THE ECONOMICS OF GETTING HIGH: DECISIONS MADE BY COMMON GULLS DROPPING COCKLES TO OPEN THEM

Common gulls drop cockles onto hard sand to open them. Gulls dropped cockles 2.9 ± 0.36 times to open them, from a mean height of 24.68 ± 1.74 m. Drop heights decreased significantly during a dropping sequence. Opened cockles had a mean shell-length of 32.22 ± 0.59 mm, which was similar to the size of available cockles. An economic model showed that gulls should drop cockles of average size twice from a mean height of 31.22 m to maximise their net rate of energy gain. The model showed that intake rates were similar for two-drop strategies with a mean drop height of 31.22 m, irrespective of the distribution of drop heights within the sequence, provided the first drop was less than the height at which cockles reached terminal velocity. The combination of number of drops and drop heights varied between cockle sizes, but model results were general - the optimum economic dropping strategy for a given size consisted of a number of drops from a constant height. Model predictions were, therefore, significantly different from observed behaviour. A risk of kleptoparasitism was added to the economic model by assuming gulls lost their cockle to a parasite if it was dropped above a threshold height immediately prior to opening. This showed that the number of drops in a sequence increased and mean drop height decreased compared with the average economic optimum. Drop heights within a sequence were also predicted to decrease in the presence of kleptoparasitism because the final drop was constrained to be lower than the height at which kleptoparasitism occurs. Previous drops in the sequence have to be increased in height as a result to ensure the cockle experiences sufficient impact energy to open. The inclusion of a risk of kleptoparasitism, therefore, increased the correspondence between model predictions and observed behaviour. The role of kleptoparasitism in decision making by gulls in the wild is discussed.