Effects of forest fragmentation on morphological traits and fluctuating asymmetry of four understory bird species

Abstract Rapid changes in natural environments can affect habitat quality for species, leading to accelerated phenotypic alterations. These changes may occur through natural selection and environmental effects, influencing multiple ecological groups. One method to detect such alterations is the analysis of fluctuating asymmetry and other non-bilateral traits. In this study, we tested whether Atlantic Forest fragmentation influences morphological diversity and fluctuating asymmetry in four understory bird species: Automolus leucophthalmus , Chiroxiphia caudata , Conopophaga lineata , and Tachyphonus coronatus . We fitted linear regression models using landscape metrics (forest cover and functional connectivity) as predictors and fluctuating asymmetry as the response variable. We proposed two hypotheses regarding asymmetry: (1) greater forest cover reduces fluctuating asymmetry, and (2) greater functional connectivity reduces fluctuating asymmetry. We also formulated hypotheses for beak morphology: (1) in highly fragmented landscapes, smaller and more versatile beaks may be favored because they allow individuals to exploit a wider variety of food resources available in degraded habitats; and (2) in less fragmented landscapes, larger beaks may be advantageous because they enable the exploitation of more specific resources that are more abundant in continuous habitats. For Conopophaga lineata , we found significant relationships between forest cover and morphology: individuals in more forested landscapes had smaller beaks and lower tarsus asymmetry. This pattern contrasts with our initial prediction, as we expected larger beaks to be favored in less fragmented areas. No significant relationships with landscape variables were found for the other species. The tarsus is essential for locomotion and ecological performance, and its asymmetry may reflect environmental stress. Our findings suggest that habitat loss may drive rapid evolutionary changes in passerine birds, although this response was detected in only one of the four species studied, and its implications for long-term resilience remain uncertain.