Jays and acorns

A few weeks ago, on a local urban walk, I had the chance of watching some Jays collecting acorns from the oaks. It's been a good mast year and the veteran oaks at the cemetery were laden with acorns. Jays are a scarce and shy species in my city, but occasionally I come across them in the most wooded green spaces. These individuals, at least two, were collecting acorns, flying back and forth from the tree to their larder, a site out of sight beyond the road. Before disappearing into the foliage of the still green oak, they often paused on trees nearby, which is how I took the photo below, the wind pushing the Jay's crest up.

Caching behaviour

Jays are omnivorous and opportunistic in their food habits, but during autumn they become dedicated food hoarders. They are resident birds that depend on their stored harvest to survive the winter and early spring, and are acorn specialists. They have a pouch under their tongues and can carry up to nine acorns in a single trip, although in 80% of trips they will carry one or two. Their acorn reserves will be scattered, a single acorn buried in a shallow hole in the ground, around their home range, and the reserves will be steadily used. You might see Jays in small parties - often detected by their screeching calls -  but they tend to be mindful before they bury each acorn, away from the prying eyes and ears from other Jays, which will be all too happy to pilfer other individual's hoards. Experiments by Nicola Clayton's group have shown that their caching behaviour is dependent of potential pilferers being around and they will cache preferentially behind an opaque screen when other individual is watching, if they are within earshot of other individual, they will cache in quieter substrates (sand) rather than noisy gravel.

Oak foresters

Jays can store several thousand acorns per year, and rely on their spatial memory skills to retrieve them.  They have a mutualistic relationship with oaks, being crucial dispersers of acorns. As not all acorns will be consumed by jays, oak long-distance dispersal depends mainly on Jays, and the forgotten or unused acorns are perfectly planted to successfully germinate. The dispersal distance,estimated using radiotransmitters inserted in acorns provided to Jays in feeders in the landscape ranged from a few meters up to about half a km.

A Jay with an acorn it has just retrieved. It took about minute to find it. Watch the full clip here:

Complex cognitive abilities

It is striking that Jays manage to remember the location of thousands of caches. As other caching birds, they have excellent spatial memory, and perform very well in spatial memory tasks. Experiments have also shown they have episodic memory, an ability to recall specific events and details, both when caching and when pilfering. They might also prefer to cache near vertical structures, which might facilitate recall.

Jay with peanut. Jays are part of the bird community of towns and cities, particularly when tree cover and food resources (oaks or bird feeders) are available. 

More information

Kurek, P., Dobrowolska, D. and Wiatrowska, B., 2019. Dispersal distance and burial mode of acorns in Eurasian Jays Garrulus glandarius in European temperate forests. Acta Ornithologica, 53(2), pp.155-162.

Legg, E. W. & Clayton, N. S. Eurasian jays (Garrulus glandarius) conceal caches from onlookers. Anim. Cogn. 17, 1223–1226 (2014).

Madge, S. and G. M. Kirwan (2024). Eurasian Jay (Garrulus glandarius), version 3.0. In Birds of the World (B. K. Keeney, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.eurjay1.03

Pons, J. & Pausas, J. G. Acorn dispersal estimated by radio-tracking. Oecologia 153, 903–911 (2007).

Shaw, R. C. & Clayton, N. S. Careful cachers and prying pilferers: Eurasian jays (Garrulus glandarius) limit auditory information available to competitors. Proc. Biol. Sci. 280, 20122238 (2013).   

Rooks hoarding acorns

It is a mast year, with a bumper crop of acorns across the city. October is peak acorn season and last week I saw the first Rooks carrying acorns. Acorns are packed with energy, and several bird species take advantage of them. Woodpigeons, presumably swallow them whole from the trees, but Rooks and Jays collect the harvest and store it for use during the winter. Both species are scatter hoarders, and cache individual acorns on the ground, and use their extraordinary spatial memory to recover them later in the year, when other food resources are scarce. Although the Jays are best known for this behaviour, Rooks are also amazing acorn hoarders. Rooks can transport acorns - and other food items - in a pouch under their tongue, which obviously bulges as they fly over with their pouch full. The number of acorns they can carry depends on the acorn size, and varies from 2 to 7. They prefer to cache the acorns on grass, and can fly up to 4 km from oaks to suitable grassland. Once they find a good spot, they drop all the acorns they are carrying and bury each one by one, by first making a hole in the ground with their bills, and then hammering the acorn in and covering it with grass, leaves or soil. Later on, they will visit the caching sites in the winter, find their stored acorns and crack them open to feed. 

A vocal Rook on an oak canopy, surrounded by plenty of acorns.

Rooks, unlike Jays, are very social and engage in communal acorn collecting, becoming very vocal when landing on the oaks canopy. They prefer to gather acorns with other Rooks, and individuals appear to join other individuals gathering acorns by flying in the opposite direction of individuals with full bills. When it comes to caching though, Rooks prefer to be alone, to avoid cleptoparasism, when other individuals try and steal their stored acorns.

Rooks displaying, the individual on the right, with distended sublingual pouch, passed an acorn to one on the left, presumably they are a mated pair.

More information

Waite, D. R. K. Food caching and recovery by farmland corvids. Bird Study 32, 45–49 (1985)

Källander, H., 2007. Food hoarding and use of stored food by rooks Corvus frugilegus. Bird Study, 54(2), pp.192-198.

The Crow and the Squirrel

At this time of the year, Grey Squirrels are very busy burying seeds for the long winter. Crows also cache food, and there is an interesting dynamics between both species. Crows are known for eavesdropping on each other, and they avoid caching food if they see other crows around that might steal their food if they know where is stored. But Carrion Crows are good at stealing squirrel food as well. They walk nonchalantly nearby, while the squirrel buries some food, and later they retrieve it and either they eat it or store it somewhere else. Well, the squirrel seems to realise it is just not working for her, and I have often watched how the squirrel goes nuts and starts chasing the crows all over the place (top shot).

Carrion Crow storing food

Crow having a good look at where the squirrel is hiding its food.

These photos are today's, but I managed to get a video of a squirrel chasing the crows last winter.

Food hoarding Coal Tits

One of the things I enjoy the most about watching birds is that even familiar species often surprise me with behaviour I have not seen before. I watched birds feeding on seed and peanut feeders a few days ago. A pair of Coal Tits, Periparus ater, showed their usual energetic prowess: a constant back and forth between the feeders, quickly snatching a seed and then flying away, to return within a few seconds later for another one. This time though, I could see where the Coal Tits were going and they were not flying away from the feeder to eat their seed in solitude, without the hassle of the larger birds, instead, they were storing the seeds: on the ground, pushing the seed into the soil, in the cracks in the pavement, on branches, in a conifer bush or in pots.
 Although corvids are more widely known to store food, food storing is common in Tits (parids). Marsh, Willow and Crested Tits are regular food storers, (although Great and Blue Tits are not). Parids store insects and other invertebrates (after decapitating them), seeds and nuts. They might store hundreds to thousands of items per day, using a different hiding place per item and even covering the hidden item with a piece of bark or stone. They might store the items a short distance away from where they were found, or up to 100 m away. They may often retrieve the food after a few days, but possibly much longer as not readily available food items (such as caterpillars) are often seen consumed by tits in the winter. A Japanese species, the Varied Tit, can feed its nestlings up to 5% of stored seeds from the previous summer-autumn. Parids display good memory not only spatial, to go to the exact place where the food was stored, but also to remember which caches have already been retrieved. Food storing in such small birds might contribute to survival when food is strongly seasonal or unpredictable and it can be retrieved when its consumption will make the largest contribution to survival.
 I managed a very short clip of a Coal Tit storing a seed.


But I wasn't the only one watching. This blue tit was also very interested in the Coal Tits.

After watching it, the Blue Tit came right up to a Coal Tit about to hide a seed - it is just visible in its beak in the photo below. After a brief hesitation, the Coal Tit decided to go somewhere else and left just after I took the following shot. Sorry about picture quality today, but all the shots were taken through glass.

More information
David F Sherry (1989). Food storing in the Paridae Wilson Bulletin, 101 (2), 289-304

Caching carrion crow

Many species of the crow family store food when it is plentiful for future use in harder times, a behaviour called caching. Some species rely on stored food more or less all year round, but others might do it occasionally. Carrion crows, Corvus corone, in particular, hoard food when they find some discarded human food, or other, occasionally abundant supply of edible items such as acorns. Today, I was walking along a path when I noticed a carrion crow on the verge. Carrion crows around here are usually very wary of humans and expected it would fly away. It didn't. I stopped with my back to the crow, took my camera slowly out of my bag and took some photos. The crow (above) was indeed very preoccupied with a few pieces of food on its beak, putting them down carefully, covering one with leaves, and then moving a few steps and repeating the procedure with another piece. I have watched carrion crows caching food - including chocolate! - before. What happens to this stored food? Do crows remember where they stored it? are these valuable resources to turn into in times of hardship?
  R.K. Waite carried out observations of caching crows, rooks and magpies in fields near copses. The main items they cached was acorns, and this behaviour was most common in the autumn, although carrion crows also cached large earthworms when they were plentiful. The birds carried the items in their bill or in a pouch under the tongue and hammered the acorn in a hole they had pecked on the ground, covering it afterwards with leafs, tufts of grass, or soil. These corvids are scatter-hoarders, they do not use the same exact location every time, but the stored food was distributed in many sites, often on the fields away from the trees. By January, there were no acorns to be seen on the ground. It is unclear if Carrion Crows remember the exact location of each cached item, although this has been suggested for Rooks, but they might retain a memory of the general area. Waite found out two different retrieving behaviours. In the first type, individuals foraging in the field for invertebrates came across a cached item, apparently, just by chance, and ate it, they carried out looking for invertebrates afterwards. In the second type of behaviour individuals appeared to be actively searching for cached items during winter:

On 14 occasions in late winter, flocks of Rooks or pairs of Carrion Crows or Magpies were seen to recover cached acorns in a quite different way. First, acorns were found about 10 times more quickly. Most observations were of birds foraging on fields rarely used at other times, while some birds found more than one acorn during a foraging bout and only eight out of 56 birds took any invertebrates. Second, searching occurred on days when temperatures were significantly below average and invertebrate availability was reduced

Waite's analysis shows that recovering cached items is a profitable way of foraging. The time spend storing the food should the taking into account as a cost, but as this happens where there is not a lot of competition, the nesting season is over, and there is a surplus of food, this cost is offset by the benefits of retrieving the food in cold days when foraging for earthworms and other invertebrates is not very profitable. Waite wondered if Rooks, Carrion crows and Magpies acorn caching behaviour might make them inadvertent, but better foresters than the Jay, as inevitably, some acorns will be forgotten or just not needed and will germinate in the spring in the fields, away from established woodland.

More information
Waite, R. (1985). Food caching and recovery by farmland corvids Bird Study, 32 (1), 45-49 DOI: 10.1080/00063658509476854