Pic: Lisa Geoghehan / Alamy Stock Photo
Words: Kate Risely, of the British Trust for Ornithology (BTO)
For a few weeks in autumn, Jays seem to be everywhere, and according to BirdTrack they are twice as likely to be recorded at this time of year than in the spring and summer. They are eye-catching birds, noticeable for their pink, black, blue and white colours that are so unusual for a crow.
What’s behind this flurry of activity? In autumn, Jays collect acorns and store them for the winter, and this is a massive operation. Birds can travel long distances out of their home ranges to productive oak trees and spend hours every day ferrying acorns back and forth.
They are able to carry several acorns at a time in their gullet, as well as a larger one in their bill, and these are secreted in nooks and crannies in trees, under roots and moss and in the ground. It has been calculated that a single Jay can relocate 5,000 acorns in a few short autumn weeks, and with a British population of 170,000 pairs of Jays, it’s no wonder they intrude on our consciousness during this busy time.
Over the course of the winter the birds will revisit their many little hiding places and eat the acorns, but a proportion are never retrieved; this is the part of the process that benefits the oak tree, as millions of acorns are dispersed and planted by willing helpers.
Acorn crops can vary hugely from autumn to autumn, or even fail entirely, depending on weather conditions earlier in the year and pest levels. Jays are by nature sedentary, but if they realise they won’t have an acorn store to see them through the winter they can move long distances, and in large numbers.
Jays often make these ‘eruptive’ migrations in Scandinavia, where they are unlikely to survive without a winter larder, and in years of particularly poor acorn crops across Europe these movements may even be noticeable in Britain. One such year was 1983, when thousands of birds were observed migrating into and across Britain and even reaching Cornwall, where 800 Jays were seen together in a single field.
Responses to crop failure are not always so noticeable, but with modern online tools, patterns can clearly be seen. The BTO Garden BirdWatch results pages show how numbers of Jays visiting garden bird feeders in autumn vary from year to year, and it’s interesting to compare these to the reports of tree seed crops on the Nature’s Calendar website. In years when reports of acorns in the countryside are low, more Jays are seen in gardens, demonstrating how birds fall back on human-provided foods when their preferred natural foods are in short supply.
While Jays are mainly reliant on acorns and other seeds in winter, at other times of year they mainly forage on the ground for invertebrates, and have been known to take small mammals, other birds, bats and even fish. This generalist lifestyle has enabled them expand from their woodland strongholds into more suburban areas, following the general decrease in human persecution.
This success has led to an increased population, with numbers at their highest levels since population monitoring began in the 1960s.