How many? New aerial surveys of Northern Royal Albatrosses and Northern Giant Petrels in the Chatham Islands

Motuhara/Forty-Fours from the air, photograph by Barry Baker

Peter Frost (Science Support Service, Whanganui, New Zealand) has reported to the Department of Conservation’s Conservation Service Programme on recent aerial surveys of Northern Royal Albatrosses Diomedea sanfordi and Northern Giant Petrels Macronectes halli on the Chatham Islands.

The report’s Executive Summary follows:

This report covers two aerial surveys of Northern Royal Albatross/Toroa Diomedea sanfordi nesting on Rangitautahi, Te Awanui and Motuhara in the Chatham Islands archipelago during the 2022–23 breeding season. The first survey was carried out on 20 December 2022, approximately mid-way during the birds’ incubation period; the second survey was flown on 16 August 2023, about two weeks before the first chicks were expected to fledge. The number of Northern Giant Petrel/Pāngurunguru Macronectes halli seen in the aerial photographs were also counted on all three islands.

The aim of the surveys and subsequent analyses of the images was to determine the number of birds apparently sitting on nests, and therefore breeding, early in the breeding season, and the number of chicks present in the weeks prior to fledging. From these data, maximum apparent nesting success could be estimated for the 2022–23 breeding season.

Both aerial surveys coincided with the presence of a two-person research team on Motuhara immediately prior to the surveys. The teams counted the overall number of nesting albatrosses in December 2022 (i.e., those birds incubating eggs and the number of recently failed nests), and the number of near-fledging chicks in August 2023. During the latter period the numbers of nesting Northern Giant Petrel were also counted. For both species, these ground counts provided a partial control on the numbers estimated from the aerial photographs.

In December 2022, there were 4,149 apparently occupied Northern Royal Albatross nests across the three islands, 1,744 (42% of the total) on Motuhara, 1,508 (36%) on Rangitautahi and 897 (22%) on Te Awanui. The number calculated for Motuhara is higher than the number of active nests counted there on the ground a week earlier (1,498 nests), most likely because the assumption that the classified birds in the close-up images are a random subset of the whole is flawed. Just under half of the birds seen in the close-up images of Motuhara is higher than the number of active nests counted there on the ground a week earlier (1498 nests), most likely because the assumption that the classified birds in the close-up images are Northern a random subset of the whole is flawed. Just under half of the birds seen in the close-up images of Motuhara could not be classified.

From ground surveys on all three islands since 2017, 7–8% of toroa nests are known to have failed up to mid-incubation. Applying this to the estimates derived from the aerial and ground surveys in December 2022, around 4,330–4,370 pairs of toroa bred on the Chatham Is during the 2022–23 breeding season.

Combining the August 2023 ground count of chicks on Motuhara (1,211, compared with 1204 from the aerial survey), and the aerial survey estimates for Rangitautahi (782) and Te Awanui (587), gives 2,580 near-fledging chicks in 2023.

Overall nesting success for the 2022–23 breeding season is therefore around 59–60%, slightly lower than that calculated solely from the aerial photographic analyses (64%). Both estimates are substantially higher than those recorded in recent years (42–55%).

A total of 2,128 Northern Giant Petrel were [sic] counted from the aerial photographs,of which 93% were on Motuhara. There is much uncertainty around numbers derived from aerial photographic analyses, but those obtained in August 2023 are broadly similar to those counted in earlier years.

Access this and other ACAP Species Infographics here

Read about a recent ground survey of Motuhara here.

Reference:

Frost, P.G.H. 2023. Nesting Success of Northern Royal Albatross/Toroa Diomedea sanfordi on the Chatham Islands: 2022-23 Breeding Season.  Whanganui: Science Support Service, 16 pp.

John Cooper, Emeritus Information Officer, Agreement on the Conservation of Albatrosses and Petrels, 02 May 2024