Red-billed Tropicbird

Monitoring the effect of cat removal on reproductive success in Red-billed Tropicbird colonies on Saba, 2013-2014: first season of results

One of the most deleterious invasive introduced predators worldwide is the domestic cat which has been found responsible for many island extinctions worldwide. Cats can live off both natural prey and garbage and can be a particularly serious threat to ground-nesting bird populations. Saba is an important location for the Red-billed Tropicbird, Phaethon aethereus and feral cats are thought to be the main reason for the low breeding success in the southern coastal colonies of this bird.

To make proper decisions in invasive predator management, information is needed on the effects of cat removal on the tropicbird breeding success and the possible resulting increase in egg predation by rats in the case of any “mesopredator release effects”. In this study we collected the first season of data needed to assess the effect of cat removal on the breeding success of the tropicbird on Saba. Two tropicbird nesting colonies (Great Level and Tent) were monitored in terms of egg and chick predation, under different net cat-removal intensities and the resulting survival was compared to survival prior to cat removal (as documented elsewhere).

In total, Saba Conservation Foundation removed 19 cats from the entire study area, of which eleven adult cats were removed from the Great Level colony and only six adult cats and two kittens were removed from the Tent colony. The gut contents of the 17 of these 19 feral cats consisted of natural prey (grasshoppers, rats, chickens, anoles and crickets), bait placed in the trap or even plant material. In the previous season 18 cats had already been removed (12 trapped, 6 shot) from the Great Level area.

During the period of September 2013 to May 2014, 46 occupied tropicbird nests were monitored, 27 at Tent, 15 at Great Level and 4 at Fort Bay. Fort Bay was not used in the data analysis. Egg-laying was documented in 34 of these nests. Observed egg failures were due to a variety of causes such as failure to hatch , broken eggs, including the breaking of an egg by an adult, and the disappearance of the whole nest due to heavy rainfall. Egg survival did not show a significant difference between the two colonies. In total 23 chicks were born, of which at least 15 died. Chick survival did show a significant difference between the two colonies, whereas prior to cat removal both had had zero chick survival. The breeding success of the tropicbirds and percentage of chicks fledged did appear to increase encouragingly in the breeding colony where cats had been more intensely culled (Great Level; 28 of initial 35 adult cats removed during two trapping seasons). The success on Great Level is notable, because in the breeding season of 2011/2012 the breeding success had been zero percent for several years.

Around the Tent colony only six adult cats were removed this season (total of 7 removed during two trapping seasons), which was insufficient to effectively increase breeding success in the tropicbird. A comparison of camera-trap densities showed that effective cat density at Tent by the end of trapping remained 4-5 times higher than at Great Level where 28 of the initial 35 adult cats had been removed. In total four black rats were observed on the camera traps but only appeared to be scavenging and no active egg predation was observed. These preliminary results suggest that cat removal seems to improve fledgling survival at no appreciable expense in terms of egg predation and that risks of any hypothetical “mesopredator release effects” are limited. Due to the low sample sizes in this first season, and natural fluctuations in breeding success which are normal in seabirds, clearly happenstance or other causative factors could equally explain the results obtained. Therefore, more definitive conclusions will depend on a more extensive and multi-year effort. 

 

Key recommendations:

  • Continue with and expand feral cat removal from the main tropicbird nesting colonies.
  • Simultaneously monitor nesting success and fledgling survival to develop a more robust data set over a longer time-frame. With an expanded sample size, the benefits in terms of net fledgling survival and any risks of potential “mesopredator release effects” can be more firmly assessed.
  • Many cats were documented to be wary of traps. Trapping was also very labour-intensive and entailed both trapping and handling stress. For these reasons additional, more effective yet humane methods (such as predator baiting or shooting) should be used. These methods have proven to be key to effective control of invasive predators worldwide.
  • As long as legislation and control of cat importation, keeping and sterilization remain less than strictly implemented and failsafe solutions remain wanting, we recommend to focus removal efforts towards key tropicbird nesting colonies shortly before or during the main nesting season each year.

    This research was funded as part of the Wageningen University BO research program (BO-11-011. 05- 029) under project number 4308701028 (A Debrot, PI). 

Date
2015
Data type
Research report
Report number
C103/15
Geographic location
Saba

Raw data of red-billed tropicbirds on St.Eustatius by STENAPA

Red-billed tropicbird peak attendance and nesting success on St.Eustatius. 

Please contact Hannah Madden for more information.

Date
2016
Data type
Raw data
Theme
Research and monitoring
Geographic location
St. Eustatius
Author

Assessment of the Breeding Success of Red-billed Tropicbirds on St. Eustatius

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

Date
2013
Data type
Other resources
Theme
Research and monitoring
Geographic location
St. Eustatius

Breeding success of Red-billed tropic birds at pilot hill, St.Eustatius - a follow up study (2013-2014)

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.

Date
2014
Data type
Other resources
Theme
Research and monitoring
Geographic location
St. Eustatius
Author

BioNews 10 - November 2013

This month’s issue highlights the Saba Bank Expedition that took place from 19 to 27 October. As a follow up to the Saba Bank survey in 2011, an international team of experts investigated the ecological functionality of the Bank, focussing on corals, fish, genetics, nutrients and dissolved organic carbon. The first glance at the results appears promising with coral cover increasing and algae cover decreasing, but final results will be published by next year.

The publication of a study on the island of Saba shows that invasive predators, such as feral cats, can have a large impact on breeding colonies of Red-billed Tropicbirds. Cat densities were investigated and varied largely between elevation levels. In some parts of the island, overall cat health turned out to be very poor. This already influenced the decision to stop releasing spayed/neutered cats back into the wild. Hopefully this will aid in decreasing the overall feral cat population and save the beautiful Red- billed Tropicbird, which has become an iconic symbol for the island of Saba.

Other content:

Date
2013
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Saba
Saba bank
St. Eustatius
St. Maarten
Author

Predation threats to the Red- billed Tropicbird breeding colony of Saba: focus on cats

Summary:

Feral domestic cats (Felis catus) are recognized as one of the most devastating alien predator species in the world and are a major threat to nesting colonies of the Red-billed Tropicbird (Phaethon aethereus), on Saba island, Dutch Caribbean. Cats and rats are both known to impact nesting seabirds and hence are both potential threats to the tropicbird on Saba. However, whereas the tropicbird has coexisted with rats for centuries, cats have only recently become a problem (since about 2000). Several studies from the region suggest that the tropicbird may be less-vulnerable to rats but cats have been unequivocally implicated in the depredation of tropicbird nests on Saba (unpublished data, Michiel Boeken). In this study we collected baseline data on cat and rat distribution, and cat diet and health. We also conducted 83 questionnaire interviews with Saba residents to assess their views on cats, rats, tropicbirds and the acceptability of different management options.

Two methods were used to assess cat density distribution. We used baited camera traps (73 successful 2-night deployments divided among 4 habitat categories) as an index of relative density. We also used scat densities (collected from 15,474 meters of transect from eleven trails) from which to extrapolate and compare relative cat densities in different habitats. For the study of diet we collected and analysed a total of 94 cat scats and studied the intestinal contents of 13 sacrificed feral cats.

Both scat densities and camera trap recordings showed large and statistically significant differences in cat density on trails between habitat zones. Cat densities were lowest in the lush forest habitat found at higher elevations (mean: 4 cats/km2) on the island and highest at lower coastal elevations. Densities were particularly high (mean: 286 cats/km2) in the small area surrounding the island landfill where food, consisting of human refuse and garbage was abundant. These densities are for areas along hiking- and goat-trails. These are actively selected by cats. Therefore, densities along trails cannot be simply extrapolated to the rest of the island.

While at the landfill daily incineration of garbage takes place, un-incinerated garbage is left open overnight almost every day. This provides feral cats with an ample food source. Construction of a vermin- proof, concrete overnight storage pen could greatly reduce food availability to cats and rats. Dry woodland and coastal scrub trail habitat had intermediate cat densities (respectively, 107 and 166, cat/km2). Rat density as documented using camera traps was highest in the forest habitat where food, water and shelter for rats was particularly abundant. Rat and cat density were markedly inversely related. Cats were concentrated at lower elevations and in more open areas where tropicbirds principally nest.

Diet analysis showed the feral cats on Saba are an opportunistic predator-scavenger consuming various categories of prey (mammals, birds, reptiles, and insects) as well as anthropogenic waste. Food composition of the feral cats of Saba differed significantly between habitats. Overall, rats were the most important food species of the cat, followed by reptiles and birds. However at the landfill where cat population densities were highest, birds and rats were strongly reduced in the diet, while garbage and reptiles were of greatly increased importance. Scats collected in the forest zone suggested a lower importance of rats in the diet of cats than at lower arid elevations. The cat scats collected in the urban environment had no prey species represented and were apparently all from well-fed house cats. The opportunistic and flexible food habits documented for cats in this study allows them to easily switch to seasonally abundant prey (for instance during the seabird breeding seasons).

Preliminary veterinary assessments on cats removed from the landfill showed the animals to be in overall poor health. This suggest that releasing neutered cats back into the wild without any further supporting care may be much less humane than typically assumed. Based on these results and taking into account the welfare concerns of the tropicbirds preyed upon by cats, the Saba Foundation for Prevention of Cruelty to Animals (SFPCA) has decided to discontinue its practice of releasing neutered unwanted cats into the wild (Trap-Neuter-Release, or TNR).

Public views regarding cats, tropicbirds and management options as assessed using a simple questionnaire and 83 questionnaire returns were obtained. Around 30% of the participants owned cats of which only about 5% were not neutered. A significant majority of respondents (66%) believed feral cats on Saba are an environmental problem. In this there was no difference between natives and expat residents. Most (44%) believed that cats were principally a problem in being a threat to wildlife. Diseases and parasites was quoted as the second-most perceived problem with feral cats (30%). Feral cats and rats scored as the two most important perceived threats to the tropicbird (respectively, 65% and 70%). When asked “how many saved tropicbirds do you think justifies the death of one cat”, 38% of Sabans and 22% of expats did not answer the question. Of those that did answer, 36% of Sabans and 48% of expats, valued the life of a tropicbird more than that of a single cat. The remainder considered cats somewhat more important than one tropicbirds but only few (5% natives, 18% expats) considered feral cats more important than the combined sum of all their tropicbird prey. Between 70-80% of respondents thought registration, neutering and removal of cats from breeding colonies was a good idea. When asked if euthanization would be acceptable to them, a significant majority (80%) found it to be an acceptable method for use in cat control. Finally, 43% even thought that total eradication of all cats (domestic and feral) from the island would be a good idea. Of the participants upwards of 80% stated that rats were also an environmental problem and more measures to control rats are supported by 75% of those interviewed. Awareness and willingness of the Saban resident population towards measures against cats and rats are clearly high. This means that there exists a wide management scope to implement measures with which to address these problems.

For more than 15 years government rat control has used brodifacoum as the main rodenticide, but rats remain a widespread and unrelenting problem on Saba. This suggests that the rats might well have become partly resistant to this anticoagulant toxin and that the time has come to alternate to a different rodenticide. Alternating use of rodenticides is the internationally recommended practice for rat control. It is already being practiced on nearby St. Eustatius where rat problems are much less acute than on Saba (but where feral fruit trees are also less abundant and landfill practices are also less favourable to vermin).

Key management recommendations:

  • Upgrade the 2004 Saba “Island Ordinance on Identification and Registration of Livestock and Domestic Animals” to prohibit the importation and keeping of unneutered cats.
  • Capacitate the SFPCA to enforce the mandatory registration of domestic animals.
  • Construct a vermin-proof concrete overnight pen at the Saba landfill (for secure storage of the garbage that might not be incinerated the same day).
  • From now on euthanize all unwanted and uncared-for stray and feral nuisance cats.
  • Judiciously use humane euthanization to address the acute overpopulation of cats in particularly sensitive areas (such as the mapped seabird colonies) as this is quite acceptable to Saban residents.
  • Start use of a new alternative rodenticide, following the example from St. Eustatius.
  • Management measures need to be accompanied by an (inter)active and effective communication plan to keep public support levels high.

Key recommendations for research:

  • Investigate the role of rats as predators, not only with respect to seabirds but also in the forested zone where they are most abundant and may seriously impact native forest species.
  • Assess the effect of cat removal on tropicbird breeding success and on rat population density, as well as the broader predator-prey relationships on the island. 
Date
2014
Data type
Research report
Theme
Research and monitoring
Report number
C011/14
Geographic location
Saba

Interactions between invasive mammals and their effects on Red-billed Tropicbird (Phaethon aethereus) nesting productivity

Abstract:

Red-billed Tropicbirds (Phaethon aethereus) are cavity-nesting seabirds with important nesting colonies on Saba and Sint Eustatius in the Dutch Caribbean. Breeding productivity has been steadily declining and nest failure is primarily attributed to the presence of invasive mammalian predators. Camera traps were deployed to determine the extent of invasive species presence and impact. Cats, rats, and goats as well as native land crabs and lizards were present on both islands and actively visiting nest sites. The positive relationship between goat visitation and cat visitation suggests that goats may be facilitating cat predation by increasing visibility and accessibility to nests. Disturbance from goats may be causing nest abandonment by breeding adults, which warrants further study. A significant proportion of nests surveyed (n=106) were failures (n=71). It can be inferred that invasive mammalian presence at those sites is contributing to nest failure. There is not enough evidence to draw any conclusions on the possible impact of native predators. Further and more extensive monitoring is recommended to better quantify impacts and invasive species behaviour for eradication programme design. 

Date
2013
Data type
Research report
Theme
Research and monitoring
Geographic location
Saba
Author

BioNews 4 - April 2013

This month’s issue focuses on the conservation efforts for the Red-billed Tropicbird, which is taking place on the islands of Saba and St. Eustatius. Preliminary studies last year indicated that at two study sites on Saba breeding success was zero. One of the prime culprits appears to be feral cats. All the more reason to bring this species to the spotlight and ask your attention for the plight of this charismatic and locally endangered sea bird.

Amongst others, you will find in this fourth issue:

Date
2013
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Curacao
Saba
St. Eustatius
St. Maarten
Author

Assessment of the Breeding Success of Red-billed Tropicbirds on St. Eustatius

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. 

Date
2013
Data type
Research report
Theme
Research and monitoring
Geographic location
St. Eustatius

Monitoring Tropicbirds, an introductory guide

This manual emphasises on practical methods, and some methods which (though highly accurate) are too labour-intensive or time-consuming for general monitoring use have been omitted. The methods presented in this manual deal only with assessment of population sizes, population changes, and the numbers of chicks produced by breeding pairs. It is also important to monitor other population parameters such as adult survival rates, diet, rate of food-delivery to chicks, or growth- rates of chicks. However, the methods required for monitoring survival rates, in particular, are labour-intensive for widespread use and are thus beyond the scope of this document. Some other measures and suggestions worth considering have been provided, but these largely go beyond monitoring and into the realm of research.

Please use this document as a guide, but remember there has been little published work on tropicbird species. Consequently, the opportunity to find new and important things about their biology, population ecology and conservation awaits you. 

Date
2010
Data type
Monitoring protocol
Theme
Education and outreach
Research and monitoring
Geographic location
Saba
St. Eustatius
Author