Saturday, November 20, 2010

Kawhia Harbour bird census

Hello! This is my inaugural blog, so I should start with a welcome greeting I suppose. I am constantly finding myself in fascinating places doing interesting things with very cool birds, so I though this blog would be a great outlet for my enthusiasm and and opportunity to share the things I see with others. So, enjoy my posts, which I hope to make every few weeks or so, or whenever something interesting happens!

Today was the Kawhia/Aotea Harbour bird census, conducted every 6 months by the New Zealand Department of Conservation (DOC) and the New Zealand Ornithological Society (OSNZ). The aim of the census is to identify and count all the birds we see while walking or boating certain routes around the Kawhia and Aotea Harbours. Counts are compared between seasons and years, and are used to monitor bird populations. This year I was assigned to count the water birds that roost on the shores of Te Motu Island, a small sandy island within Kawhia Harbour.

After convening at the Hamilton DOC office at 7am, Bruce Postill (who works for DOC), Ann Buckmaster and I drove for an hour south west of Hamilton to the sleepy coastal community of Kawhia. The windy road led us down from the misty hills east of Mt. Pirongia to an impressive natural harbour. At ~5km x 15km, Kawhia Harbour is one of 6 large, natural harbours on the west coast of the North Island (Hokianga, Kaipara, Manukau, Raglan, and Aotea being the others). As such, it is important habitat for water birds.

Te Motu island

Caspian Tern colony (note the chick in the bottom right)
Once we arrived at Kawhia, we took a small DOC boat across to the island and jumped overboard into the cool, knee-deep water and begun our counting. First we started at a sand spit at the north end of Te Motu, which even at first glace had quite a few birds on it. As we approached the high ground on the spit a large group of Caspian Terns took to the air, which we quickly counted at 65 individuals. I noticed a small, white fluffy chick had started to wander around, and we so we quickly got in a little closer to see what was going on. To our great surprise we stumbled across quite a few large, mottled eggs lying in shallow nests in the sand. In total we counted 32 nests, 45 eggs and 17 chicks!

This colony was a fantastic discovery, as it didn't exist a year ago when I was last on the island. The tides, wind, and storms that blow in off the Tasman Sea helped to shape this spit into something habitable for the terns. Hopefully the colony will make it through the breeding season - Caspain Terns colonies and notorious for getting inundated by summer storms!

Caspian Tern chicks

To add to the excitement we also found 2 White-fronted Tern nests in the same location, each with a single egg, and one Red-billed Gull nest with 2 eggs. The maculation (spotting and patterning) on the eggs seems to vary not only between species, but also between eggs in the same clutch!

Caspian Tern eggs

These Caspian Tern nests nearly all had 2 eggs/chicks in them.

White-fronted Tern nest and egg

These nests only had a single egg each. White-fronted terns are smaller than Caspian Terns...

Black-backed Gull eggs

...and these nests all had 3 eggs/chicks. Black-backed Gulls are the largest of the gull and tern species on the island.

Black-backed Gull chick

Hehe, what a cute little birdie, very good camouflage provided by the markings on the head.

Red-billed Gull eggs

Red-billed Gulls (aka Silver Gulls) are smaller than Black-backed Gulls, and also lay fewer eggs. Hmmm, seems to be a pattern here...(har har, yes, that was an intentional pun)

Red-billed Gull nest

So there seems to be a positive relationship between the bird's size and the number of eggs laid in each clutch, at least in the bird family Laridae (to which gulls and terns belong).

Relationship between bird size and clutch size in the family Laridae

I think I'll have to do a bit of burrowing at the library to find out if this pattern holds true with other gulls and terns, and if the pattern is consistent with other avian families. I'll be sure to make a post when I figure this one out!

So, back to the island! We moved around the west side to where we saw a large flock of shorebirds (aka waders) land when we approached the island on the boat. As we got closer we realised that it was an immense flock of Bar-tailed Godwits, probably numbering in the thousands! These birds spend the northern winter in New Zealand, flying thousands of kilometers from Alaska and Siberia, sometimes directly across the Pacific non-stop (!!). They moved around nervously, at times lifting off as a single mass that shimmered and waved in cohesive motion. A stunning sight to anyone's eyes, even non bird-enthusiasts! When they settled down again we had a go at estimating numbers, which is no easy task. We eventually came to a consensus, and wrote down 2,500 birds on our data sheet. Mixed in with them was a single female Ruddy Turnstone, 4 Red Knots, and a pair of New Zealand Dotterel. Not too shabby!

As an added bonus, when we rounded the south east corner of the island we flushed a small group of large waders, which I managed to get in my binoculars as they flew off - Whimbrel! This was exactly the same place where we saw a single bird last year, but add a half-dozen more to that and you've got a very good find indeed. I suspect that this sighting will be a high-count for the entire country as only about 20 are thought to spend the northern winters down here. Excellent!

In all we only counted 12 different species, but some fantastic sightings non-the-less. The boat came and picked us up around noon and we headed by to Kawhia for a much needed cup of tea and to tell the others of our successes. A good day out!

OSNZ bird counters