Ocean control of the breeding regime of the sooty tern in the southwest Indian Ocean

Abstract

Abstract:Food availability, which is often seasonal, is regarded as a key factor in the breeding success of seabirds. In oceanic tropical areas, the resources are mostly patchy and ephemeral at the surface, and the seasonality is less marked than at higher latitudes. Such a situation influences greatly the breeding strategies of the oceanic seabird species. We conducted a comparative study of the breeding phenology of the sooty tern (Sterna fuscata) in relation to the local and regional oceanographic conditions around the four major colonies (Europa, Juan de Nova, Lys and Bird Islands) of the southwest Indian Ocean. Over the 1997-2003 period, around all the studied locations, the sea-surface temperature (SST) and the chlorophyll concentration in the Mozambique Channel and the Seychelles area showed clear seasonal differences related to the southern climate and the monsoon phenomena. The breeding activity is synchronized at each studied colony, but the timings are very different. Seasonal reproduction occurs in austral winter at Europa and Bird Island and in austral summer at Juan de Nova; at Lys Island the reproduction is non-seasonal. For the seasonal colonies, there is a large monthly change in SST just before the beginning of reproduction, which is a proxy indicating the annual phytoplankton bloom. This variation is accompanied by the development of oceanic features such as fronts that favour aggregation of prey, and may also play an important role in the presence of schools of surface tuna, which are very important for the foraging success of sooty terns. Conversely, around Lys Island the seasonal variations of the marine environment do not lead to pronounced development of oceanic structures, and consequently, the longer-lasting phytoplankton bloom could explain the non-seasonal breeding regime there. Further studies will help discern the advantages and disadvantages of seasonal and non-seasonal reproduction regime in response to unpredictable fluctuations of the marine environment.

Protection of marine birds and turtles at St Brandon’s Rock, Indian Ocean, requires conservation of the entire atoll

“…We can therefore assume that breeding is not always synchronous between different islets and species at SBR, a condition that may be advantageous, but should be protected and managed as such (see Rocamora et al 2003, Feare et al 2007, Jaquemet et al 2007, Jaquemet et al 2008, and Catry et al 2009 for more on the breeding phenology of seabirds in the western Indian Ocean). Visits to some of the islets by tourists or for other reasons could be restricted to certain periods of the year, but this would require regular monitoring.…”

Climatological Effects on the Breeding of Terns

“…We also made use of two climatic indices: Multi-variate ENSO Index (MEI) and Dipole Mode Index (DMI). Further details on the provenance, derivation and interpretation of all our data can be found in refs [2,5]. …”

“…By recording wing span and using recognised growth rates for terns [6], and subtracting a typical incubation period of 28 days, we deduce the likely laying dates. Typically all the birds in a colony lay within a short period; the typical standard deviation of observations being ~8 days (see [5] for full details). In the case of Bird Island, we make use of historical records of observations by Feare [6,7] The particularly interesting aspect of these results (Fig.…”

Within-colony determinants of Sooty Tern post-fledging survival revisited: effect of body mass and relative time of fledging

“…In Seychelles, egg laying by Sooty Terns Onychoprion fuscatus is highly synchronous (Feare 1976a, Jacquemet et al 2007, as evidenced by 75% of the estimated 395,000 eggs on Bird Island being laid within a nineday period in the 1973 breeding season (Feare 1976a), the only year that the pattern of laying has been closely followed. A corollary of synchrony is that pairs that laid during peak egg-laying reared more chicks to fledging than those that laid early or late in the season (Feare 1976a).…”

“…A corollary of synchrony is that pairs that laid during peak egg-laying reared more chicks to fledging than those that laid early or late in the season (Feare 1976a). Three factors could account for this: social behaviour within the colony, involving aggression of adults and larger young to smaller chicks that enter their territories (Feare 1976a), a social component in the location of feeding opportunities that are unpredictable in space and time (Ashmole 1963), and a seasonal effect of food availability, which Jacquemet et al (2007) demonstrated from remote sensing of sea-surface temperatures and chlorophyll concentrations in surface waters around Seychelles.…”