Red-billed Tropicbird

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be less-vulnerable to rats but cats have been unequivocally implicated in the depredation of tropicbird nests on Saba (unpublished data, Michiel Boeken).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