Seed selection by crossbills within cones of Scots Pine
Description
Crossbills Loxia species have been observed to abandon some seeds within the cone, which would appear to be a sub-optimal foraging strategy unless these represented less profitable seeds. We undertook a detailed field assessment of crossbill foraging on Scots Pine Pinus sylvestris in order to determine the cues potentially used to select seeds and whether these were based on the mass of the seed, its position within the cone, or on cone characteristic such as thickness. Analysis of Scots Pine cones dropped by foraging crossbills showed that they left behind over four seeds per cone, representing approximately one fifth of those estimated to have been originally present. We found no evidence that seeds were left behind because they were at positions which usually contained lighter seeds. Although seeds were usually smaller underneath the basal and distal cone scales, seeds were not only left behind within these positions. Nor did crossbills start or stop foraging at a fixed position within the cone. Rather, crossbills selectively foraged on heavier seeds, regardless of their position within the cone, leaving behind the majority of seeds under 2 mg. Crossbills foraged on less well-defended cones, which contained lighter seeds, thus any seeds overlooked would tend to be lighter than the population average. However, the magnitude of the difference in seed mass suggests additional foraging behaviour was involved. Selectively foraging on heavier seeds would be highly advantageous, avoiding wasting energy extracting lighter or empty seeds. This set of data files comprises 6 Excel files. Each file contains 2 worksheets. The first is the data that was used for analysis in the manuscript; ‘Seed selection by crossbills within cones of Scots Pine’. The second worksheet gives a key to the meaning of the title for each column in the dataset, and a brief explanation of how the data was obtained. Full details are given in the manuscript. The file 'GPS tree positions' gives the locations of each point in the study sites. These locations are not used in the data analysis. The file 'Ages of cohorts of cones' explains how the age of cones (cone age) was quantified relative to the point in time that they were first accessible to crossbills. This term is used within all data sets and throughout the manuscript. The file ‘Cones cut from the canopy vs fed upon by crossbills’ contains records of cones either cut directly from the canopy of Scots Pine trees or collected from the forest floor after being dropped by foraging crossbills. The file ‘Undamaged cones vs cones fed upon by crossbills’ makes a similar comparison to cones which had naturally fallen to the forest floor. In the files ‘Seeds remaining within different positions of the cone’ and ‘Seeds remaining beneath scales split by crossbills’, seeds are recorded according to the position within which they remained in the cone.