Year-round use of anthropogenic food sources in human modified landscapes by adult and young Kelp Gulls

Abstract

Predictable anthropogenic food subsidies attract species with generalist and opportunistic feeding habits, often resulting in conflicts with human populations. We assessed the spatio-temporal distribution and abundance of Kelp Gulls (Larus dominicanus) during the annual cycle at anthropogenic food sources located along 70 km of urban and agricultural-livestock landscapes in the lower Chubut River valley, Argentina. We quantified the seasonal abundance of adult and young Kelp Gulls through monthly counts from July 2021 to June 2022 at six identified anthropogenic food sources, complemented with strip transect sampling along cultivated land and cattle grazing areas. In addition, we analysed the differential use of waste types by adult and young Kelp Gulls at a mixed livestock waste dump where different food remains (cattle remains, poultry remains and urban waste) are disposed in independent pits. The total number of Kelp Gulls counted each month along the river valley was variable, with a mean number of 2585 ± 822.7 individuals (range = 276 in December and 8958 in June). The highest gull abundance was recorded at a pig farm (mean = 1784.5 ± 640.1 individuals). The transect survey showed a relatively low use by gulls of the cultivated land and cattle grazing areas, with a mean of 29.7 ± 11.2 individuals recorded per survey (range = 0–96). Kelp Gull abundance patterns recorded in the river valley throughout the annual cycle evidenced a contrasting seasonal use of anthropogenic food sources between the breeding and non-breeding seasons, being clearly less abundant during the former, when gulls move to their main breeding grounds and adjacent marine habitats in coastal Chubut. At the mixed livestock waste dump, Kelp Gull numbers varied among the three waste patches, with higher numbers and a significantly higher proportion of adults at the cattle remains pit. This study shows the high trophic plasticity of Kelp Gulls and their use of alternative foraging habitats. Further monitoring and evaluations of the use by Kelp Gulls of predictable anthropogenic food sources along the Chubut River valley, particularly those related to the growing livestock production, are needed to support management decisions.