Morphological, Genetic, and Ecological Insights into the Puerto Rican Tody: Investigating Variation Between and Within Dry Forest and Rainforest Populations

Abstract

Environmental variation plays an important role in shaping how species persist and adapt, particularly on islands—isolated areas where ecological conditions can vary dramatically over short distances, and anthropogenic pressures are often intensified. These factors contribute to conditions where bird species typically have small populations and limited genetic diversity, resulting in a reduced capacity to adapt to environmental changes compared to their mainland counterparts. This dissertation examines how both spatial and temporal environmental variation influences populations of a small, endemic island bird, the Puerto Rican Tody (Todus mexicanus), in Guánica Dry Forest and El Yunque Rainforest. Although not currently threatened, the species faces increasing challenges from climate change, forest fragmentation, and invasive predators, making it a valuable system for understanding how environmental change shapes island birds. Incorporating morphology, population genetics, disease ecology, and breeding ecology, this research provides an integrated view of how tody populations vary between and within dry forest and rainforest habitats, and how they may respond to future environmental changes. The first study focuses on long-term morphological change in the dry forest population spanning nearly 50 years. Absolute bill surface area significantly increased over time in correlation with rising temperatures, consistent with Allen's rule, which predicts larger appendages in warmer climates to enhance heat dissipation. In contrast, tarsus length remained unchanged. When examining overall body size, body mass did not decline as expected under Bergmann's rule, contrasting established geographic patterns in the species, where todies in the hotter dry forest are smaller than those in the cooler rainforest. The second study uses whole- genome sequencing to examine genetic structure, genetic diversity, and demographic history across dry forest and rainforest populations. Despite observed phenotypic differences in morphology, physiology, and behavior between habitats, results revealed only subtle population structure and low overall genetic differentiation, suggesting that trait variation may be driven by phenotypic plasticity. Historic demographic analyses indicated that both populations experienced long-term bottlenecks, which if continued could reduce evolutionary potential and increase vulnerability to future environmental change. The third study examines the prevalence, distribution, and potential impacts of the turkey chigger (Neoschoengastia americana), a previously undocumented ectoparasite in Puerto Rico. Mites were found exclusively on todies in the dry forest, with over half of individuals parasitized, while no mites were observed in the rainforest population. The parasite was widespread across the dry forest, though some areas consistently had mite-free individuals across the two-year period. Despite high prevalence in the dry forest, mite presence did not significantly affect body condition as measured by scaled mass indices. The last study evaluates how the small Indian mongoose (Urva auropunctata), an invasive predator, has shaped nest predation and breeding behavior among the rainforest todies over a 53-year period. While historical data showed high rates of mongoose nest predation, recent observations from our study revealed a significant decline in nest predation, as well as behavioral changes—notably the construction of nest burrows higher up on dirt banks, a behavior previously linked to reduced predation risk. The decline in mongoose predation may reflect shifts in mongoose abundance due to management efforts, changes in tody nesting behavior, or broader ecological factors. Continued monitoring and management of invasive predators remain essential for the long-term conservation of native birds in Puerto Rico. Together, these studies provide a comprehensive view of how environmental variation across space and time affects an endemic island bird. The findings emphasize the importance of long-term, integrative research across different habitat types in understanding species responses to environmental change. While the Puerto Rican Tody appears resilient in some respects, ongoing environmental pressures present continued challenges, particularly for the dry forest population, which is experiencing rising temperatures in an already hot, arid climate, and the spread of novel parasites with unknown long-term impacts. Moreover, the findings of this work have broader relevance for understanding and conserving other endemic island bird species.