Friday, 2 December 2011

Caching carrion crow

ResearchBlogging.orgMany 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

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