand on their work, both geographically and in terms of which plants are favoured by particular bird species.
Participants selected one of the shrubs in their garden to watch. When a bird arrived to feed, participants recorded:
- the time,
- a tally of the number of berries that the bird attempted to take,
- the number of berries the bird actually ate, and
- any additional behaviours, like interactions with other birds.
Berry depletion survey
This component investigated how the availability of berries on different plants changed over the course of the winter.
Participants kept a regular count of the number of unripe, ripe or damaged berries on plants throughout the winter to find out how quickly they disappear. They recorded berries on different plants separately.
Because berry depletion rates might vary between parts of the same plant e.g. berries on the top of a shrub may be taken preferentially by birds, participants could also record berries on different branches on the same plant.
Results
The results have increased the depth and breadth of our understanding, and enabled us to further improve the efficacy of the wildlife gardening advice we publish about providing berries for birds.