Effects of decreased anthropogenic food availability on an opportunistic gull: evidence for a size‐mediated response in breeding females

Abstract

Some opportunistic vertebrates exploit, and may largely rely upon, food generated by human activities. Better understanding the influence of this additional anthropogenic food on species' ecology would inform sustainable waste management. In the Balearic Archipelago of Spain, closure of an open‐air landfill site provided an experimental setting to measure the effect of removing anthropogenic food on the average body mass, breeding parameters and body condition of opportunistic Yellow‐legged Gulls Larus michahellis. After landfill closure there was a significant decline in the average body mass of breeding females and males (−10.4 and −7.8%, respectively), in average egg volume (−4.8%), and a shift in the modal clutch size from three to two eggs. Body condition decreased after landfill closure in both sexes. In breeding females, the drop in body weight was greater for birds with a low body size index. The differential response to a reduction of anthropogenic food between small and large birds suggests that food of anthropogenic origin contributes to tempering the effects of natural selection, making the long‐term demographic effects of changes in food supply difficult to predict.

Population control of an overabundant species achieved through consecutive anthropogenic perturbations

“…Nevertheless, when food is in very short supply it can even affect body mass and local adult survival in long-lived seabirds (Oro and Furness 2002). This appears to be the case for our population, given the recorded drop in YLG body condition (Steigerwald et al 2015) along with a slight decrease in local survival rates after landfill closure. Gulls usually form spatially structured populations and given that previous studies have shown that dispersal occurs following environmental perturbations (Oro et al 2004, Ferna´ndez-Chaco´n et al 2013, we suggest that food limitation has triggered not only mortality but also permanent emigration (through breeding dispersal) to neighboring populations (Bosch 2000, Oro 2003.…”

“…Previous studies on gulls found severe impacts on breeding performance under different levels of food availability (Pons 1992, Pons and Migot 1995, Oro 1996. Particularly, our colony suffered a significant decrease in breeding performance (clutch size and egg volume) after landfill closure (Steigerwald et al 2015). Moreover, Steigerwald et al (2015) suggested that a dietary switch, which we confirmed, might not have been sufficient to fully cover the energy requirements of the local population.…”

“…Particularly, our colony suffered a significant decrease in breeding performance (clutch size and egg volume) after landfill closure (Steigerwald et al 2015). Moreover, Steigerwald et al (2015) suggested that a dietary switch, which we confirmed, might not have been sufficient to fully cover the energy requirements of the local population. Foregoing reproduction to guarantee adult survival acts as a buffer mechanism against environmental variability and allows animals to cope with moderate environmental perturbations.…”

“…Breeding adults were trapped during the incubation period using a tent spring trap (see Steigerwald et al 2015). We chose to trap individuals in accessible nests along the main south path of the island (4.5 km long) to increase re-sighting probabilities in the following years.…”

“…Life-history theory predicts that under limited food access, long-lived organisms will forego reproduction to avoid negatively affecting their future survival (Pugesek andDiem 1990, Oro et al 1999). Thus, we expected that food limitation would trigger a dietary shift and differential resource exploitation, affecting breeding performance (Oro et al 1995, Steigerwald et al 2015 but not individual survival probability. Moreover, we expected that the inadvertent poisoning event would cause an increase in mortality rate due to the accidental poisoning of individuals.…”