Trophic interactions during chick rearing between Kelp Gulls and Cayenne and Royal Terns

Terns are one of the most affected groups by gulls, through predation, kleptoparasitism and competition for nesting sites. These interactions have been reported in Patagonia, where Cayenne and Royal terns (Thalasseus sandvicensis eurygnathus and T. maximus, respectively) nest in mixed-species colonies with the Kelp Gull (Larus dominicanus). However, trophic interactions between these species have not been yet evaluated. During 2014, we assessed their diet during the chick stage at the Bahía San Blas Protected area, through the analysis of Kelp gull chick stomach samples (n=66) and direct observation of prey deliveries (DO) in Cayenne and Royal terns (n=1747 and 1014, respectively), in addition to stable isotope analysis (SIA) of chick whole blood (n=10 per species). Engraulis anchoita was the main prey shared by the three species. In both tern species, this was one of the three most frequent prey (~30% DO; ~ 50% SIA), together with two silverside species, while in the Kelp Gull it showed a frequency of ~35% (both methods) followed in similar frequencies by Cynoscion guatucupa derived from recreational fishery waste. Both analysis indicated that the overlap between the trophic niches of Kelp Gulls and each tern species was low (Czekanowski Index<0,20, in both cases; SEAc<10%, in both cases). The trophic overlap does not seem to be relevant, but the interaction between these species should be assessed particularly in scenarios where anthropogenic resources are not available for the Kelp Gull.