Day 35: Wrybill – Anarhynchus
frontalis
Right-beak politics
If you look closely at the photo of the wrybill below you
may notice something awry. The wrybill is the only bird species whose beak
bends sideways and always to the right! Asymmetry is relatively rare in
animals. Most animals develop with bilateral symmetry with cues along an
anterior-posterior axis determining body part placement. Additonal signals are
needed to override this symmetry and of there must be an selection pressure
favouring such abnormality. Obvious asymmetries in nature include the massive
claws of fiddler crabs and the spiral shells of snails (there is even
asymmetrical snake that eats asymmetrical snails!). Within the birds the
crossbills are notable for the mandibles which cross over, enabling them to tweeze
apart pine cones.
The wrybill is a New Zealand member of the Charadriidae, the
family that includes lapwings and plovers and like many plovers it prefers
stony coastal areas where it uses its dextral beak to probe under rocks. The
wrybill rears its camouflaged eggs among the pebbles and uses deceptive behaviour to protect them, feigning distress to lure gullible predators away
from its nest. This is a strategy seen in other species such as the killdeer (Charadrius vociferus) which holds one
wing as if broken to entice predators.
Day 36: African jacana – Actophilornis
africanus
These feet are made for walking
The most noticeable attribute of a jacana is its feet. The
jacanas are eight species of ‘aberrant’ waders that have specialised to
traverse leafy waterways. As with the coots, the jacanas use their huge feet to
spread their weight allowing them to walk on precarious surfaces such as water
hyacinths and water lilies whilst it picks invertebrates from the foliage. This
ability to (almost) walk on water also gives them the name Jesus bird. The
floating mats are their full-time home to the extent that the even build their
nests on the vegetation, laying eggs camouflaged to resemble water weed.
Also like coots they have bright fontal-shields which are
presumably used in sexual selection given their striking colouration. Jacanas
are rather unusual in that the smaller males are responsible for incubating the
eggs allowing the females of some species to be polyandrous. Reproductive
ecology of this type is rare in the vertebrates. For the most part, females are
the limiting sex because they invest more in their offspring and so males must
compete for mating opportunities. In birds, some of the investment burden is
shifted away from female internal development meaning males must play a larger
role in parental care to ensure reproductive success. This predicts the large
amount of monogamy and biparental care in the class.
Why a few shore birds including jacanas and phalaropes
should shift to male care is relatively unknown. In fish, such as stickleback
and seahorses, males care for the eggs fry essentially because the female has time
to escape whilst the male fertilises so he is left with the task of their
rearing. This is not obviously the case for jacanas so the reasoning is
probably an idiosyncratic occurrence in their breeding biology which does not
occur in other birds. One suggestion is that extremely high egg predation (due
to being a floating buffet and all) means that females should spread their
investment around rather than put all their eggs in one basket (couldn't resist).
With eggs being more evenly distributed around males, females must now compete to put their eggs in the males' respective baskets (as it very much were).
Day 37: Long-billed curlew – Numenius
americanus
Length matters
I was hoping that the long-billed
curlew would hold some beak-related record. Unfortunately it has neither the
longest beak (Australian pelican, Pelicanus
conspicillatus) nor the longest beak to body ratio (sword-billed
hummingbird, Ensifera ensifera).
Having said this it is still a very impressive implement! Like the Eurasian
curlew (Numenius arquatus) the
long-billed curlew is able to use it beak to probe deep into the sediment of
salt marshes and mudflats capturing bivalve molluscs and annelid worms.
Visiting these habitats in winter
is (personally) one of the most brilliant wildlife spectacles because there are
so many wader (and other) species milling around, doing what they do best.
Curlews, with the rest of the snipe family (Scolapacidae), provide a much-cited
example of niche differentiation. The sediment of the marshes and mudflats hold
a bonanza of food, various invertebrates distributed through the sediment
column. Different species have adapted to access different levels and thus
different prey sources. We have seen that curlews can probe the deepest; next
comes the godwits (Limosa spp) with
their long, straight bills, then the Tringa
sandpipers such as the redshank and greenshank. At the smaller end of the
spectrum are the Calidris sandpipers
ranging in size from knots to stints. Additionally there are more unusually
specialised birds such as the avocet and waders occupying slightly different
habitats such as the beach-loving sanderling (Calidris alba), the rock-braving purple sandpiper (Calidris maritima) and the pier-friendly
turnstone (Arenaria interpes).
Day 38: Collared pratincole – Glareola
pratincola
One pratincole does not make a summer
I once casually asked my friend to identify the birds on her
top. She said they were swallow but I knew them to be pratincoles (yes, I am an
insufferable know-it-all). Her mistake was an easy one to make because of the
striking convergent evolution. Swallows (Hirundinidae) are aerial insectivores
which use their slender and aerodynamic wings to snatch flying insects from the
air. Well, it turns out that pratincoles do too! I did not know this for sure
before looking into it and I began to second-guess myself when my friend and
ornithological maestro questioned my statement of ecological convergence. Well it seems that, as I predicted, the apparent similarities are not mere coincidence. Like
swallows, pratincoles (Glareola) have
long tapered wings and forked tails which enable them to manoeuvre through the air and grab
insects with their short beaks and wide gapes. The collared pratincole is the
only species to breed mainly in Europe so may be the most familiar. It is a bird of
open country which gives it its name pratincole from the Latin pratum incola which means meadow resident.
The
pratincoles share the family Glareolidae with the coursers (Cursorius
& Rhinoptilus), shore birds much more at home running around then
flying. This is the last family of ‘wader’ that we encounter before we move
onto the charadriiform suborder Lari: the gulls, terns and auks.
