A year in seabird breeding

 This White Tern Chick will transform into a fairy-white adult bird

While it is that time to map out goals for the year ahead, it is also a good time to look back on the year. On Cousin Island Special Reserve it is time to look at the breeding success of some of the birds found on the island. Breeding success is an important parameter when monitoring seabird colonies, as it can be a useful indicator of the health and status of marine ecosystems, and just of the breeding colony in general.

Nature Seychelles staff working on Cousin Island followed the breeding of four species on a weekly basis during each species’ breeding period. These four species include the White-tailed Tropicbirds (Phaethon lepturus), White terns (Gygis alba), Lesser Noddys (Anous tenuirostris) and Brown Noddys (Anous stolidus).

When walking around Cousin, it is noticeable that species occupy different sections of the surrounding habitat. For instance, the White-tailed tropicbirds, which do not build nests, can be found at the base of trees or in little nooks on the ground. Their feet are located far back on their body, making them exceptional divers, but also making it difficult to balance on tree branches.

 Life starts as a balancing act for white terns

White terns also have an interesting way of reproducing. Instead of building nests, eggs are laid in precarious looking spots, in grooves of tree branches or in the V-shape formed by two splitting branches. Lesser Noddys build small nests made predominantly of leaves throughout the pisonia dominated forest, while Brown Noddys mostly nest on the open, rocky surfaces found on the island.

For each species, nests were selected once an egg was present. Then, each week, each nest was checked on and the status was recorded. Following some of these species is difficult, especially the Lesser Noddys, which nest higher up in the trees. A pole with a jerry-rigged mirror is used to see in these nests in order to accurately record the egg/chicks progress.

With each species having different tactics, some birds had higher breeding success rates than others.

 The white-tailed tropicbird is more adept to finding ready made homes on the ground

In 2015, the White Tern had the least amount of grown chicks to survive out of the four species. Only a small percentage of the followed nests had chicks that grew large enough to fly away (fledglings). This species also had the highest egg failure. Considering where they lay their eggs, it is not too surprising that many White Tern eggs did not hatch. The good news is that this year’s breeding success for the White Tern was higher than last year’s. Although we don’t know exactly why there is this difference in success between 2014 and 2015, the more chicks that fledge, the better the possibility of the population to increase.

For the Lesser Noddys, this year’s fledging success was similar to that seen last year, one in every three fledged. However the Brown Noddys had a much lower success rate than that seen in 2014. The Brown Noddy success was lower mostly due to many of the eggs not even hatching and to a lot of the chicks not surviving at very early stages. There were no obvious reasons for such high mortalities.

 

The most successful parenting goes to the White-tailed Tropicbirds. Their breeding success was very similar to that seen in 2014 with a breeding success of nearly 40%, it was the highest of the four species. When looking at hatching success though, these tropicbirds had the second highest egg failure also at 40%.

 Brown Noddy and Chick feeling at home on the rocks

What does this all mean? When looking at breeding success throughout the years, we are able to see if there are any trends or patterns seen for the different species. White-tailed Topicbirds have a rather steady trend, with White Terns and Lesser Noddys showing a slight decline in numbers. For all species, there are a lot of oscillations found throughout the years, which may be a reflection of environmental fluctuations.

There are a lot of factors which can influence breeding success, such as food availability, density of the nesting colony, predation, and many other factors. If one of the factors is out of balance, such as if there are low fish stocks one year, then breeding success may be affected. On the other hand, breeding success may just vary naturally.

Nature Seychelles’ years of monitoring seabirds on Cousin Island reveals data sets and trends which are important in guiding the organisation’s conservation work on this designated marine protected and important bird area.

By Cheryl Sanchez
Science Officer, Cousin Island Special Reserve

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