Breeding success

Abstract

The island of Bonaire is a nesting location for at least four tern species: a subspecies of the Least Tern (Sternula antillarum
antillarum), the Common Tern (Sterna hirundo), the Royal Tern (Thalasseus maximus), and a subspecies of the Sandwich
Tern (Thalasseus sandvicensis eurygnathus). The island is also a significant nesting site for the Caribbean Least Tern (Sternula
antillarum) population. Our main objectives were to: a) measure and compare breeding success at five known nesting sites on
northern Bonaire, b) document and compare the impact of natural and introduced predators on each site, and c) give management
recommendations for increasing breeding success based on our results. Our nest counts from 2014 indicate a significant
decline in nest abundance compared to historical observations from the 1950s, matching previous studies and observations
from the last two decades. Among the five nesting sites in our study, terns at the two island sites had the largest number of
breeding pairs and achieved the greatest success, fledging a maximum of ~0.8 chicks per nest, compared to all other sites which
were connected to the shoreline. We recorded rats and cats as predators at the peninsula site and field observations suggested
that predation by Laughing Gulls (Leucophaeus atricilla) might also be occurring at two sites. Both natural and artificial islands
on hypersaline lagoons provide good nesting sites for terns on Bonaire as they are protected from mammalian predators. However,
recreational disturbance remains the single most serious and pervasive threat to the future of seabird nesting on Bonaire
and requires concerted action. We propose a list of management actions to increase the numbers of nesting terns throughout
the sites studied. Increasing protection from predators and human disturbance by making artificial nesting islands will provide
the potential for Bonaire, and its sister islands, to become major refuges for southern Caribbean metapopulations of these four
tern species.

We assessed the breeding success of Red-billed Tropicbirds Phaethon aethereus on St. Eustatius, particularly in relation to predation at the nest. We conducted weekly surveys at five sites during 2012-2013 and measured chick and adult morphometrics. Apparent nest success ranged from 55-100% across five breeding areas, while apparent fledge success ranged from 63-100% at those same locations. We used cameras and baited rat traps to document the presence of predators at nest sites. Predation rates captured on cameras were low (ca. 20 images of predators from ca. 263,000 images over 11 weeks). Cameras documented cats and rats at accessible nests. Although we could not confirm the cause of egg loss or the death of some chicks, the presence of cats and rats suggests that additional effort be expended to accurately measure their impact.
 
Retrieved from http://www.statiapark.org on April 13, 2015

Assessment of the breeding success of Red-billed Tropicbirds (Phaethon aethereus) at the Pilot Hill site on St. Eustatius, particularly in relation to predation at the nest. We conducted weekly surveys during 2013-2014 and measured chick and adult morphometrics. Overall apparent hatching success was 64.6%, while apparent fledging success was 75.6%. We used cameras to document the presence of predators at nest sites and wax bait blocks to estimate rat density. Camera traps documented rats opportunistically scavenging eggs that were left unattended for even short periods of time. In total we documented nine predation events of eggs by rats at eight individual nesting cavities.

Executive Summary:

The eleventh field season of the Aruba Tern Project was completed in 2009. The study continued its stated aim of producing a quantitative description of the status and distribution of nesting terns and laughing gulls within San Nicolas Bay and around the island of Aruba.

The 2009 field season began during the third week of June, which was later than in most previous years. Consequently, we were unable to gain accurate egg counts, egg measures and population counts for those species nesting early in the season, primarily Cayenne terns and sooty terns. We were also unable to confirm the nesting status of regionally threatened roseate terns and some common terns. Despite these challenges we were able to generate population estimates for each species, although the margin of statistical error and reliability of the estimates, for the earlier nesting species, was more variable.

We observed a continuation of the gradual increase in the number of nesting bridled terns. We also recorded average or greater than average numbers of sooty tern, common tern, brown noddy and laughing gull. The numbers of Cayenne terns were within the range of numbers experienced in previous years. The nesting island for Cayenne terns shifted from island 3 to island 2 and there were no Cayenne terns observed nesting on the islets opposite Oranjestad. The absence of Cayenne terns near Oranjestad was unusual but may be an observer bias as we were not on the island to see whether birds nested in that area and failed before we arrived.

The egg size, clutch size, chick growth and juvenile and adult survivorship was within the range recorded for all species in previous years. In general, and despite the challenges associated with relatively late data collection, we believe that 2009 represents a slightly better than average year for all tern species nesting on Aruba. The increase of nesting laughing gulls must be the result of immigration as the local population could not account for the scale of increase observed in this species. Laughing gulls continue to remain a serious threat to ground- nesting terns, most notably Cayenne terns. 

Aruba, and particularly the San Nicolas Bay islands are unique within the Caribbean and most probably the world. In 2009 San Nicolas Bay contained approximately 25% of the world’s population of Cayenne terns, over 90% of the Caribbean population of common terns, 25% of the Caribbean’s black noddy, 3% of the Caribbean’s brown noddy, >1% of the Caribbean’s bridled terns and 4% of the Caribbean’s laughing gull. We are unaware of any other location in the world that contains the diversity and abundance of terns observed in San Nicolas Bay. For this reason BirdLife International (2008) has declared the Bay islands to be a globally important bird area (IBA).

The project was conducted in collaboration with the Aruba Veterinary Service, the Department of Agriculture, Husbandry and Fisheries, and the Government of Aruba, whose support is gratefully appreciated. We thank Valero Aruba Refinery for funding the study and also providing logistical and lodging support. We thank Caribe Alaska for their continued support and encouragement with this project. 

Abstract:

We assessed the breeding success of Red-billed Tropicbirds Phaethon aethereus on St. Eustatius, particularly in relation to predation at the nest. We conducted weekly surveys at five sites during 2012-2013 and measured chick and adult morphometrics. Apparent nest success ranged from 55- 100% across five breeding areas, while apparent fledge success ranged from 63-100% at those same locations. We used cameras and baited rat traps to document the presence of predators at nest sites. Predation rates captured on cameras were low (ca. 200 images of predators from ca. 263,000 images over 11 weeks). Cameras documented cats and rats at accessible nests. Although we could not confirm the cause of egg loss or the death of some chicks, the presence of cats and rats suggests that additional effort be expended to accurately measure their impact.