Browsing by Author "Kastner, Martin"

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    Ecology and conservation of the Såli (Micronesian Starling, Aplonis opaca) in the presence of the invasive Brown Treesnake (Boiga irregularis) in Guam, Mariana Islands
    Kastner, Martin (Virginia Tech, 2024-10-21)
    The world's islands are of singular importance from a conservation perspective because they hold a disproportionate share of global biodiversity. However, island ecosystems are fragile, and the introduction of invasive species poses an especially grave threat to their species. My research took place on the island of Guam, which has been particularly hard hit by invasive species in recent history. The introduction of the invasive Brown Treesnake (BTS; Boiga irregularis) to Guam in the mid-20th century caused the extinction of the vast majority of its avifauna. Of the remaining bird species, the native Såli (Aplonis opaca) is a forest starling now limited to breeding and roosting in certain urbanized areas on the island. Previous research has shown that although Såli adults appear to have high survival, their fledglings are highly vulnerable to predators. My research focused on refining our understanding of how BTS are impacting Såli ecology and survival, and how BTS management can benefit Såli conservation. In my first chapter, I conducted the first field study to investigate the influence of both prey and predator sizes on the frequency of ingestion success by snakes. I found that nearly half of ingestion attempts by BTS on fledgling Såli were unsuccessful, and that BTS routinely pushed the upper limits of their capacity to swallow prey. These results suggest that relatively small BTS pose a greater predation risk to birds than previously recognized, and broaden the size range of BTS that should be targeted in management efforts for avian conservation. In my second chapter, I investigated a potential life-history trade-off for Såli between the proximity of nest sites to foraging areas and the risk of fledgling predation by BTS. I found that Såli nesting closer to the forest make more yearly nesting attempts, but that fledgling survival was higher the farther their nest site was from the forest. Såli therefore balance potential fecundity with predation risk to fledglings in their selection of nest sites. In my third chapter, I examined the survival response of fledgling Såli to the suppression of BTS. I found that snake control did not lead to a change in fledgling survival from the pre-treatment baseline or in comparison to areas where snakes were not controlled, and that there was no shift from snake predation to other causes of mortality. Changes to the strategies or tools employed will therefore be required in order for future BTS control efforts within similar habitats to effectively improve avian survival. This body of work demonstrates that BTS are affecting Såli in unexpected ways, and offers new guidance on how to manage damaging invasive species for native species conservation.
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    Gape-limited invasive predator frequently kills avian prey that are too large to swallow
    Kastner, Martin; Goetz, Scott M.; Baker, Kayla M.; Siers, Shane R.; Paxton, Eben H.; Nafus, Melia G.; Rogers, Haldre S. (Wiley, 2024-07-25)
    Gape-limited predators (e.g., snakes, many fish) are not generally expected to pose a predation threat to prey that are too large for them to swallow. However, the extent to which snakes predate on prey that exceed their gape limitation remains largely unknown. We conducted the first study to investigate the influence of both prey and predator sizes on the frequency of ingestion success by snakes in a natural system. We combined survival monitoring of an avian prey species (Aplonis opaca) via radio-telemetry with a survey of the size distribution of their major predator (Boiga irregularis) on Guam. This allowed us to assess (1) the frequency of unsuccessful ingestion by the predator, (2) whether the size of the prey predicts ingestion success, (3) whether the size of the predator predicts ingestion success, and (4) the relationship between prey and predator sizes in successful ingestion attempts. We found that nearly half (47.95%) of ingestion attempts by snakes on fledgling birds were unsuccessful, and no instances where unsuccessful ingestion caused the mortality of the snake. Attempts to consume smaller fledglings were as likely to be unsuccessful as attempts to swallow larger fledglings. However, snakes that successfully ingested fledglings were among the largest snakes in the population, and larger than average conspecifics attracted to endothermic prey. The smallest snakes that successfully ingested fledglings attained remarkably high relative prey mass values for their species, consuming prey weighing up to 79.9% of their own mass. Our study indicates that B. irregularis routinely predate prey that are too large for them to successfully ingest, which causes mortality to the prey but poses little risk to the predator. The potential reward for snakes in consuming oversized prey may outweigh the inherent risks, while instances of predation that do not result in consumption may have considerable impacts on prey populations.