HULSCHER J & P DRIESSEN (2010) The importance of hoarding walnuts and acorns for Rooks Corvus frugilegus. LIMOSA 83 (2): 49-60.

Hoarding walnuts and acorns in autumn is common practice in Rooks. In this study we try to quantify the number of recovered nuts and acorns in relation to the daily food requirements of the individual Rook. The study was performed in the NE part of The Netherlands near the village of Haren. Nuts and acorns were collected by Rooks in the village and buried some 0.5-1.0 km away in adjacent meadows intersected by ditches (Fig. 1). Each autumn the ditches are dredged and the sludge is deposited in ridges along their banks. Rooks buried nuts and acorns in the fields, but particularly in the ridges. Hoarding started late August and continued until late October. Recovery and consumption of nuts and acorns started in the very beginning of the hoarding period and continued until April when the grass started to grow and the ridges were removed by farmers. During two seasons we collected the clearly visible shell remains of walnuts and acorns in particular fields and ridges, about twice each month. Shell remains were dried and weighed; split but otherwise complete walnut shells were measured (Fig. 2). Based on the shell weight of dried samples of freshly collected walnuts and acorns of known size, we converted the collected shell remains into numbers and dry weight of flesh eaten (Figs. 7-9). Counts made throughout the season quantified the number of Rooks involved in hoarding and eating nuts and acorns (Fig. 3). In walnut trees, Rooks searched for nuts which were visible in the partly opened peel. Nuts were peeled completely before being transported to the caching site, except early in the season when small numbers of unpeeled fruit were transported. Some nuts were already eaten in the tree. Rooks probably selected against empty nuts (Tables 1, 2). Nuts recovered in the field were hammered open. In most cases nuts were split into two halves, but early in the season they were frequently opened by hammering a hole in the shell (Fig. 4, 5). We argue that splitting the nut is easier when the shell becomes weathered.
      In 2007/08, the number of walnuts eaten was highest in late October, corresponding with the end of the hoarding period. Most acorns had already been eaten by then (Fig. 6). Shell remains were more often found on ridges along the ditches than in fields, by a factor 5-9 for walnuts and by a factor of 3-14 for acorns (Table 4). Densities of eaten fruit found by us averaged 365 /ha for walnuts and 719 /ha for acorns. The total number of walnuts consumed per bird over the season was calculated to be equivalent with the food requirement for 13 days, and of acorns for 8 days, during winter time. In this study recovery rate did not increase during cold days, as we did notice in 2005. Despite the loss of some nuts and acorns to fellow Rooks or Carrion Crows Corvus corone, hoarding will be retained when the remaining fruit is sufficiently abundant to bridge adverse periods when other food is scarce or unavailable.

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