I have a cleverly designed, multi-purpose tool that fits into a credit card slot in my wallet. This metal tool is roughly rectangular. Three of the corners of the tool are projected out to form a slot-head screwdriver, a Phillips-head screwdriver and a micro screwdriver one can use to repair glasses. Various parts of the center of the card are removed to make a bottle opener and slots to loosen or tighten five sizes of hex nuts. But sometimes the tool can’t produce enough torque to remove a stubborn screw; a good, old-fashioned screwdriver is needed. Similarly, a ratchet or adjustable wrench may be needed to loosen really tight bolts.
I describe this tool as a metaphor for the old adage, which in its entirety is “A jack of all trades is a master of none but sometimes better than a master of one”. Is it better to be a specialist or a generalist?
One of the most important tools that a bird has is its beak. Depending on its shape, a beak can be used to tear flesh, deftly capture small insects, extract nectar from the base of long flowers, crush seeds and even, with the aid of the baleen-like tongue of a duck, filter microscopic algae from the water.
The bill of a European Starling falls on the generalist side of the spectrum of beak function. A starling’s bill is relatively long, stout at the base narrowing to a fine tip. It serves pretty well as a forceps, as a crushing tool and a probing tool for these birds with a notably broad diet.
Most birds have bills that tend to fall on the more specialized end of the spectrum of bill function. The bill of a chickadee is great for capturing small caterpillars in the summer but not powerful enough to crush sunflower seeds. These adaptable birds can feed on sunflower seeds but have to hold a seed against a hard surface with their toes and use their bill as a chisel along the suture line of the seed to get at the tender kernel inside. Compare chickadees with House Finches or Evening Grosbeaks that can rapidly eat large numbers of sunflower seeds with their crushing bills.
To me, the most specialized bills of our Maine birds occur in the two species of crossbills: White-winged Crossbill and Red Crossbill. The upper bill is curved in one lateral direction and the lower bill in the opposite lateral direction. In some individuals, the upper beak bends to the right; in other individuals it bends to the left.
This crossed bill is a perfect tool for extracting seeds from the cones of coniferous trees. Let’s consider the cone of a spruce or fir tree. The cone has a number of overlapping woody scales. These scales serve to protect the seeds, which are located at the base of each scale. Most birds have no chance of extracting a seed from the base of the scale.
Not so for a crossbill. These birds insert their bill between two skills and then open their bills. The bill essentially pries the two scales bracts apart. Then, using its long tongue, a crossbill can reach down to the base of the bract and extract a seed.
The crossed bill is, as you would expect, useless in more standard feeding activities. Crossbills are therefore dependent on the cones of spruce and other conifers.
We know that conifers vary greatly in cone production from year-to-year and from place-to-place. In some years, a particular species in a local region will show the phenomenon of masting, the production of a profusion of cones. We believe this masting is a way to overwhelm seed predators like squirrels, crossbills and some insects.
The dependence of crossbills on cones means these birds live a nomadic existence, wandering widely to find a masting population of conifers when local cones are depleted.