Effect of urbanization on the individual condition of the regionally threatened Olrog`s gull (Larus atlanticus)

Abstract

Extinction of species and population declines are occurring globally as a consequence of human activities, including the occurrence of urbanization processes transforming the structure of natural habitats and exposing organisms to novel environmental challenges. That is the current scenario in the northern coasts of Argentina, as well as Uruguay and Southern Brazil, regularly used as winter quarters by the Olrog's gull (Larus atlanticus). This regionally threatened species was formerly known to be specialized in crabs but now to have incorporated a range of anthropogenic items in its diet. As part of an ongoing long-term study on the foraging ecology and health status of the Olrog's gull we investigated the impact of urbanization on the body condition of free-living individuals, measuring a range of blood parameters as indicators of body condition. Eighty-two gulls (46 juveniles and 36 adults) were captured at two sites with radically different levels of urbanization. Body weight, and levels of hematocrit, red and white blood cells, mean cell volume, heterophils, lymphocytes, eosinophils, monocytes, basophils, heterophils-lymphocytes ratio, glucose, uric acid, total protein, cholesterol, and triglyceride were determined. Most of parameters didn't show significant differences between the two populations. However, independently of sex and age, individuals from areas with low urbanization showed higher values of uric acid (1052.53±452.35 μmol/l), and lower levels of cholesterol (6.47±1.76 mmol/l) than individuals from highly impacted areas (455.08±305.69 μmol/l and 8.35±3.07 mmol/l, respectively). Adults from areas with low urbanization showed lower values of hematocrit than adults from impacted areas (39.14±7.47% and 44.00±4.68%, respectively). These differences in blood parameters could be considered as proxy indicators of health condition in Olrog's gulls exposed to urbanization in winter quarters. Further studies should focus on the effect of individual condition on the species recruitment and breeding success, taking into account the endemism, reduced population size and fragile conservation status of the species.