Currently, I am on chapter 6 of Vera’s book Grazing Ecology and Forest History, and there’s a great sub-chapter (6.2.2) that explores the relationship between the Eurasian jay (Garrulus glandarius) and the oak. I learned a lot from reading this specific segment of the book, and thus hope that others also find the following information useful.
Firstly, one can certainly recognise that oak trees in Europe (Quercus petraea and Quercus robur) do not generally regenerate in woodland. Whilst acorns do germinate, after a period of usually less than a decade, they cease to be, as they are out-competed by shade-tolerant species, such as hornbeam (Carpinus betulus) and beech (Fagus sylvatica). Instead, oaks oft regenerate amongst thorny scrub (in grasslands, grazed areas, and within mantle and fringe vegetation bordering woodland), be the scrub comprised of hawthorn (Crataegus spp.), blackthorn (Prunus spinosa), holly (Ilex aquifolium), gorse (Ulex europaeus), or otherwise. In many an instance, one can perhaps wonder how the young oaks reached and regenerated within such scrub, and most notably so if there are no sizeable oaks within the immediate area. One explanation for this is, as explained by Vera, and courtesy of an array of sources, the Eurasian jay.
As an oak releases its fruit crop (acorns) into the harsh environment during September to November, the jay will collect many of the acorns (the healthy, ripe, good-sized ones, generally-speaking), and bury them at a distance from the tree that may vary from tens of metres to a few miles. When a jay collects only one or two acorns, it may bury them only a short distance from the point at which it gathered them from, but if the jay carries five or six acorns (in its gullet, throat, and beak – the largest being in its beak), then the acorns may be buried thousands of metres away. All of the acorns collected in one ‘trip’ will be buried in close proximity to one another, at a maximum spacing distance of around 15m (usually 0.5-2m spacings). At the level of a single jay, this dispersal of acorns may be relatively small, but when amplified to a group of jays 65 strong, research has indicated that (over the course of four weeks) up to 500,000 acorns may be dispersed. This dispersal is also very significant at distances from nearby oaks, with approximately 5,000 acorns per hectare when the nearest fruit-producing oak is around 200m away.
Jays also have a strong preference of burying their acorns in open areas with loose soil. Usually, this will be open areas outside of forests within grasslands and amongst thorny scrub (this explains why oaks may pop up amongst thick scrub), though if substantial forest clearings manifest (due to tree mortality – normally courtesy of windthrow and the forest edge effect), then they too may be alluring sites in which the jay may cache its treasure. Interestingly, open areas complete with loose soil and areas of thick scrub are akin to medieval wood pastures, where grazing would have occurred.
Once the acorn has been buried by the jay, it will cover the site with soil and leaves, in order to hide it from sight. In order to re-find it, the jay will remember the vertical structures of vegetation in the immediate area – their memory of where they have planted acorns is, in fact, very good. Interestingly, a jay will only be able to find acorns it has buried – it will not be able to locate acorns buried by other jays, unless that other jay is also present at the time of digging up the acorn. Typically, the acorns they bury will be eaten throughout the year, though the months of April through to August mark the period when fewest acorns are dug up and eaten. This drop in predation by the jay fits in well with the period of acorn germination, with the stem emerging during May and the first few leaves unfolding in June.
This development of the small leaf crown co-incides with when jays will begin to search for acorns with a little more intent, as it is during this time that they will be feeding and training their young to fend for themselves. When a seedling (that is still green in the stem, and thus not lignified) is found from an acorn that the parent jay had buried, it will pull it up and eat the acorn, before depositing the little seedlng back into the ground rather crudely. Its young will, actually rather hilariously, mirror this behaviour, though with far less finesse, and pull up not only oak seedlings but anything else they find as well. This removal of the acorn does however not have a huge impact upon the oak seedling, as the strong and extensive tap root developed rapidly after germination means that the little oak seedling is securely anchored into the ground and can fend for itself, even if the acorn (provides energy for seedling growth) is lacking. Essentially, the uprooting is a trade-off that the oak tolerates well, so to be able to grow in the full sunlight conditions the jay opts to bury its acorns in (and where they will grow best, as the oak is not shade tolerant). Research has even shown that many oaks bear the scar of early uprooting by a jay, though since that time they have developed into healthy young trees. Importantly, this disturbance event only happens once (usually), as jays will only uproot small seedling that they buried. Jays are very ‘untrusting’ of ‘foreign’ or ‘alien’ acorns and oak seedlings, and thus will avoid them, by-and-large.
The symbiotic nature of the relationship between the oak and the jay is cemented when one recognises that both benefit very well from the association. As remarked above, the oak is able to regenerate in ideal conditions, though as the jay selects only the healthiest and larger acorns there’s also a direct filter placed upon acorn populations, that ensures only the strongest acorns ever have a good chance of germinating. Furthermore, the wide dispersal of acorns by the jay ensures that oak populations can spread quite markedly, with small satellite groves emerging at sometimes great distances from the parent tree. On the side of the jay, the provisioning of an important food source is critical, as without acorns the jay, in Europe, may have to find other significant sources of food with good nutritional value. Of course, it is also important that the right conditions are available for the jay – this means open areas of grassland, or mantle and fringe vegetation. Closed forests are not suitable for the jay, and it will thus not bury its acorns there. In fact, even though forest stands of oak produce 10-times more acorns than sparsely-populated open-grown oak pasture landscapes, the density of acorns within the soil buried by jays is 50-times greater within the open landscapes. Such landscapes are generated by grazing, according to Vera, and whilst modern grazing has been created by man (up until the decline on grazing, in recent times), historically the wild boar (Sus scrofa) would have provided good conditions (due to its disturbance of the soil in grasslands, which were created by large roaming herbivores – the auroch, deer, and wild horse, for example) for where the jay could bury its acorn collection.
Source: Vera, F. (2000) Grazing Ecology and Forest History. UK: CABI Publishing.