350 pairs of Brown noddies are estimated to breed on Denis Island

In February 2016 a Brown noddy (Anous stolidus) census was conducted on Denis Island by Green Islands Foundation (GIF) staff and volunteers to estimate the size of the breeding population. Brown noddies are commonly ground nesting but are seen far more frequently nesting in the crowns of coconut trees on Denis Island, possibly due to the presence of mammal predators in the recent past before the eradications of rats and cats. Systematic observations were carried out in order to calculate a minimum count of nesting individuals found on the island during the north-west monsoon season (December-March).

The census was carried using the Seabird Monitoring Handbook for Seychelles 2nd Edition as a guide (Burger & Lawrence 2003). Over the course of 3 weeks all the potential nesting trees were inspected for the presence of Brown noddies. Nesting trees largely consisted of tall green coconut trees, with a few orange coconut trees and casuarinas trees used as well. The presence of an adult noddy on the nest, evidence of defecation on the surrounding leaves, or a chick either moving or begging for food were all used as indicators for active nests.

Throughout the island, the number of trees with active nests was recorded, along with the number of active nests in each tree. The data were used to assess a minimum estimate for the island breeding population, the average number of active nests per tree and where the highest concentration of Brown noddies was located. QGIS was used to calculate average nesting density per hectare. The height of each active nesting tree was recorded to see if there is an optimum height on which Brown noddies made their nests.

In total, 179 trees were recorded as nesting sites, carrying a total of 343 active nests. The minimum Brown noddy population estimate is therefore 343 breeding pairs (equaling 686 individual adult birds, thus excluding eggs and developing chicks). This estimate of ca. 350 breeding Brown noddy pairs matches the earlier estimate of ca. 600-750 individuals from July-September 2014 (reported by A. Labiche).

Figure 3. Brown noddy densities from the 2016 survey, the existing paths helped dividing the island into different areas for observations.
Brown noddy densities from the 2016 survey, the existing paths helped dividing the island into different areas for observations.

The human-habituated areas had the highest densities (nests per hectare) of Brown noddies: around the hotel, tortoise pen, chalets (particularly along the north-west coast) and the houses north of the Airstrip (see Figure 3). A small number of nesting pairs was found along the west coast and around the sooty tern area in the south-west of the island. The majority of the forest area was unused by Brown noddies. The average number of nests per active tree over all checked locations was just under two nests.

On average, the highest number of active nests per tree was seen in trees between 14 and 16 metres tall. Thus, this height range seems to be preferred by the birds for nesting. It is likely that noddies choose trees that are in rather open areas and/or stick out above the surrounding trees. Brown noddies are known to be gregarious; feeding in large flocks and this social behaviour appears to lead to more pairs breeding in the same tree.

Our survey indicates that there is a well-established breeding population of Brown noddies on Denis Island. The eradication of invasive predators has led to a higher number of chicks surviving to adulthood even when they fall out of their tree: the parents are forced to feed the chick on the ground, but without any predators present they still succeed to raise the chick. We are looking forward to seeing our first ground-nesting breeding pairs in the near future. We hope that the Brown noddy breeding population on Denis Island will continue to grow if food resources allow for it. It is spectacular to hear all the noise of begging chicks in the trees around the hotel, and parents flying around while calling out particularly after sunset. Sometimes, the guests are surprised that it’s not frogs but birds making these weird noises!

By Nick Burnham & Janske van de Crommenacker

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