Blog by Dr Ron Summers, Principal Conservation Scientist, RSPB Centre for Conservation Science.

The woodland landscape in Britain has changed enormously over the last century, most notably through the planting of large areas of North American conifers (Sitka spruce and lodgepole pine) and larches from Europe and Japan. Prior to these plantations, the native Scots pine was the only conifer available to common crossbills when irrupting crossbills arrived from the European continent. 

On the continent, common crossbills are associated with the Norway spruce, a species to which they are adapted. The availability of a different suite of conifers in Britain provides an opportunity for common crossbills to exploit a wider and novel range of food species.

In this new study, we investigated the physical properties of cones from different conifers to determine the strength of likely defence mechanisms against crossbills, bearing in mind that crossbills usually cut the peduncle to remove a cone from the tree, prise or split the scales to access the seed and remove the seed coat to obtain the kernel.  

Japanese larch had the thickest peduncle and Norway spruce the narrowest.  Lodgepole pines had the greatest defence against cone removal by being sessile, and because cones grow in clumps, the bases of cones are mutually shielded.  Scots and lodgpole pines had the longest and thickest scales, which were positively related to cone size.  Sitka spruce and larch scales were short, and in the case of Sitka spruces, they were paper thin.  The seed coat of larches was thickest, and that of Sitka spruce were thinnest. Overall, Sitka spruces seeds were least defended against crossbills.

 

Photo: A female common crossbill foraging on a Sitka spruce cone.  This is the most profitable conifer for common crossbills in Britain because the seeds are protected only by paper-thin scales. by Ron Summers.  

A major part of seed protection is the array of overlapping scales which have to open apart to shed the seed. Therefore, access to seeds for birds is greatly enhanced when the scales open. Scots pine and Norway spruce scales open in spring, lodgepole pines open in winter, and Sitka spruce in autumn through to spring, though there is partial closure in winter if the weather is damp or snowy. 

Feeding trials on captive common crossbills confirmed that common crossbills have great difficulty in opening closed Scots pine cones and also some difficulty with closed lodgepole pine cones. Further, it took longer to extract lodgepole pine seeds from large open cones (i.e. those with thicker scales) than small cones (Fig. 1).  For Sitka spruce, for which there is no relationship between scale thickness and cone size, there was a small effect of the size of open cones on seed extraction rate (Fig.1).  It is probably trickier to handle large cones than small ones.  There was also a small difference between extraction rates of open and closed cones.  

Figure 1. Relationships between seed extraction rate and cone size for lodgepole pine cones whose scales were open, and Sitka spruce cones whose scales were either open or closed.  For Japanese larch, there was no significant relationship between extraction rate and cone size.

 Comparing feeding rates when replicating the cone conditions in winter (open or closed for the different conifers), the energy intake was fastest for Japanese larch and Sitka spruce, assuming that all seeds contain a kernel. However, in the absence of cross-fertilisation, kernels do not develop. Taking into account the percentage of empty seeds, Sitka spruce proved to be the most profitable, even although the seeds were the smallest. The analysis took into account the differing energy content of kernels among conifers, being higher in the two spruce species than the two pine species.

Full details of the paper:

McNab, E., Summers, R.W., Harrison, G. & Park, K.J. 2019. How important are different non-native conifers in Britain to Common Crossbills Loxia c. curvirostra? Bird Study.  Doi:10.1080/00063657.2019.1614143

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