Establishment of a new breeding colony of Gould’s petrel (Pterodroma leucoptera leucoptera) through the creation of artificial nesting habitat and the translocation of nestlings

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Establishment of a new breeding colony of Gould’s petrel (Pterodroma leucoptera leucoptera) through the creation of artificial nesting habitat and the translocation of nestlings David Priddel*, Nicholas Carlile, Robert Wheeler Department of Environment and Conservation (NSW), P.O. Box 1967, Hurstville, NSW 2220, Australia

A R T I C L E I N F O

A B S T R A C T

Article history:

Gould’s petrel (Pterodroma leucoptera leucoptera) was restricted, essentially, to a single breed-

Received 6 May 2005

ing locality – Cabbage Tree Island, Australia. As a safeguard against extinction, an addi-

Received in revised form

tional breeding colony was established on nearby Boondelbah Island, where artificial

30 September 2005

nesting habitat was created by installing 100 plastic nest boxes. Over two years, a total of

Accepted 14 October 2005

200 nestling Gould’s petrels were translocated from Cabbage Tree Island to these boxes.

Available online 1 December 2005

Colonies on both islands were then monitored for a further four years. Selection of nestlings for transfer was based on prior knowledge of growth, plumage development and

Keywords:

emergence from the burrow, and aimed to select only birds that were 11–28 days from

Burrow-nesting seabird

fledging (DBF) in 1999 and 11–22 DBF in 2000. Of the first 100 nestlings translocated to Boon-

Re-introduction

delbah Island (in March 1999), 95 fledged successfully 8–27 days after transfer (mean = 17.3

Nest box

days). Of the second 100 nestlings translocated (in March 2000) all successfully fledged 9–22

Artificial habitat

days after transfer (mean = 15.1 days). The removal of young had no discernible effect on

Site fidelity

the subsequent breeding productivity of the donor pairs. In all, 41 Gould’s petrels have been recorded at the new colony on Boondelbah Island, where at least 27 nest boxes have been visited. Ten translocated fledglings (nine male, one female) have returned to the translocation site, taking up nest boxes that were, on average, 5.5 m from the box from which they fledged. An additional 27 non-translocated birds, of unknown origin, have also nested in nest boxes on Boondelbah, along with four birds previously known from Cabbage Tree Island. Two nestlings transferred to Boondelbah Island have returned to Cabbage Tree Island. Within five years of the first translocation, the newly established colony on Boondelbah Island has produced a total of 24 eggs and 14 fledglings. The translocation technique developed for Gould’s petrel has broad applicability, being readily adaptable for other burrow-nesting seabirds.
2005 Elsevier Ltd. All rights reserved.

1.

Introduction

Translocation can be a powerful tool for the conservation of threatened fauna, particularly insular species. It can be used to re-establish extirpated populations, expand species ranges to former distributions, increase the diversity of depleted

communities or relocate critically endangered species to habitats that are free of threats. Translocation of seabirds has been attempted only occasionally (Kress and Nettleship, 1988; Serventy et al., 1989; Bell, 1995; Miskelly and Gummer, 2004; Miskelly and Taylor, 2004), with varying success. Procellariiformes (albatross and petrels) are highly philopatric

* Corresponding author: Tel.: +61 02 9585 6504; fax: +61 02 9585 6606. E-mail address: [email protected] (D. Priddel). 0006-3207/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2005.10.023

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(Warham, 1990), and a key factor affecting the outcome of translocations of these species is the age at which nestlings are removed from their natal site (Fisher, 1971; Serventy et al., 1989). The older the chicks are when moved the more likely they are to have developed a strong philopatry toward their natal site, and the less likely they are to return to the translocation site. Conversely, the younger they are the less likely they are to survive the translocation process. The Gould’s petrel (Pterodroma leucoptera) is a globally threatened species that breeds only in Australia and New Caledonia. The Australian subspecies P. l. leucoptera was restricted, essentially, to a single breeding location – Cabbage Tree Island at the entrance to Port Stephens, New South Wales. In 1992, the population ashore on Cabbage Tree Island was estimated at about 1500 birds, a decline of 26% since 1970 (Priddel et al., 1995). A breeding population of about 200 pairs fledged less than 50 young per annum. Mortality of adults, from predation by pied currawongs (Strepera graculina) and entanglement in birdlime trees (Pisonia umbellifera), exceeded 50 individuals per annum (Priddel and Carlile, 1997b). The underlying cause of this unsustainable situation was the severe degradation of the nesting habitat on Cabbage Tree Island by the introduced European rabbit (Oryctolagus cuniculus). A successful recovery program for Gould’s petrel, involving annual culling of currawongs and removal of birdlime trees, commenced in 1993. Rabbits were eradicated from Cabbage Tree Island in 1997 (Priddel et al., 2000). By 2000, the population of Gould’s petrel on Cabbage Tree Island exceeded 800 breeding pairs, and the annual output of young was greater than 400 individuals (NSW National Parks and Wildlife Service, 2001; Carlile et al., 2003). Despite these significant conservation gains, the colony on Cabbage Tree Island remained vulnerable to extinction through random stochastic events or a single act of vandalism. The close proximity of Cabbage Tree Island to urban areas on the Australian mainland heightened the potential risk. Invasion by alien predators (particularly cats or rats) could rapidly drive the population into decline. An outbreak of fire, either from a lightning strike or campfire, could also devastate the population if it occurred during the incubation period when breeding adults were ashore. Recognising the potential threat, the recovery plan for Gould’s petrel (NSW National Parks and Wildlife Service, 2001) recommended the establishment of an additional breeding colony on another island as a safeguard against such eventualities. This initiative was not unprecedented. A colony of about 19 breeding pairs of Atlantic puffin (Fratercula artica) was successfully established on Eastern Egg Rock, Maine, USA, by translocating nestlings from Great Island, Newfoundland (Kress and Nettleship, 1988; Kress, 1998). This achievement, however, involved a long-term effort moving 954 young over a period of 13 years. We aimed to replicate this success, but with much reduced effort and less impact on the donor colony. Earlier studies demonstrated that Gould’s petrels could not only breed successfully in partially buried, plastic nest boxes, but breeding success in these artificial nests was greater than that in natural nests (Priddel and Carlile, 1995). Additionally, an experimental translocation, conducted in 1995, demonstrated that Gould’s petrel nestlings could be translocated successfully (Priddel and Carlile, 2001). This experiment

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collected and utilised data on chick provisioning rates, meal size, growth rate, plumage development and emergence from the nest. It also developed criteria to select chicks of optimal age for translocation (11–28 days before fledging). Separation of nestling and parents, and the associated use of handfeeding, resulted in a small increase in fledging weight, but caused no reduction in fledgling success and no change in the time of departure of fledglings (Priddel and Carlile, 2001). By 1998, three of the 30 nestlings transferred in 1995 had returned to their release site on Cabbage Tree Island (Priddel and Carlile, 2001). This paper describes the successful establishment of a new breeding colony of Gould’s petrel through the creation of artificial nesting habitat and the subsequent translocation of 200 nestlings. It documents the translocation procedure and examines the outcomes based on data from five years of monitoring at both the release site (Boondelbah Island) and the donor colony (Cabbage Tree Island). It also updates information concerning the return of birds involved in the experimental translocation undertaken in 1995.

2.

Methods

2.1.

Study sites

Cabbage Tree Island (3240 0 S, 15214 0 E) is located 1.5 km offshore from Port Stephens, New South Wales, Australia. The 30-ha Island is a nature reserve administered by the New South Wales Department of Environment and Conservation. The island is roughly aligned north–south with the western side sloping steeply to 123 m above sea level. The eastern face falls precipitously into the sea. Several basaltic dykes dissect the toscanite bedrock from east to west, the two largest forming pronounced gullies containing extensive areas of rock scree. Rainforest dominated by deciduous figs (Ficus superba) and cabbage tree palms (Livistona australis) covers much of the western slopes of the island and dense stands of spinyheaded mat-rush (Lomandra longifolia) dominate much of the remainder (Priddel and Carlile, 2004b). The nesting grounds of the Gould’s petrel lie principally within the two steep gullies on the western side of the island (Fullagar, 1976). Typically, the petrels nest underground in natural cavities within the rock scree, but also occasionally in boulder crevices, hollow fallen palm trunks, among the buttresses of fig trees and under fallen palm fronds. Unlike many other small Procellariiformes Gould’s petrels do not dig earth burrows. Boondelbah Island (3242 0 S, 15214 0 E) is located about 1.4 km south of Cabbage Tree Island. The 9.3-ha island is a nature reserve shaped in the form of a mesa (Morris, 1976). Steep cliffs of toscanite rise on all sides to 55 m above sea level and are highly fractured along vertical bedding planes. Several basaltic dykes dissect the Island. The main north– south dyke is heavily eroded at the southern end, where it forms a deep bay and a natural arch. At the northern end it forms a deep ravine. Siliceous soils on the southern and eastern parts of the island give way to more humic and waterlogged soils in seepage areas (Priddel and Carlile, 2004a). These wet areas are dominated by knobby club-rush (Isolepis nodosa). Elsewhere, the vegetation is dominated by spiny-

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headed mat-rush. Scattered, wind-sheared trees of deciduous fig, tuckeroo (Cupaniopsis anacardioides) and soft corkwood (Duboisia myoporoides) occur along the ridges, in the central valley and on the north and west cliff faces. Introduced prickly pear (Opuntia stricta) and bitou bush (Chrysanthemoides monilifera) occur in patches throughout the island. Approximately 12 pairs of Gould’s petrel nest on Boondelbah Island (Priddel and Carlile, 1997a). The population is small because natural cavities suitable for nesting are scarce, being restricted to a small rock pile in a narrow ravine on the western shore and a few isolated rock piles on the eastern side of the southern bay (Priddel and Carlile, 1997a). Thus, translocation of birds to this island was an augmentation of an existing population rather than the introduction of a species to a new environment (IUCN, 1987).

2.2.

Creation of artificial habitat

If a sizeable colony of Gould’s petrel (50–100 pairs) was to be established on Boondelbah Island, additional nesting habitat needed to be created. Accordingly, in October 1998, one hundred nest boxes were installed within the steep ravine on the northern side of the island. These nest boxes had been designed specifically for burrow-nesting seabirds (Priddel and Carlile, 1995). The boxes were positioned 1.0–1.5 m apart and partially buried in soil. In all, the boxes extended over an area of about 150 m2. To facilitate future management of the colony each nest box was marked with a unique number. Birds could enter the nest chamber via a tunnel consisting of several plastic PVC pipe fittings (see Priddel and Carlile, 1995). The installation of the nest boxes displaced no other bird species and did not compromise existing conservation values.

2.3.

Selection of nestlings for translocation

A total of 200 nestlings were translocated from Cabbage Tree Island to Boondelbah Island: 100 in March 1999 and 100 in March 2000. The optimal time for translocating nestlings is after they have reached maximum mass, but before they first emerge from their burrow. For Gould’s petrels this corresponds to about 11–28 days before fledging (DBF) (Priddel and Carlile, 2001). The criteria used to identify nestlings of this age were based on plumage and mass. Nestlings were selected only if they exhibited loss of mesoptile (downy) feathers from (1) the face only; or (2) face and tail; or (3) face, tail, wing and no more than two of the following: chest, back, shoulders or rump, provided body mass was >220 g (see Priddel and Carlile (2001) for a full explanation). An additional parameter – wing length (flattened straightened wing; Lowe, 1989) – was evaluated during the 1999 translocation and added to the selection criteria in 2000. In this year, nestlings were selected only if they met the criteria for plumage and mass, and their wing length was >160 mm. This refinement was designed to eliminate the least developed individuals (those >22 DBF), thereby shortening the duration of handfeeding and so reducing costs.

2.4.

Translocation

On 22 March 1999 and 2000, previously monitored nest sites on Cabbage Tree Island were visited and the nestling in-

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spected. Nestlings were weighed, measured and assessed for plumage development. Nests where nestlings met the selection criteria were flagged for easy recognition. Early the following morning, burrows containing preselected nestlings were revisited. Each nestling was removed from its burrow and placed in a waxed cardboard carton (72 · 40 · 30 cm) partitioned into six separate chambers (24 · 20 · 30 cm). Air holes provided ventilation. We aimed to transfer nestlings between islands when temperatures were mild. In 1999, warm ambient temperatures were forecast for the day of translocation, so birds were moved before dawn. In 2000, conditions on the day of translocation were overcast and temperatures cool, so birds were moved after dawn. An inflatable boat (Zodiac Mk II) was used to ferry the birds from Cabbage Tree Island to a waiting launch, which then transported them to the second island. The birds were again transferred to the inflatable for the landing on Boondelbah. Each bird was then placed singularly within the nesting chamber of a nest box. The maximum time in transit (from natal nest to nest box) was three hours. A vented barrier was fitted across the external entrance of each nest box to prevent the translocated nestlings from prematurely wandering from the safety of their allotted box. These barriers were removed after three days.

2.5.

Handfeeding of translocated individuals

Whereas Atlantic puffin nestlings will pick up food off the burrow floor (Kress and Nettleship, 1988) Gould’s petrel, like most other Procellariiformes, will only accept food inserted directly into the oesophagus. Translocated Gould’s petrel nestlings were fed whole fish and sectioned pieces of squid. Other conservation programs involving handfeeding of young birds (e.g., Maxwell and Jamieson, 1997) have used hand puppets resembling parent birds to feed chicks. Such measures, aimed at preventing imprinting on humans, were regarded as unnecessary in this instance as only near-fledged young were involved. All food was of a grade suitable for human consumption. It was stored frozen and thawed in seawater immediately before use. Unused thawed portions were discarded. In 1999, translocated nestlings were fed approximately 25 g of food every second day or thereabouts, an amount similar to that which parents normally feed to nestlings 18–42 days before fledging but in excess of that fed to older nestlings (Priddel and Carlile, 2001). Excess feeding was likely to produce heavier fledglings (Priddel and Carlile, 2001). For other Procellariiformes such as Manx shearwater (Puffinus puffinus), sooty shearwater (P. griseus), and common diving petrel (Pelecanoides urinatrix) heavier fledglings have higher post-fledging survival (and therefore return rates) than lighter fledglings (Perrins et al., 1973; Sagar and Horning, 1998; Miskelly and Taylor, 2004). However, there is likely to be a limit to the optimal maximum weight, so we were cautious not to increase fledging weight excessively. Generally, until fully satiated, younger nestlings readily accepted the food offered to them, whereas near-fledged nestlings (<8 DBF) were usually reluctant to feed and, if fed forcibly, often regurgitated the entire meal when they were

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returned to their nest. In light of these observations and the heavy fledging weight of the 1999 cohort (see Results), the feeding technique was modified for 2000. In this year, nestlings were fed as much as they would accept readily (up to a maximum of 50 g), but once they had acquired a wing length of 205 mm (equivalent to about 8 DBF, see Results) they were no longer fed. This cessation of feeding induced a short period of starvation similar to that which occurs naturally before fledging (Priddel and Carlile, 2001). In both years, wing length and mass of each translocated individual were measured every 1–2 days, and the transition to adult plumage recorded. In 2000, 30 fledglings on Cabbage Tree Island were captured on the surface as they were preparing to depart the island. These birds were weighed and their wing length measured. Fledglings that had previously been assessed and rejected for translocation were excluded from this sample. Fledging mass and wing length were compared between translocated and non-translocated birds.

2.6.

Subsequent breeding success of donor pairs

The breeding outcome of pairs from which nestlings had been removed (for the purpose of translocation to Boondelbah Island) was investigated in the season following removal. Like other Procellariiformes Gould’s petrels are long-lived, pair for life and use the same nest site year after year (unpublished data), so it was assumed that pairs using a nest site in one year were the same pair in the following year. Nests from which nestlings were taken in March 1999 and 2000 were inspected for the presence of nestlings in March 2000 and 2001, respectively. Fledgling production (the number of fledglings as a proportion of the number of nests) in these donor nests was compared with fledgling production from a sample of 100 randomly selected nests that had also produced fledglings the previous year.

2.7.

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variable random transects, covered only 40% of the nesting habitat, so it was unlikely that all birds returning in any one year would be detected. Annual monitoring of all known nest sites (NSW National Parks and Wildlife Service, 2001) and opportunistic nocturnal searches for birds on the surface provided additional opportunities of finding returning birds. Records of translocated individuals were examined to establish whether these birds returned to their natal site or to their release site. The age when each individual was first recaptured was determined, and the distance between the site of their recovery and the nest box from which they fledged was calculated.

3.

Results

3.1.

Survival of translocated nestlings

In 1999, 95 of the 100 nestlings translocated to Boondelbah Island fledged successfully. One nestling died from gradual loss of condition after its feathers became heavily soiled with proventricular oil regurgitated while the bird was being fed for the first time. Further occurrences of this problem were avoided by discontinuing to feed birds at the first sign of oil rising in the oesophagus. Another four nestlings died during an exceptionally warm day (ambient temperatures exceeded 30 C), three days after they were transferred. These birds each moved from the chamber of their nest box, where they had been placed, into the distal end of the tunnel that was blocked by a barrier and exposed to direct sunlight. It is believed that high temperatures within the tunnels proved lethal to the nestlings. In the second translocation, the position of birds within the nest boxes was checked approximately every hour while ever the barriers were in place and ambient temperatures exceeded 28 C. Any bird found in the tunnel was returned to the nest chamber. In this year (2000), all 100 translocated nestlings fledged successfully.

Breeding activity on Boondelbah Island 3.2.

The nest boxes on Boondelbah Island were monitored regularly during each breeding season between 1999–2000 and 2003–2004. Each nest box was inspected at least once during the courtship period (October) to identify those boxes that had been visited and to locate any birds prospecting for nest sites. The boxes were also inspected several times during the incubation period (November–December) to locate birds sitting on eggs. The sex of these birds was determined by cloacal examination (Serventy, 1956). A final inspection shortly before fledging (March) determined if breeding had been successful. All adults and nestlings were banded with individually numbered metal bands supplied by the Australian Bird and Bat Banding Scheme.

2.8.

Return of translocated birds to Cabbage Tree Island

Annual surveys of the nesting habitat on Cabbage Tree Island to locate incubating adults (Priddel and Carlile, 1997b; NSW National Parks and Wildlife Service, 2001) provided a means of detecting translocated birds that returned to breed. In any one year, however, these surveys, based on a series of

Duration to fledging

In 1999, the translocated nestlings fledged 8–27 days (mean ± SD, 17.3 ± 4.2 days) after they were transferred (Table 1). Only three birds (3.2%) were outside the targeted range of ages (11–28 DBF), and these fledged up to 3 days earlier than intended (Fig. 1). Those nestlings that fledged last (23–27 days after translocation) had wing lengths <160 mm at the time of transfer (Fig. 2). In 2000, an additional selection criterion (wing length >160 mm) was used to ensure a closer age range of individuals. In this year, nestlings fledged 9–22 days (15.1 ± 2.9 days) after they were transferred (Table 1). The maximum time to fledging was reduced by 5 days and the mean by 2.2 days. Four birds (4.0%) were outside the targeted range of ages (11–22 DBF), and these fledged up to two days earlier than intended (Fig. 1). The barrier across the external entrance to each nest box prevented translocated nestlings from leaving the nest box during the first 3 days following their transfer to Boondelbah Island. The removal of these barriers did not precipitate an exodus of birds from the boxes. It was a further 5 days before any nestling departed.

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Table 1 – Time to fledging, fledging mass and wing length of Gould’s petrel nestlings translocated to Boondelbah Island (1999 and 2000) and fledging mass and wing length of non-translocated nestlings on Cabbage Tree Island (2000) Year

Time to fledging (days) Fledging mass (g) Fledging wing length (mm)

1999 2000 1999 2000 1999 2000

Translocated nestlings

Non-translocated nestlings

n

Mean ± SD

Range

95 100 69 90 69 89

17.3 ± 4.2 15.1 ± 2.9 184.1 ± 18.7 178.5 ± 14.4 222.0 ± 5.5 223.6 ± 5.3

8–27 9–22 147–245 154–230 210–236 210–235

n

Mean ± SD

Range

30

172.8 ± 13.9

158–207

30

220.4 ± 5.0

208–229

All pairs produced fledglings in the preceding year. Breeding productivity is the proportion of nests that produced fledglings.

-------------------------------- intended range 1999 ----------------------------------------------------------------------------------------------------------- intended range 2000 -----------------------------

Number of fledglings

18 16 14 12 10 8 6 4 2 0 8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Days after translocation Fig. 1 – Interval between transfer and fledging of Gould’s petrels translocated from Cabbage Tree Island to Boondelbah Island (1999, open histograms; 2000, solid histograms).

3.3.

Feeding

In 1999, a total of 14.7 kg of food, in 611 meals, was fed to the 95 translocated nestlings that survived to fledging. Average meal size (±SD) was 24.1 ± 6.5 g (range 5–50 g). On average, each bird was fed every 2.7 days and received 155 g of food from 6.4 meals – a mean intake of 9.0 g per day. In 2000, 10.0 kg of food was fed to the translocated nestlings in a total of 688 meals. Average meal size was 14.6 ± 4.8 g (range 3–42 g). On average, each bird was fed every 2.2 days and received 100 g of food from 6.9 meals – a mean intake of 6.6 g per day.

3.4.

Wing length

At the time of transfer, the wing length of the 2000 cohort (mean ± SD, 181.4 ± 9.9 mm; range, 164–201 mm) was greater than that of the 1999 cohort (173.7 ± 17.3 mm; 135–219 mm; single factor ANOVA, F1,198 = 14.083, p < 0.001). This result is expected given that, in 2000, a minimum wing length was used as a selection criterion to ensure a shorter period between translocation and fledging. Mean wing length as a function of age (DBF) is shown in Fig. 2. Wing length at the time of fledging was similar for both cohorts (1999, 222.0 ± 5.5 mm; 2000, 223.6 ± 5.3 mm; F1,156 = 3.118, p = 0.079).

3.5.

Mass

At the time of transfer, the 2000 cohort weighed less than the 1999 cohort (251.3 ± 21.8 g, 210–305 mm cf. 263.3 ± 23.6 g, 211– 318 g; F1,198 = 14.786, p < 0.001). Again this was expected given that, on average, older nestlings were selected in 2000 and mass decreases between 28 DBF and fledging (Priddel and Carlile, 2001). Mass as a function of age (DBF) is shown in Fig. 3. The mass of individual nestlings fluctuated greatly during the period the birds were being fed (up to 8 DBF), thereafter it declined steadily, at a mean rate of 4.5 g per day in 1999 and 5.2 g per day in 2000. The greater rate of mass loss in 2000 was a consequence of a change in feeding regime that eliminated feeding during the last 8 days before fledging. There was a significant difference among the fledging mass of translocated birds in 1999, translocated birds in

Wing length (mm)

230

210

190

170

150

130

24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9

8

7

6

5

4

3

2

1

0

Days before fledging

Fig. 2 – Mean wing length of translocated Gould’s petrels between the time of transfer and fledging (1999, open circles (n = 95); 2000, closed circles (n = 100)).

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270

Mass (g)

250

230

210

190

170

24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9

8

7

6

5

4

3

2

1

0

Days before fledging

Fig. 3 – Mean mass of translocated Gould’s petrels between the time of transfer and fledging (1999, open circles (n = 95); 2000, closed circles (n = 100)).

2000 and non-translocated birds in 2000 (one-way ANOVA, F3,221 = 8.46, p < 0.01). Each sample was significantly different (Newman–Keuls multiple range test for unequal sample sizes; Zar, 1974), with birds translocated in 1999 the heaviest and non-translocated birds the lightest (Table 1).

3.6.

Subsequent breeding success of donor pairs

A comparison of the subsequent breeding productivity (the proportion of nests that produced fledglings) of pairs from which nestlings were removed and pairs that retained their nestling is shown in Table 2. Pairs from which nestlings were removed the previous year were as successful in raising a fledging as pairs which had previously retained their nestling (1999–2000: v2 = 0.834, p > 0.25; 2000–2001: v2 = 2.075, p > 0.10). Thus, the removal of nestlings for translocation did not lessen the future productivity of the donor colony.

3.7.

Breeding activity on Boondelbah Island

Since the boxes were installed, a total of 21 pairs have been found in nest boxes on Boondelbah Island (Table 3). These 21 pairs comprised 10 translocated individuals (9 males, 1 female), 31 non-translocated individuals and one unknown. All translocated birds paired with non-translocated birds. The first translocated birds to return did so in 2002–2003 (four males, one female). Two pairs containing a translocated bird each laid an egg, one of which was successfully reared to fledging (Table 3). The following year (2003–2004) 10 translocated birds (nine males, one female) were recorded on Boondelbah Island. Eight of these pairs laid eggs, of which six produced fledglings. No translocated fledgling that has returned to Boondelbah Island has bred in the same box from

which it fledged, but all have used boxes close by (mean distance ± SD, 5.5 ± 3.0 m; range 1.3–11.6 m). Among the non-translocated birds on Boondelbah Island were four birds that were previously banded on Cabbage Tree Island; all had partners of unknown origin or age. Two of the Cabbage Tree Island birds fledged in 1995 and one in 1997 (all males), the fourth bird (a female) was first recorded in 2000 as a paired adult in a nest without an egg. These records provide the first evidence of movement of this species between the two islands. Neither of the two birds from the 1995 cohort had been involved in the translocation experiment conducted that year. One bred on Boondelbah Island in 2001–2002 (presumably for the first time), when approaching 7 years of age, and has bred there every year since. The other 1995 bird has been paired with the same female since 2002–2003, but the pair has yet to produce an egg. The single bird from the 1997 cohort was first recaptured on Cabbage Tree Island in December 1999 thence on Boondelbah Island in December 2003 where, at 7 years of age, it successfully produced a fledgling. The Cabbage Tree Island female first laid an egg on Boondelbah in 2003–2004, but this failed to hatch. Overall, a total of 24 eggs have been laid in the nest boxes on Boondelbah Island, and 14 fledglings produced (Table 3). The first egg was produced in 1999–2000, just one year after the boxes were installed. The resulting well-developed chick was killed when a rock fall destroyed the nest box. No other nest boxes were damaged. The crushed box was replaced, and the same pair has returned to breed within it in each subsequent year. The origin of these birds is unknown. The first fledglings produced by the new colony fledged during the 2001–2002 breeding season (Table 3). The 2003–2004 breeding season was the most productive, with 21 nest boxes occupied, 14 eggs laid and nine fledglings produced (Table 3). Another

Table 2 – Subsequent breeding productivity of pairs from which fledglings were removed for translocation and pairs that retained fledglings Year 1999–2000 1999–2000 2000–2001 2000–2001

Previous year’s nestling

n

Failed to lay

Egg only

Removed Retained Removed Retained

100 100 100 100

12 7 20 14

16 28 15 32

Fledgling 72 65 65 54

Breeding productivity 0.72 0.65 0.65 0.54

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Table 3 – Breeding activity at the new colony on Boondelbah Island (1999–2000 to 2003–2004) Year 1999–2000

2000–2001

2001–2002

2002–2003

2003–2004

0 0 1 0

0 0 1 0

0 1 1 2

5 6 2 3

6 7b 5c 9

2 0

2 0

7 1

13 3

27 4

0 0

0 0

0 0

4 1

9 1

1

1

3

3

6

0 0

0 0

0 0

1 1

7 1

0

0

2

2

3

0 0

0 0

0 0

0 1

6 0

Nest box use Nest material only Nest + bird(s) Nest + bird(s) + egg Nest + bird(s) + egg + fledgling Birds identified Not translocated Unknown origin CTI Translocated Males Females Pairs with eggsa No translocated bird One translocated bird Males Females Pairs with fledglinga No translocated bird One translocated bird Males Females

a No pair comprised two translocated birds. b Some birds were found in more than one nest box. c One nest with egg was occupied by two pairs.

six boxes were visited by petrels, as evidenced by the presence of a depression and nesting material in the nest chamber, but no birds were present during inspections.

period during which 90% of translocated birds fledged (Fig. 1). As yet, only relatively late fledging birds from 1999 and early fledging birds from 2000 have returned.

3.8.

3.9.

Characteristics of returning birds

Fledging mass, wing length and time to fledging (DBF) of returning birds are shown in Table 4. Although small numbers of returning birds preclude a rigorous analysis, the data provide some useful insights. The fledging mass of returning individuals spanned a substantial range (147–200 g). Reassuringly, the lightest translocated fledgling to leave Boondelbah Island successfully returned and bred. Similarly, there was no apparent bias in the size (mean and range) of wing length at fledging of returning birds (Table 4). Returning birds fledged 12–24 days after being transferred to Boondelbah Island, a

Table 4 – Fledging mass, wing length and time to fledging of all translocated Gould’s petrel nestlings that returned to Boondelbah Island

Fledging mass (g) Fledging wing length (mm) Time to fledging (days)

Year

n

Mean ± SD

Range

1999 2000 1999 2000 1999 2000

5 5 5 5 5 5

175.0 ± 20.1 180.6 ± 12.4 223.2 ± 4.0 220.4 ± 6.8 22.0 ± 1.6 14.6 ± 1.7

147–198 166–200 220–230 211–229 20–24 12–16

Return of translocated birds to Cabbage Tree Island

Two birds that were transferred to Boondelbah Island have since been detected on Cabbage Tree Island, having returned to within 85 and 50 m of their natal nests. One was first detected during the 2002–2003 breeding season, the other during 2003–2004, at three and four years of age, respectively. Neither is known to have bred and their sex is unknown. Both birds were among the earliest individuals to fledge after the translocation (11 and 13 days). All except one of the translocated birds that have returned to Boondelbah Island fledged later than 13 days after transfer (Table 4). If this preliminary trend were to be verified by additional data, it may be prudent to modify the selection criteria to omit those birds closest to fledgling (i.e., 11–13 DBF). Of 30 birds translocated between gullies on Cabbage Tree Island during the experimental translocation in 1995, seven are known to have returned; one to its natal gully and six to the gully where they were released. The bird that returned to its natal gully has been recorded there on five occasions, eventually breeding in a burrow 25 m from where it hatched. This bird fledged 7 days after it was translocated. The six birds that returned to their release sites fledged 5–20 days (mean ± SD, 10.3 ± 5.5 days) after translocation. The mean distance (±SD) between the box from which these birds fledged

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and the box in which they bred was 26.0 ± 10.9 m (range 10– 80 m).

4.

Discussion

4.1.

Creation of artificial nesting habitat

This study has demonstrated that by creating artificial nest sites it is possible to establish a new colony of burrow-nesting petrels in an area previously devoid of suitable nesting habitat. Artificial burrows have also been used to increase the availability of nesting sites for the Bermuda petrel (Pterodroma cahow) and Madeiran storm petrel (Oceanodroma castro) (Wingate, 1978; Bolton et al., 2004). Other burrow-nesting Procellariiformes to have nested successfully in artificial structures include: the dark-rumped petrel (Pterodroma phaeopygia); Cory’s shearwater (Calonectris diomedea); wedge-tailed shearwater (Puffinus pacificus); British storm petrel (Hydrobates pelagicus) and Leach’s storm petrel (Oceanodroma leucorhoa) (Byrd, 1979; Podolsky and Kress, 1989a; Bolton, 1996; Ramos et al., 1997; Podolsky and Kress, 1989b). Hence, the use of nest boxes to create artificial nesting habitat is likely to have broad applicability. A drawback of this technique is that nest boxes require periodic maintenance to ensure their continued suitability as nesting sites. Consequently it is preferable to use natural nesting habitat if this is available. If artificial habitat is the only option, the use of durable plastic instead of timber for box construction greatly increases the lifespan of the structure and minimises any ongoing maintenance requirements. Procellariiformes exhibit a high degree of both mate and nest fidelity (Warham, 1990) so it is likely that the birds occupying the nest boxes on Boondelbah Island were breeding for the first time. Inexperienced breeders generally have lower breeding success than do established breeders (Wooller et al., 1990), so poor breeding success in the initial years of colony establishment is to be expected. However, breeding success in the new colony on Boondelbah Island (0.58, 14 fledglings from 24 eggs) has been as high as the annual breeding success in natural nests on Cabbage Tree Island during the same period (0.46–0.57, unpublished data). This was due to a lower incidence of egg damage, presumably resulting from the increased protection afforded by the box. Madeiran storm petrels nesting in artificial burrows on Praia Islet in the Azores archipelago also had higher annual breeding success than pairs using natural nest sites (0.42–0.64 cf. 0.15–0.29; Bolton et al., 2004). Breeding success on Boondelbah Island can reasonably be expected to rise still further as breeders become more experienced.

4.2.

Survival of translocated nestlings

The survival rate of translocated Gould’s petrel nestlings (95% in 1999, 100% in 2000) was a marked improvement on earlier translocations of burrow-nesting seabirds, many of which resulted in high mortality of the translocated individuals. For example, only 62% of fluttering shearwaters (Puffinus gavia) survived to fledging after being translocated from Long Island to Maud Island, New Zealand, between 1991 and 1993 (Bell, 1995). Similarly, only 50% of common diving petrels fledged

1 2 8 ( 2 0 0 6 ) 5 5 3 –5 6 3

successfully following translocation to Mana Island, New Zealand, between 1997 and 1999 (Taylor, 2000a). These previous translocations involved moving nestlings of various ages. Translocating young nestlings requires them to be handfed over a protracted period, necessitating that the diet provide the full complement of nutrients required for growth and development. The dietary needs of seabirds are not well understood, and dietary inadequacies may have contributed to the high death rate in several translocations. The method of feeding possibly also contributed to the loss of nestlings. Most workers, particularly those dealing with species that feed on crustaceans, fed nestlings by delivering a slurry directly into the crop through a plastic tube. This method required the food to be highly processed, introducing the risk of bacterial contamination of the food or feeding apparatus, particularly when working in remote field situations. Some deaths of translocated fluttering shearwaters showed symptoms of food poisoning (Bell, 1995). Other workers have attempted to overcome the problems associated with handfeeding by selecting replacement foods that duplicate the nutritional content of the natural diet (e.g., Thomas et al., 1998), by using tinned food and by maintaining good hygiene (Miskelly and Williams, 2002). In developing techniques to translocate Gould’s petrels we opted to minimise most of the potential problems associated with handfeeding by translocating only nestlings that were close to fledging. Near-fledged Procellariiformes require very few feeds prior to fledging, particularly since they generally fast during their last days ashore. Gould’s petrels, for example, fast for 10 days prior to fledging, and during the last 20 days in the nest, nestlings typically receive only four meals from their parents (Priddel and Carlile, 2001). Selecting near-fledged birds not only minimises the number of feeds required, it also lessens the importance of any dietary inadequacies because the amount of food provided artificially is small relative to that already provided by the parents. Although the translocated nestlings were fed for a short period only, the quantity of food they received during this time strongly influenced their fledging mass. In 1999, translocated birds were fed up until the time they fledged, resulting in particularly heavy fledglings. The mean fledging mass of these individuals (184.1 g) was similar to that of nestlings that received food both from their parents and from handfeeding during the 1995 experimental translocation (186.9 g; Priddel and Carlile, 2001). The introduction of a short period of starvation in 2000 produced translocated fledglings that were lighter (178.5 ± 14.4 g) than those in 1999, but still heavier than non-translocated fledglings (172.8 ± 13.9 g). The approach of translocating only near-fledged nestlings has proved successful for Gould’s petrel and for subsequent translocations involving other burrow-nesting seabirds. A translocation of 240 near-fledged fairy prions (Pachyptila turtur) from Takapourewa to Mana Island during 2002–2004 recorded no mortality of translocated nestlings (Miskelly and Williams, 2002; Miskelly and Gummer, 2003, 2004). A translocation of 14 near-fledged Bermuda petrels to Nonsuch Island from four nearby islets in 2004 also achieved 100% fledging success (J. Madeiros, unpublished data). Minimising the time

B I O L O G I C A L C O N S E RVAT I O N

that translocated nestlings need to be handfed has the added benefit of reducing the cost of the translocation.

4.3.

Selection of nestlings for translocation

While translocating near-fledged nestlings reduces the risk of mortality, it is important that the translocated individuals have not yet acquired a strong philopatry towards their natal site. We believe that this philopatry develops from the time the nestlings first emerge from their burrows. For Gould’s petrels this typically occurs at about 10 days before fledging (Priddel and Carlile, 2001). Therefore, selecting birds of appropriate age for translocation is critical and consequently the success of any translocation is highly dependent upon the ability to age nestlings accurately. The selection criteria used to age Gould’s petrel nestlings in 1999 were based on plumage and mass, and targeted birds that were 11–28 DBF. Only 3% of translocated individuals fell outside this range. In 2000, wing length was used as an additional selection criterion to narrow the band of targeted ages to 11–22 DBF. Only 4% of translocated individuals fell outside this range. Thus, the combination of mesoptile feather loss, mass and wing length provided a simple and reliable means of ageing nestlings for the purposes of translocation.

4.4.

Subsequent breeding at natal sites

Translocations involving rare or threatened species often raise concerns about the potential impact that the removal of young may have on recruitment and the subsequent breeding performance of the donor pairs. In the year following each translocation of Gould’s petrels, the pairs from which nestlings were removed the previous year were as successful in raising a fledgling as pairs that had previously retained their nestling. This result demonstrates that the impact of nestling removal did not extend beyond the loss of potential recruitment arising from the individuals removed.

4.5.

Subsequent breeding at translocation site

Five years after the first translocation of Gould’s petrel to Boondelbah Island, a colony of 21 pairs has been established, and 14 fledglings produced. The new colony currently contains at least ten translocated birds, from a total of 195 translocated fledglings – a return rate, thus far, of 5.1%. Two fledglings have returned to Cabbage Tree Island, giving a current overall recruitment rate of 6.1%. These rates are expected to rise as additional birds return to breed. Recruitment rates for Gould’s petrels on Cabbage Tree Island are not known. Currently, the sex ratio of returning birds is heavily skewed toward males. Assuming equal sex ratio of translocated nestlings, the return rate for males is currently 9.2% and females 1.0%. This disparity suggests that female Gould’s petrels may reach sexually maturity later than males. In other Procellariiformes both sexes first breed at about the same age (Mougin et al., 1986; Wooller et al., 1988). Long-term monitoring on Boondelbah Island may eventually confirm whether or not there is a difference between sexes in the time taken to reach sexual maturity. Alternative explanations to account for the biased sex ratio are that the selection process favoured males,

1 2 8 ( 2 0 0 6 ) 5 5 3 –5 6 3

561

or that survivorship was sex dependent. Adult males are marginally larger than females (O’Dwyer, 2004) but the overlap is so great that sexual discrimination based on the criteria used to select nestlings for translocation is unlikely. This possibility can be examined in future translocations by sexing all translocated nestlings using molecular techniques (see Griffiths et al., 1998). Nothing is known about comparative survivorship of the sexes. Return rates for previous seabird translocations have been generally lower and colony establishment slower. Of 157 short-tailed shearwater (Puffinus tenuirostris) nestlings translocated to Fisher Island in Bass Strait, Australia, during the late 1960s only five are known to have returned during the following 20 years (Serventy et al., 1989). A total of 954 Atlantic puffins were translocated to Eastern Egg Rock, Maine, USA between 1973 and 1986 (Kress and Nettleship, 1988; Kress, 1998). Return rates varied between cohorts (0–56%), but it was not until 1981 that the first breeding took place. By 1985 the colony had grown to 19 pair and has stabilised at this size. A total of 249 black petrels (Pterodroma parkinsoni) were transferred to Little Barrier Island, New Zealand, from nearby Great Barrier Island between 1986 and 1990 (Taylor, 2000a). Only two have been found at the translocation site during the following 10 years. A total of 308 fluttering shearwaters were translocated to Maud Island from Long Island, New Zealand between 1991 and 1996 (Bell and Bell, 1996). In the eleventh year after the first translocation 12 pairs, comprising both translocated and non-translocated birds, nested and produced nine fledglings (Brian Bell personal communication). Of 90 common diving petrels fledged from a translocation site on Mana Island, New Zealand (Taylor, 2000b) 18 had returned five years later, with one pair prospecting in a burrow (Miskelly and Taylor, 2004). The relatively rapid establishment of a breeding colony of Gould’s petrel on Boondelbah Island was due to an unprecedented return rate of translocated individuals. This high rate of return was a consequence of both the low mortality of translocated nestlings and the birds’ strong philopatry toward Boondelbah Island. We believe these highly desirable outcomes were the result of selecting only those nestlings that were at the optimal age for translocation, i.e., near-fledged individuals that had not yet emerged from their burrow. The ability to accurately age nestlings, therefore, is critical to the success of any translocation.

4.6.

Use of audio cues

Only 24% of adult birds found in nest boxes on Boondelbah Island were translocated there as nestlings; the rest arrived independently. The role that audio cues play in attracting prospecting birds is uncertain. Two pairs bred in nest boxes on Boondelbah Island, and several other pairs visited boxes, before the first return of a translocated bird. The only audio cues for these birds would have been the calls of near-fledged birds toward the end of the previous breeding season, when most adults are at sea. The first pair to breed in the nest boxes on Boondelbah Island could not have been attracted there by conspecifics as they hatched an egg before any nestlings were translocated to the site. Regardless of what lured them to the site, the presence of non-translocated birds has significantly

562

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hastened the establishment of the colony above that which could be achieved through the return of translocated birds alone. The continued attraction to the site of non-translocated individuals is likely and this will increase the potential for the colony’s prolonged growth and survival. Other seabird translocations have incorporated the use of audio systems to broadcast the calls of the translocated species (e.g., Bell and Bell, 1996; Taylor, 2000a). The deployment of these systems aimed to attract both returning translocated birds and other birds in the vicinity. The usefulness of audio systems as an alternative or adjunct to translocation of nestlings has not been adequately evaluated. Audio cues and the provision of artificial burrows have successfully established new populations of both dark-rumped petrel and Leach’s storm petrel (Podolsky and Kress, 1989a,b). Comparative sites without audio stimuli were not colonised. The combination of audio cues and artificial burrows has also been used to increase the size of a breeding population of Madeiran storm petrels, with occupied burrows being closer to audio speakers than those burrows that remained unoccupied (Bolton et al., 2004). On Boondelbah Island, the colony of Gould’s petrel established without the use of audio cues. The closeness of Cabbage Tree and Boondelbah Islands or the presence of a small number of conspecifics breeding elsewhere on Boondelbah Island may have negated the need for any additional audio stimulus. Serventy et al. (1989), however, proposed that the closeness of donor and recipient colonies was a reason why the attempted translocation of short-tailed shearwaters to Fisher Island failed, suggesting that the high level of activity at the donor colony attracted birds away from the release site. It would be interesting to compare the outcome of the translocation to Boondelbah Island with one to a translocation site that was more distant from the main colony on Cabbage Tree Island. Where practicable, future translocations of seabirds should be conducted experimentally by broadcasting audio cues at some sites and not at others. Monitoring subsequent colony establishment at each site could then assess the usefulness of this technique as an adjunct, or alternative, to translocation.

4.7.

Conclusion

The Gould’s petrel was essentially confined to a single breeding locality – Cabbage Tree Island. The establishment of a second colony on Boondelbah Island has greatly enhanced the conservation status of the species by providing a safeguard against extinction occurring through a single random stochastic event or act of vandalism. Further conservation gains could be made by establishing additional colonies on neighbouring islands where the species does not currently occur. Procellariiformes are amongst the most threatened of all taxa (BirdLife International, 2000), and of the 104 species whose nesting habitats are well documented 78 (75%) nest in burrows or cervices (Schreiber and Burger, 2001; Bolton et al., 2004). The creation of artificial habitat and the translocation of nestlings can be useful tools for the successful conservation of these species. Establishing new populations of burrow-nesting seabirds on islands where introduced preda-

1 2 8 ( 2 0 0 6 ) 5 5 3 –5 6 3

tors have been eliminated can not only restore extirpated colonies, but also expand species’ ranges to their former distribution and increase the diversity of nesting seabird communities. Although there are some aspects of seabird translocations that have yet to be fully understood, such as the benefits of using audio cues, the successful establishment of a second colony of Gould’s petrel on Boondelbah Island has demonstrated that translocations of burrow-nesting species are highly practicable. Moreover, this study has shown that such conservation initiatives can now be achieved with little or no loss of translocated birds, no reduction in the subsequent reproductive output of the donor parents and with the knowledge that a high proportion of translocated birds will return to the new colony.

Acknowledgements The project was partially funded by the Endangered Species Program – a program of the Natural Heritage Trust administered by Environment Australia (now the Dept of Environment and Heritage). George Malalakis and Richard Coulbourne helped install the nest boxes on Boondelbah Island. Michael Jarman, Michael Murphy, Lisa O’Neill, Terry O’Dwyer, Adam Bester and Fransciso Bravo assisted in either the translocation or subsequent feeding of translocated nestlings. Chris Norman, Pro-Dive Nelson Bay, assisted in sea transport. Research was conducted under Animal Ethics Approval No. 010816/01 from the NSW National Parks and Wildlife Service (NPWS), NPWS Scientific Investigation Licences A2635 and C147, and Australian Bird and Bat Banding Scheme (ABBBS) licences 8010, 1208, and 1373.

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