Blog by Dr Ron Summers, Principal Conservation Scientist, RSPB Centre for Conservation Science
Crossbills are finches with a peculiar bill, adapted to prise open the scales of closed and opening conifer cones. The lower mandible is offset either to the left or right of the upper mandible, which is straight. This results in the tip of the lower mandible overlapping the tip of the upper one.
When feeding, crossbills often snip the cone from the tree, carry it in their bill to a stout twig, and, holding the cone lengthways along the twig with one foot, systematically prise open the scales. While a gap between two scales is held open by the spreading action of the upper and lower mandibles, the tongue reaches to the core of the cone to get the seed. The crossbill then de-husks the seed using the edge of the bill and ridges in the upper palate before swallowing the kernel. Generally, a cone is well worked before being dropped to the ground.
Photo: Scots pine cones from which crossbills have extracted seeds. Photo by Ron Summers.
The female cones become visible in summer and development takes a further two years before they shed seed. When they first appear, the tiny soft and red cones are only a few millimetres long and grow to about 8 mm in length before stopping growth for the winter. In the following spring, cone growth starts again, and cones reach their full size by July.
They are still green and sappy, and it is only at this stage that crossbills start to take seeds from cones. Seed removal continues through to the following summer during which time the cones harden and become browner, resulting in tough, woody cones. In the third spring, the cone scales spring back in the warming weather and the seeds shake loose from the grip between the scales and slowly spin down to the ground on papery wings.
Photo: Three cohorts of female Scots pine cones: at the top, a small red cone starts its growth on the end of a shoot. In the second spring / summer, there is a burst of growth during May to July, by which time cones attain their full size. They are still green at this stage and the seeds are soft and white. In the third spring, the scales of the mature woody cones spring back to release the seeds. Photo by Ron Summers.
I took part in an international study that examined the timing of shedding seed and how this may impact crossbills under the projected scenario of climate change. In Abernethy Forest, in Highland Scotland, most seed is shed in May. The dates by which half the seeds were shed ranged from 27 April to 13 June during 1992 to 2007, and the mean was 25 May.
Crucial to this seasonal pattern of feeding by crossbills is the timing of seed fall in relation to the availability of seeds in the next cohort of cones. Currently, there is an overlap in July when residual seeds are still available in the current cohort and developing seeds are available in the green cones of the next cohort.
If spring temperatures rise and seeds are shed earlier, a gap will grow between the availability of seeds in these two cohorts. This already takes place in Spain, which has much higher spring temperatures. In this case, crossbills switch to other pine species (Aleppo pine and mountain pine) that grow in the Mediterranean region. Such pines do not occur in northern Europe so it is likely that those crossbill species that are most closely associated with Scots pine (Scottish and parrot crossbills) will face food shortages.
Photo: The days on which half the seeds were shed from Scots pine cones in four parts (different symbols) of Abernethy Forest in relation to the maximum April temperature. Day 121 refers to 1 May. Seeds are shed earlier in warmer springs. Data from Sweden also followed this pattern.
Full paper: Mezquida, E.T., Svenning, J.-C., Summers, R.W. & Benkman, C.W. 2107. Higher spring temperatures increase food scarcity and limit the current and future distribution of crossbills. Diversity and Distributions 2107, 1-12. DOI 10.1111/ddi12694.