Sunday, February 12, 2012

New Zealand: Boulders and Birds

On March 11, 2011, we drove down from the heights of Mount Cook to the South Island's east coast, where we stopped off for a look at a truly peculiar spot: the boulders of Koekohe Beach, near Moeraki.  A set of fifty or so giant rounded pebbles may seem like an odd sort of tourist attraction, but I have never seen anything quite like the Moeraki boulders anywhere else.

The boulders are essentially a set of giant 60-million-year old concretions - hollow balls of mud and sand cemented together with calcite - that formed in the mud of the seafloor during the Paleocene.  They may have taken 4-5 million years or more to reach their present size.

I won't repeat  the details of their history and geology (read about them in what appears to be one of the better Wikipedia articles).  They are not, in fact, unique to this beach - apparently there are smaller versions a lot closer to my own home, at Kettle Point, Ontario (a fact I should have known; Kettle Point is a famous raptor migration spot).  Nonetheless, they are certainly worth a look.

For one thing, of course, they are really large...  smaller ones appear to have been carted off as souvenirs, but the area is protected so don't try this now (even assuming you have a beach-worthy forklift to hand).

Also, they are truly weird.

The stones are slowly weathering, and a number have broken open - revealing a hollow Rubik's Cube interior that looks every bit as alien as the complete boulders.  This is, according to Wikipedia at least, the result of cracks within the concretion becoming filled with a calcite mortar marking off the separate sections, or septaria, within the boulder.

Some, like the one at right, have lost their mortar, and fallen almost completely into their component bits.

The smooth faces of the septaria provide gathering places for limpets.  Going by range, these are  Cellana strigilis redimiculum, the only one of the six endemic members of this widespread Indo-Pacific genus [Famiy Nacellidae] on the southeast coast of the South Island, here at almost the northern extremity of its range (C. strigilis ranges south to New Zealand's subantarctic islands; if you are interested in the details a thesis on the subject by Sharyn Goldstein is available online).

The outer surfaces of the boulders carry clusters of rock barnacles and Little Black Mussels (Xenostrobus pulex) where they are washed by the tide.

Around the boulders, and along the sandy beach leading to them, remains of marine life lay among the wrack. Strands of beach-washed kelp stretched out along the tide line...

...interspersed with tangled mats of other brown algae (the top photo is possibly bladder kelp (Macrocystis pyrifera) though I'm far from sure)...

...and dried bits of sponge (there may be over 1500 species of sponge in New Zealand, with some 95% of them endemic to the region).

Not someone's tires exploded on the beach, but the leathery fronds of bull kelp (perhaps Durvillaea antarctica).

Shells of various sorts littered the sand along the beach.  The large snail on the left appears to be Southern Cat's-Eye (Modella granulosa, Turbinidae), based on photographs on the excellent New Zealand Mollusca web site.  The name refers not to the shell but to its shiny operculum, the plate that closes in the living animal.

Over the course of its life this mussel, presumably a green-lipped mussel (Perna canaliculus), picked up some passengers.

A sort of cockle or venus clam - rather beachworn, so I'm not sure exactly what it is.

These attractive bivalves may be Common Pipi (Donax deltoides).

The shells on the beach attract oystercatchers, anxious to see if any of them still contain something edible, and gulls waiting to see if the oystercatchers have any luck.  Here a Red-billed Gull (Chroicocephalus scopulinus) eyes a Variable Oystercatcher (Haematopus unicolor) as it puts its can-opener of a bill to good use.

This oystercatcher is enjoying a bit more privacy.

Oystercatchers are beautiful things, especially to Canadian inlanders like me who almost never get to see them.

This is the other oystercatcher of mainland New Zealand, the South Island Pied (Haematopus finschii).  Its bill appears thinner than the Variable Oystercatcher's, but what difference does this make to the feeding preferences of the two species? 

For that question, I turned to a 1974 paper by Dr Allan Baker, curator of birds at the Royal Ontario Museum in Toronto (where I did my Master's thesis), a transplanted New Zealander and the world's leading authority on oystercatchers: Prey-specific feeding methods of New Zealand oystercatchers, Notornis 21: 219-233.  According to Allan, oystercatchers have specific techniques depending on which sort of mollusk they are trying to deal with, but in general Pieds concentrate on bivalves like clams and oysters while Variables are better-adapted to prying limpets and chitons from their rocky perches.  Obviously the Variable Oystercatchers at Moeraki, happily eating mussels, haven't read the paper (not fair; Allan of course notes a lot of overlap between the two species, but I couldn't resist).

Here are the two species in flight.

With the two oystercatchers I found all three species of New Zealand gull: the largest, the Kelp Gull (Larus dominicanus)...

...the ubiquitous Red-billed Gull...

...and the prettiest of the lot, the Black-billed Gull (Chroicocephalus bulleri), found on both islands but far commoner in the South.

Here are the Red-billed and Black-billed together, for comparison.... 

And here, joining them, is a White-fronted Tern (Sterna striata). 

And finally here is Eileen, demonstrating why she left most of the beach-walking to her heavier-set husband, who was far less likely to be blown out to sea in the process!

No comments:

Post a Comment