Positive association between rat abundance and breeding success of the Critically Endangered Mariana Crow Corvus kubaryi

Abstract

SummaryDiagnosing the causes of population declines of highly endangered species is vital to ensure that appropriate conservation measures are undertaken. Although experiments can provide critical information on potential causes, these can be time consuming and costly, and so it is important to determine the most plausible hypothesis for such tests. The last wild population of the Critically Endangered Mariana Crow Corvus kubaryi has been declining rapidly on Rota in the Mariana Islands. Because nest predation by introduced rats Rattus spp. has been implicated in the declines of other Pacific island birds, we tested for a correlation between breeding success of Mariana Crows and abundance of rats R. diardii surrounding their nest sites. If rats are a contributing factor in this species’s decline, we predicted that breeding success would be lower in areas where rats were more abundant. In complete contrast to this prediction, we found that breeding success was significantly higher where rats were most abundant, suggesting that rats are unlikely to be a major driving force behind the decline in this species. The positive association between crow breeding success and rat abundance suggests these species may be responding to similar habitat needs or a shared predator. This study illustrates the importance of gathering sufficient knowledge on the ecology of a species prior to undertaking costly, logistically difficult and time consuming experiments.

Population status and nest success of the Critically Endangered Mariana CrowCorvus kubaryion Rota, Northern Mariana Islands

“…Nest filming showed crows and starlings consuming the nest contents, although the sample size was small. Another study has revealed that high abundance of introduced rats around Mariana Crow nests did not decrease breeding success, suggesting that rats may not be important nest predators for this species (Amar and Esselstyn 2013.). Indeed Amar and Esselstyn (2013) found a significant positive relationship between rat abundance and Mariana Crow breeding success; it was proposed that this relationship may be due to the abundance of a shared predator, the feral cat.…”

“…Another study has revealed that high abundance of introduced rats around Mariana Crow nests did not decrease breeding success, suggesting that rats may not be important nest predators for this species (Amar and Esselstyn 2013.). Indeed Amar and Esselstyn (2013) found a significant positive relationship between rat abundance and Mariana Crow breeding success; it was proposed that this relationship may be due to the abundance of a shared predator, the feral cat. Predation by feral cats has also been proposed for the widespread declines of other bird species on Rota (Amar et al 2008).…”

Invasive rats strengthen predation pressure on bird eggs in a South Pacific island rainforest

“…In Hawaii, rodent control decreased predation on artificial and real nests placed on trees and the ground to mimic O’ahu ‘Elepaio ( Chasiempsis sandwichensis ibidis ) nests by 45% and 55%, respectively ( VanderWerf 2001 ). However, the nature of rat impacts on birds is not always clear, and in some cases, bird populations may remain unaffected ( Amar and Esselstyn 2014 ; Hervías et al 2014 ). It is important to remember that the impacts of rats can vary according to ecological context (bird species, habitat, presence of other invasive predators, etc.…”