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. |
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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