Bad year for Bird Island’s Sooty Terns confirmed

In my post of 15 September I reported that Bird Island’s Sooty Terns appeared to be having a bad year. The first indication that all was not well revealed itself in the late arrival of the birds, to the extent that the first eggs were laid in late June, almost a month later than usual. After eggs had been laid we had found that incubating birds were absent for long periods, leaving their mates to care for the eggs for longer shifts than normal. This led to some of the partners that were left incubating to desert their eggs after they had been left without relief for nine days or more.

Remains of dead chicks following food shortage for Bird Island's Sooty Terns in 2015
Remains of dead chicks following food shortage for Bird Island’s Sooty Terns in 2015

In addition, most of the first chicks to hatch died when only 2-3 days old and the ground in the colony was a sad sight, with dead young chicks scattered among the deserted eggs – bad for the birds but good for the scavenging land crabs! Many adults did continue to incubate, however, and chicks that hatched after this round of mortality seemed to fare better. By the end of August these later chicks were losing their down and growing their juvenile feathers.

In late September Christine Larose returned to Bird Island to count the chicks present at dusk in the squares that we had marked out in the colony (see previous post). She found that there had been further chick mortality, with corpses all over the colony. Her counts revealed that since our first counts of the eggs just before hatching began, only 21 % of these eggs had produced chicks on the point of flying. This compares with 58 % success in 1973 and 2003, the only other years for which we have good data. However, we do not know how healthy these 2015 chicks were and what their chances of survival are after leaving the colony.

What can have led to this bad performance in 2015? The development of a strong El Niño in the Pacific has been well publicised in the press. El Niño manifests itself as part of an oscillation of sea surface temperature (SST) over the central Pacific. During an El Niño event the SST in the western tropical Pacific increases relative to the SST in the east. The reverse of this is La Niña, when eastern SST is warmer relative to that in the west. Strong El Niño events have wide climatic ramifications, with heavier than usual rainfall and storms in the western Pacific and droughts in the east.

The Indian Ocean has its own oscillation, the Indian Ocean Dipole. This is monitored as a Dipole Mode Index (DMI). A positive DMI is recorded when SST in the western tropical Indian Ocean is warmer relative to the SST in the east, and a negative DMI is the reverse. In March-April 2015 the index switched from negative to positive. Thereafter the SST in the western Indian Ocean continued to rise relative to the east and the DMI has remained in a strong positive phase up to the present. This warm surface water, leading to an increased rate of evaporation, doubtless contributes to the high rainfall to which Seychelles has been subjected over the last two months, and this rainfall could have been responsible for some of the chick mortality that Christine saw on her late September visit.

Earlier, however, it was the adults that were being affected and their long incubation periods suggested that food was hard to find. In 2014 we had found that, using GPS loggers to track adults on their foraging trips during the incubation phase of the breeding cycle, that long absences while foraging resulted from greatly increased distances that the birds had to travel to find food. However, we do not know how variations in SST affect food availability. Possibilities include effects of SST on the depth at which prey swim or on their survival, or on the migrations of predatory fish that Sooty Terns (and other seabirds) rely on to drive prey fish to the surface. Alternatively, prolonged heavy rainfall might lead to reduced salinity at the sea surface, which prey fish might avoid, or rain falling on the water surface might interfere with the Sooty Terns’ ability to see their prey.

The situation is clearly complicated because the short period of food shortage that we witnessed in 2014 coincided with a strong but brief negative phase of the DMI. In addition, the food shortage in 2015 does not appear to have affected the entire western Indian Ocean basin. Aurélie Duhec, who has been studying seabirds for Island Conservation Society on Farquhar atoll, about 800 km south-south-west of Seychelles, told me that this year nesting began at the normal time in the Sooty Tern colony on Goelettes Island and the birds there seem to have had a very successful breeding season. At the latitude of Farquhar, about 10° south (6° further south than Bird Island), ocean currents differ from those further north and this might have provided more reliable food sources for Goelettes’ birds.

We still have much to learn about Sooty Terns and how their food supplies are influenced by climatic and oceanic variables! And of course this includes the likely effects of climate change and of man’s use and abuse of the marine environment.

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