Blog post by Dr Ron Summers, Principal Conservation Scientist, RSPB Centre for Conservation Science.
Within northern Europe, common crossbills depend on Norway spruce seeds for food. The Norway spruce cone crop varies enormously between years and the variations are synchronous across a wide range. In years when no cones are produced, we see irruptions of crossbills to regions south of their normal range.
As Norway spruce isn’t native to the UK, previously irrupting common crossbills arriving from Fennoscandia and Russia could only forage on the tough woody cones of the native Scots pine. This was until the introduction of Norway spruce and Sitka spruce (a North American conifer) to enhance the national timber crop.
Now, Sitka spruce makes up half of the woodland area of Scotland, whilst Norway spruce comprises only three %. Sitka spruce in particular, has helped common crossbills become established in the UK as a breeding species.
A female common crossbill foraging on a Sitka spruce cone. Photo by Ron Summers
Why was I researching this?
While we were aware of the importance of spruces to crossbills, there was little known about their feeding habitats or indeed basic features of spruce cones on which they feed.
To understand this important part of these fascinating birds ecology, I measured cone sizes and seed fall, and collected spruce cones dropped by feeding crossbills to establish seed extraction efficiency. This research has recently been published in the journal Forestry.
Sitka spruce and Norway spruce cones differ in several respects. Norway spruce cones are much larger than Sitka cones. The latter are small enough for a crossbill to remove from the tree, hold against a stout branch with its foot, and pry apart scales with its crossed bills to extract the seeds. The cone is then discarded and tumbles to the ground.
However, crossbills are only able to do this for the smaller (lightest 10%) Norway spruce cones. The larger ones are too heavy, so the crossbills have to forage directly on these cones still attached to the tree.
Sitka spruce cones (above) and Norway spruce cones (below). The Sitka spruce cones are a mix of maturing (green/purple) and old cones (brown). Photos by Ron Summers.
What's the difference?
Another major difference between the two conifers is when they shed their seeds following the cones maturing in late summer. I studied this by catching seeds in plastic bins set under the canopy, with a muslin liner placed inside the bins catching the falling seeds.
Sitka spruce started shedding seed in autumn, thereby reducing the food supply for crossbills through autumn to spring, whereas Norway spruce retained their seeds until spring.
The early start to seed dispersal by Sitka spruce is a trait of many North American conifers. It is believed to be a strategy to reduce seed losses to pine squirrels, which begin removing and caching cones as soon as the cones have matured. In northern Europe, red squirrels don’t have the same caching tendency, so there has been no selective pressure on Norway spruce to disperse seed early.
To study seed extraction by crossbills on Sitka spruce cones, I had to search for foraging birds. When located, I approached the tree quietly, so that cones dropped by the birds could be observed as they fell, and then collected. I then dried the cones to extract and count remaining seeds.
I estimated the number of seeds in the cones (from cone length) before the crossbills started foraging, so by subtracting the number remaining, the number consumed was derived. The mean number of seeds taken from Sitka spruce cones was 87, and 68 from Norway spruces.
These were equivalent to 45% and 42% removed, respectively. There was a trend for fewer Sitka seeds to be taken across the period between autumn (August) and spring (March), coincident with fewer seeds being available as the seasons progressed.
Figure one: The seasonal decline in the number of seeds available (□) in Sitka spruce cones and numbers consumed in these cones (●) by common crossbills.
Incomplete extraction of seeds suggests that it is more profitable to switch to another cone to maximise average intake rate, and that the threshold for switching cones changes as the number of seeds declines.
Although optimal foraging may account for some of the variation in the number of Sitka spruce seeds taken, some of the low values may have been due to birds terminating feeding when disturbed, or when a flock decided to forage elsewhere.
Despite the seasonal decline in seed number, irregular cone production and the small seed size of Sitka spruce, this conifer is attractive to crossbills because the thin papery cone scales make it easy for crossbills to extract seeds.
This, plus its abundance in the UK, has clearly benefitted common crossbills, whose numbers exceeded 27,000 in the Scottish Highlands during 2008.
One conservation concern is that hybridisation with Scottish crossbills is now more likely when both crossbill species share other conifers, such as lodgepole pine, particularly in years when there are no Sitka spruce cones.
To date, cases of hybridisation have only been recorded between parrot and Scottish crossbills in Caledonian pinewoods.
It would probably be more difficult to detect hybridisation between Scottish and common crossbills, because it involves catching pairs of birds at nest sites, and these are more difficult to find in plantations, where the tree density is high.
The paper: 'Foraging patterns of common crossbills (Loxia curvirostra) on spruces (Picea spp.) in Scotland' has just been published in Forestry.