Pied Imperial Pigeon Counts.

Low Isles Preservation Society volunteers have been busy maintaining the Pied Imperial Pigeon (Ducula bicolor) count which has been conducted now for many years.

Dr. Julia Hazel of JCU confirms that the PIP counts at Low Isles are part of an important long-term monitoring program for these birds in Queensland. The monitoring includes counts at other PIP nesting islands and at various coastal count sites…Validity and long-term value of PIP counting depends on following the standard count procedure exactly and on everyone counting as accurately as possible. 

Between September and March a large population (est. 20,000-25,000) of pied imperial pigeons live and breed at Low Isles.

The Pied Imperial-Pigeon is a large distinctive black and white pigeon, mostly white but with black on the outer parts of its wings and on its tail tip and black bars on the underside of its tail. The beak is yellow or yellow-green and legs and feet are blue-grey. This bird is also known as the Torresian Imperial Pigeon, Torres Strait Pigeon, White Nutmeg-Pigeon, Nutmeg Pigeon, Spice Pigeon, or Australian Pied Imperial-Pigeon.

Since 1994 Lips has assisted the QPWS with the volunteer counting programs. During the season, volunteers carry out a monthly count to monitor the number of pigeons on the island.

The Pied Imperial-Pigeon is a summer migrant to northern Australia, and nests colonially on many islands of the Great Barrier Reef, Queensland. While their major breeding colonies occur on off-shore islands (most notably Low Isles off Port Douglas and North Brook off Cardwell) the birds are also seen around Townsville and Magnetic Island.

Numbers are perceived to have declined as a result of large-scale shooting in the past and clearing of their mainland feeding grounds.

Like most islands favoured by these birds, Low Isles has mangroves. The mangrove trees provide nesting sites but food is obtained from the mainland rainforest to and from which the buds fly each day.

The nest is large for a pigeon and is the centre of a feebly defended territory of a mated pair. Clutch size is one. Incubation and fledging periods are about twenty-eight days each.

Density of nests is correlated with the density of the vegetation and is different each year. The years of high nesting density corresponded with years of good fruit production particularly by species of Lauraceae; low density occurred when fruit production was poor, suggesting that food may be a regulating factor for the population. Direct disturbance by man has probably caused the bird's decline this century. (Breeding, Feeding and Status of the Torres Strait Pigeon at Low Isles, North-eastern Queensland, January 1975, Emu 75(4):189-198)

Pied Imperial-Pigeons nest in colonies on offshore islands and sometimes on the mainland. Elsewhere, including mainland Queensland their nests are solitary. The birds nest in mangroves, forest or scrub, sometimes on rocks or bare ground.

The nest itself can be a loosely woven platform of sticks or a large dense platform with a central depression made of small twigs. The nest material is collected from vegetation near the nest. When nesting on the ground they do not build a nest but gradually encircle the nesting site with mounds of excreted seeds from fruit they have eaten. Pied Imperial-Pigeons lay up to three clutches of eggs per season. Both male and female incubate the eggs on alternate days and both brood and feed the chicks, again on alternate days.

Source

A comparison of historical counts and those made during this study, of the Torresian Imperial Pigeon Ducula spilorrhoa nesting populations of the northernmost region of Australia’s Great Barrier Reef suggest that numbers have remained stable or increased slightly over the last 20 years.

The reliability of monitoring D. spilorrhoa abundance accurately has been confirmed and indicates that ongoing monitoring could be an important contribution to understanding climate change impacts on the species and on two highly vulnerable and important vegetation types, upon which this species depends.

As a result of this study the number of nesting colonies for which counts have been undertaken in this region has been expanded from 11 to 26, and six of these have provided the first indications of trends in abundance.

Torresian Imperial Pigeon Ducula spilorrhoa monitoring, population trends and species suitability as an indicator of environmental changes by Nigel Brothers and Catherine Bone. Corella, 2012, 36(3): 69-75.

...“species for which reliable historical records are available, and whose biology suggests may be sensitive indicators of climate change, need to be identified and monitored”, (Hughes 2003, p. 438). It is therefore regrettable that there are limited D. spilorrhoa historical records because this species is a potential ‘sensitive indicator’ for the following reasons:

• D. spilorrhoa is considered to be a ‘true’ frugivore (McConkey et al. 2004) and an excellent seed disperser in tropical rainforests and Price (2006) suggests that the ability of plants to adapt to the rapid climate changes that will be brought about by global warming will depend critically on the maintenance of sizable populations of frugivores.

• D. spilorrhoa populations may also be able to ameliorate the effects of fragmentation in tropical rainforests by dispersing seeds from one area of rainforest to another. Fragmentation caused by land clearing for agriculture and urban development is a widespread feature of the forests within the foraging range of D. spilorrhoa. With nesting colonies on islands distributed along virtually the entire length of the Great Barrier Reef, D. spilorrhoa have access to a significant amount of the highly fragmented and often remnant tropical rainforest habitat on which they are dependent for food.

• This species is dependent on the Great Barrier Reef island mangrove environment, which is a breeding habitat that has been identified as highly vulnerable to various predicted consequences of global warming (Lovelock and Ellison 2007). The majority of the islands occupied by D. spilorrhoa for breeding are classified as ‘low wooded islands’ (Steers 1937) consisting of mangrove forests usually growing in a lagoon environment 0.36 metres below mean sea level, making them highly vulnerable to sea level rise (Lovelock and Ellison 2007). Furthermore, species of mangroves from the family Rhizophoraceae which dominate most of the island nesting habitats cannot be coppiced, have no epicormic buds from which to re-sprout after canopy damage and so are likely to be particularly adversely affected by enhanced cyclonic frequency or intensity (Lovelock and Ellison 2007). Population monitoring may assist in providing some measure of human impact (other than climate change) on D. spilorrhoa here in Australia as well as in PNG where they spend the nonbreeding season. Abrahams et al. (1995) point out that although a common species, the nesting behaviour of this species makes it vulnerable to human disturbance and therefore a certain level of understanding about population status and trends of colonies is important in the face of continual expansion of human visitation in the nesting region.

Full paper / Brothers&BonePaper.indd (absa.asn.au)

Fun Fact 

From December 1994 to April 1995 Department of Environment and Heritage researchers collected and analysed the droppings from PIPs at Low Isles to determine what they were eating.

A total of 68 different plant species were recorded during the four-month period. Identification from seeds alone was almost impossible, so they were germinated, the seedlings allowing identification of 24 species.

The most popular items on the pigeons’ diet seemed to be Syzygium angophoroides (Yarrabah satinash), Euroschinus falcata (ribbonwood/pink poplar), Terminalia seriocarpa (damson), Buchanania arborescens (little gooseberry tree), Canarium vitiense (canarium), C. acutifolium (Melville Island white beech) and Calamus australis (hairy Mary lawyer cane). 

Source: Tropical Topics No.28 June 1995, p.7 QLD Dept of Environment and Heritage