Estimating feral cats density on Corvo island (Azores)

Oppel, S., , Hervias, S., Oliveira, N., Pipa, T., Cowen, H., Silva, C., Geraldes, P., 2012 Estimating feral cat density on Corvo island, Azores, to assess the feasability of feral cat eradication. Airo, 22: 3-11.

Feral cats have had negative effects on native biodiversity on many islands worldwide. Eradicating feral cats from islands is often feasible, and can yield great benefits to native biodiversity, especially for seabirds. Corvo Island (Azores) is an important island where feral cats limit the distribution and abundance of breeding seabirds. To assess whether the eradication of feral cats on Corvo would be feasible we used camera traps to estimate the density of feral cats. We deployed 24 camera traps at 253 locations around the island for 14 months, and identified cats detected by camera traps individually based on the coat colour. We then used spatially explicit capturere-capture models to estimate cat density for Corvo. Cat density in the uninhabited upland part of Corvo, which is dominated by cow pastures, was 0.036 (95% CI 0.025–0.054) cats/ha. The lowland part of Corvo, which is inhabited by humans and contains domestic cats, had an estimated cat density of 0.734 (0.581 – 0.927) cats/ha. Overall, we estimated that the cat population on Corvo during our study period included 163 (123 - 228) individuals. The estimated cat densities are within the range of cat densities from other islands where cats have been successfully eradicated, and we conclude that feral cat eradication on Corvo would be technically feasible. However, the co-existence of feral and domestic cats would create operational challenges, and the current lack of a legal framework to ensure that all domestic cats are sterilised would increase the risk of a feral cat population becoming re-established after eradication.

Impact of alien predators on seabirds on Corvo island (Azores)

Henriques, A. C. M. (2010). Impacto dos predadores introduzidos na ilha do Corvo no sucesso reprodutor das populações de cagarro (Calonectris diomedea borealis) (Doctoral dissertation, Universidade Técnica de Lisboa) (in Portuguese with English abstracts).

In the Corvo island, Azores, there are important species of seabirds classified as prioritary by the Annex I of the Bird Directive, for example Cory’s Shearwater Calonectris diomedea borealis, is one of the species that represents in terms of number and density the biggest expression in this archipelago in an European level. One of the major threats for the birds that reproduce in islands is the presence of predators, like rats, so, in this context, we analysed the variables that could explain the distribution of rodents, identified the accessible places of nesting of Cory’s, monitored the reproductive season and tried to understand the causes of fail in the egg and chick phase. The results point out a positive relation between rat abundance and reproductive success of Cory’s. The characteristics of the reproductive season are similar to those described for Berlengas and Selvagens, but they differ in the hatching success and chick success. We verified that in the first days of life of the chicks the predation is very significant and other variables should be taken into account, for example the predation by cats.

The Corvo Island has unique features, in terms of endemic fauna and flora, numbers of human population, introduced predators, size of the island, among others, that gave to this island the status of Biosphere Reserve. Additionally, there are important species of seabirds that only reproduce in restricted areas of Europe, and are classified as prioritary by Annex I of the Birds Directive. Nevertheless, the populations of some species are still to confirm (e.g Pterodroma feae, Oceanodroma castro) and others need more research in terms of numbers and localization of the colonies (e.g Puffinus puffinus, Puffinus assimilis). 

The more conspicuous of the seabirds here present is Cory’s Shearwater, Calonectris diomedea borealis and has the biggest expression in terms of numbers and densities in the Azores archipelago which represents 60-70% of the world population. The Corvo population is one of the major ones but the nidification areas and the actual size of the population is still not well known. One of the major causes of the decline of the seabirds, besides the fisheries, specially those ones who reproduce in cavities on the soil, are the introduced predators in islands, places that originally yield millions of seabirds. The biggest problem is that they evolved in environments that were predator free and lack defense mechanisms to face the predation of eggs and chicks by rats and cats. This causes the reproductive collapse in some species and could force the nesting places to sub-optimal conditions, that can compromise the viability of the populations. Last year, SPEA started a LIFE+ project “Safe islands for seabirds” to assess the impacts of the predators on seabirds and prepare the restoration of habitat, by ways of predator and invasive plants control and reforestation with endemic species. This project has an important ecological relevance as it can represent a important step in the developing of protocols for exotic species eradication in islands with human population, with both cats and rats as the major predators. 

The objectives of this thesis were to quantify the influence of rodents in the reproductive success of Cory’s Shearwater. We analyse the distribution of three species of rodents (Rattus rattus, R. norvegicus and Mus musculus) in different habitats and their dependence with some environmental variables. The method chosen was by means of tracking tunnels in two different seasons, one in April-May and the other in September-October, that wanted to express different food availability and different stages of the reproductive period of Cory’s in Corvo. For this analysis we decide do use an algorithmic model Random Forest for the software R 2.9.2. Our results point out the dependence of the presence of rodents with introduced anthropogenic characteristics as roads, exotic vegetation and distance to village. Also we noticed some differences between the house mice and the rats in terms of abundance and selection of habitat. 

We were able to identify different accessible colonies of Cory’s Shearwater in the island, and monitor almost 200 nests, once a week, in eight different places of Corvo. We analysed and compared times of hatching, fledging, incubation period and breeding success, with other islands (Berlenga, Selvagem Grande and others from Azores, such as Graciosa) where Cory’s shearwater reproduces, and finally we tried to understand which are the main causes of failure using the model Nest Survival for the program MARK. This program is an important ecological tool for testing different factors and the correlation between them that could influence the weekly nest survival. Also give us the weight of the model for explaining the data. 

Our results point out the synchronization of times of hatching and fledging among the different places in the Atlantic, but there’s a major difference in the hatching and fledging successes. Surprisingly the causes of failure don’t seem to be related with rat abundance, as the majority of the studies point out, ant the time with less survival probability seems to coincide with the first days after hatching when the chick is more vulnerable. Our assumption is that there is a third variable that wasn’t account for, that is influencing the abundance of rats and the reproductive success of Cory’s. This variable is the presence of cats. 

The analyse of the Life Table demonstrate the big resilience in fluctuations of the population of Cory’s Shearwater in Corvo Island, a characteristic of long lived species. 

We recommend that for the planning of the eradication protocol both cats and rats should be target of control at the same time, incorporate prioritary areas of control for rodents and to study more accurately the causes of fail that influences the seabirds success. 

Multiple predators on Corvo island

Hervías, S., A. Henriques, N. Oliveira, T. Pipa, H. Cowen, J. A. Ramos, M. Nogales, P. Geraldes, C. Silva, R. Ruiz de Ybáñez & S. Oppel. 2012. Studying the effects of multiple invasive mammals on Cory’s shearwater nest survival. Biological Invasions, DOI 10.1007/s10530-012-0274-1

The most common invasive mammals—mice, rats, and cats—have been introduced to islands around the world, where they continue to negatively affect native biodiversity. The eradication of those invasive mammals has had positive effects on many species of seabirds. However, the removal of one invasive mammal species may result in abundance changes of other species due to trophic and competitive interactions among species. Understanding the overall impact of several invasive species is a key challenge when evaluating the possible effects of eradication programmes. Here we assess the influence of the three most common invasive mammals on nest survival of Cory’s shearwater (Calonectris diomedea). We monitored six breeding colonies over 3 years and measured the activity of mice, rats and cats to examine the influence of invasive mammals on nest survival. We found that nest survival showed a similar temporal trend in all years, with lowest weekly survival probabilities shortly after chicks hatched.Cats were identified as major predators of chicks, but no measure of colony-specific cat activity was able to adequately explain variation in shearwater nest survival. Nest survival was on average 0.38 (95 % confidence interval 0.20–0.53) and varied among colonies as well as over time. We found a small positive influence of rats on nest survival, which may indicate that the presence of small rodents as alternative prey may reduce cat predation of chicks. Our findings suggest that the eradication of rodents alone may exacerbate the adverse effects of cats on shearwater nest survival.