A Cross to Bear (on the Bill)
Story by Bryan Pfeiffer
Even among the quirky cast of characters we know as birds,
the crossbill is a bit of a freak. It’s hard to decide which is more
bizarre: its bill or its breeding behavior.
Front and center is the bill. Faithful to its name, the crossbill’s
mandibles cross at the tip, sort of like a misaligned pair of needle-nosed pliers. It’s a unique departure from the avian blueprint
and no other bird shows this twisted visage.
Crossbills are finches, the males apple red with dark wings,
the females and young yellow-green. Two of the planet’s crossbill species inhabit North America: the white-winged crossbill
(with white bars on those dark wings) prefers seeds from
spruce and tamarack (and isn’t particularly fond of balsam fir);
the red crossbill, whose bill size and shape varies around the
continent, enjoys a more diverse diet, including pine and fir
species and eastern hemlock.
The crossbill turns the odd-looking beak into a mega-tool
for prying apart the scales of cones to expose the seeds within.
First, the bird bites down to open a gap in the scales, like reverse
pliers opening at the tip when you squeeze them. With its bill
now wedged behind a scale, the crossbill tilts it head to open
the gap wider as it digs deeper, sort of like when you insert a
flat-head screwdriver into the seam of a paint-can lid and twist.
But the crossbill does the screwdriver one better by spreading its
upper and lower mandibles laterally to help widen the gap even
more as it approaches its quarry. Once it gets to the base of the
scale, the crossbill lifts the seed out with its tongue (sometimes
with an assist from the hook of its upper mandible), breaks away
and discards the husk, and swallows its meal whole.
If the extraction seems laborious, crossbills pull it off with
the skill of an assembly line worker, eating as many as 3,000
conifer seeds in a day. Out there foraging in boughs full of
cones, crossbills also use their bills to grab branches and needles
as they climb, making them look a bit like parrots walking and
feeding in the conifers.
The crossbill’s determination with seeds reveals an urgency
beyond routine foraging; it allows them to breed at any time of
year. Among virtually all songbirds, day length – the photoperiod
– triggers hormone production to bring on breeding. Longer,
warmer days in spring, along with a burst of food availability
(insects, fruits, and seeds) provokes birds to sing and reproduce. For crossbills, however, the proximate trigger for breeding
is food availability. When the boughs hang heavy with cones,
crossbills can get the calories essential for reproduction – from
egg production to feeding young – even as snow coats the cones.
Those high-calorie conifer seeds stoke fires in their bellies.
Although white-winged crossbills can breed in the cold, relative
cone abundance suggests three preferences – or breeding cycles
– here in the Northeast: the first during the development of new
cones in July and August; the second from January to March,
BIRDS in focus
when spruce cones might be all that’s available; and the third in
March, when black spruce cones are hanging on after red and
white spruce cones have dropped.
Crossbills are vagabonds, wandering south some winters
from the boreal core of their range in search of cones. When
they find a source of seeds, the birds gather and feed together
in groups. During my own wanderings in Maine and Vermont
this fall I saw heavy cone crops, particularly on white and red
spruce, which means crossbills may be here in abundance
this winter. So you might not need to wait until spring to find
Bryan Pfeiffer is an author, wildlife photographer, guide, and consulting naturalist who
specializes in birds and insects. He lives in Montpelier, Vermont.
Top: White-winged crossbill; bottom: Red crossbill