Tuesday, 15 November 2016

Coral Cay Island Birds – Great Barrier Reef – Part 4 Boobies and Shearwaters

The background to our recent visit to Lady Musgrave Island on the Great Barrier Reef (GBR) was given in Coral Cay Island Birds – Great Barrier Reef – Part 1 Background.

This post covers Brown Boobies and Wedge-tailed Shearwaters, both tropical water birds and both breeding when we visited Lady Musgrave Island.

Click on photos to enlarge.

Brown Booby (Sula leucogaster)

Brown Boobies are closely related to and are very similar to Gannets – perhaps a tropical version of the Australian Gannet (Morus serrator) so often encountered along the Victorian coast.

Less pelagic than Red-footed and Masked Bobbies, the Brown Booby can be found in shallow tropical coastal waters, harbours and estuaries.

Brown Boobies are relatively sedentary in Australian waters though immature birds may disperse more widely. They turn up in Victorian coastal waters now and again when their vagrant presence causes great excitement among twitchers.

The Brown Booby relies heavily on flying fish and squid for its food. Like the Gannets it can plunge dive from great heights to capture food.

Brown Booby – the streamlined body and sharp bill - which extends to and protects the forward facing eyes - are adaptions that enable spectacular dives into the sea at great speed.
I was lucky to capture a photo of a flying fish from the deck of Alchemy 1.

We saw individual flying fish and small schools burst from the sea and fly (glide?) a surprising distance before dropping back beneath the surface again.

Brown Boobies breed on offshore islands including in the Bunker Group. The breeding season is almost continuous with peaks in spring and autumn. They were breeding on Fairfax Island just north of Lady Musgrave when we were there so we saw many flying over Lady Musgrave lagoon as birds flew out and returned from fishing expeditions.

We planned a visit to Fairfax Island however some checking showed it was closed to the public for “scientific reasons”. I suspect this is at least in part to protect nesting sea birds. The Warden on Lady Musgrave Island told me her husband had recently joined some National Parks staff to conduct a survey of birds on Fairfax Island however they had to abandon their survey when the nesting Brown Boobies left their nests and Silver Gulls moved in to attack eggs and small chicks.

Brown Booby chicks can survive near-starvation for lengthy periods and then make up lost ground. Their ability to resist starvation is needed because tropical seas are less productive than colder waters closer to polar regions. This at first seems counter intuitive as one would expect warm tropical seas to be more productive than cold seas. The underlying reason is due to warm water containing less oxygen compared with cold water. In tropical seas the water column can stratify with warm low-oxygen water on top and cold below. The stratification in turn stops nutrients from cycling into the surface waters which then cannot support as much phytoplankton nor higher level sea life including fish supported by phytoplankton. As a consequence, tropical seas do not always support large numbers of sea birds.
For breeding sea birds, the energy to not only support themselves but also raise young can be a very delicate balance and breeding may fail if sufficient food resources are not available or diminish to below a workable level as the appetite of their chicks grows.

So Brown Booby photo opportunities were limited to chance when the odd birds happened to fly close by Alchemy 1 anchored in Lady Musgrave Lagoon and when I happened to see them coming, have the camera ready and the light was right. After some persistence I managed to get a few acceptable images.

Booby wings are similar to other large sea birds – they have evolved to minimise effort by maximising the use of wind to stay aloft and roam the ocean.

This Brown Booby was almost adjacent to the yacht when it noticed me on the deck and in alarm did a rapid about-turn.

As we left Lady Musgrave lagoon early one morning a pair of Brown Boobies rested on one of the navigation markers beside the entrance to the narrow channel through the reef. Their strong green-yellow legs and large webbed feet are obvious in the photo. The large webbed feet help propel them under water, sustaining the momentum of the dive for pursuit of prey.

A pair of Brown Boobies on a navigation marker in Lady Musgrave Lagoon.

So where did the name Booby come from? It certainly sounds derogatory? For an answer here is what Fraser and Gray have to say in their excellent book “Australian Bird Names – A Complete Guide”:

“Booby was used in the sense of a “foolish fellow”, presumably for the poor beast’s trusting habits which allowed them to be easily slaughtered by sailors. The word itself appears in English from the very beginning of the 17th century, apparently from the Spanish bobo, a fool.”

Wedge-tailed Shearwater (Puffinus pacifica (Christidis & Boles 2008 Ardenna pacificus))

The Wedge-tailed Shearwater is a pelagic seabird that inhabits the tropical waters of the Indian and Pacific Oceans. The Australian East Coast birds breed on islands and cays in the Coral Sea and along the Great Barrier Reef and on offshore island as far south as Montagu Island on the NSW south coast. They rarely come south of the NSW-Victoria border so, apart from some rare vagrants, are not seen in Victorian waters.

In Victoria we have the similar but much more numerous Short-tailed Shearwater (Ardenna tenuirostris) which breeds on islands in Bass Strait and feeds in cold Southern Ocean waters to fuel its breeding effort.

Wedge-tailed Shearwaters were breeding on Lady Musgrave Island when we were there. Given the adult birds only visit the breeding colony after dark how did we know they were breeding?

A number of observations made it clear they were breeding on Lady Musgrave Island.

First, we knew the island was used for breeding by this species. A well-marked walking track through the Pisonia forest and good signage helps keep human visitors away from the Shearwater nest holes that are very vulnerable to collapse as they are dug in soft sand.

Second, it was the right time of year for birds to be incubating their one egg in the underground nests.

Thirdly, while looking at their nest holes in the Pisonia forest I could hear the distinctive mournful call of the odd incubating adult coming from underground.

Fourthly, just after sunset and before it was completely dark I could see from our yacht anchored across the lagoon from Lady Musgrave Island the silhouettes of hundreds of Shearwaters flying against the orange glow in the western sky at the NE corner of the island as they milled about for it to become dark enough to go into their nests.

In addition, the Warden on Lady Musgrave Island told me it was a very noisy event each night as the Wedge-tails came in to land and then scurry across the jungle floor to their nest holes.

Wedge-tailed Shearwater nest holes at base of a Pisonia tree.
During the day we only saw the odd bird as it passed over the lagoon on its way to deeper waters beyond the reef looking for food. The majority of birds left well before dawn and headed out to sea to hunt for the day. As we sailed/motored on our trip south we sighted, well out to sea from the mainland, the odd Wedge-tailed Shearwater - only single birds for they are not particularly gregarious and generally forage alone or are seen in small parties of four or five birds - though they do flock in large numbers at fish or other food concentrations to gather food.

Capturing photos of a Wedge-tailed Shearwater from a moving boat at sea as the odd bird happened to fly by close enough and with the sun in the right position, proved to be impossible for sharp images. At Lady Musgrave it was not until day three and four when a strong early morning easterly wind blew that some Wedge-tails flew across the lagoon and some by our yacht. I spent an hour or two standing with my camera ready on the forward deck to capture a few, just acceptable, photos of the Wedge-tailed Shearwater.

A distant shot of two Wedge-tailed Shearwaters flying over Musgrave Reef lagoon. The bird at top left shows the typical wing position with wrists held well forward and the long pointed wedge-tail.
The Wedge-tailed Shearwater’s flight follows a similar pattern to an albatross as it rises and falls using the wind to maximum advantage without the need to flap its wings. At the low point of the pattern they almost touch the water and often disappear behind waves.

Post No. 5, the last post in this series, will cover the other/remaining bird species we saw at Lady Musgrave Island.

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