Seagull predation patterns: what, when, and why?



At low tide, many seagulls prey on bivalves (clams, cockles, etc.), gastropods (snails), and hermit crabs living in gastropod shells. The seagulls can't pry open or crack the shells with their beaks, so instead they carry them up in the air and then drop them from a height onto a hard surface. The shells break open on impact, then the seagulls swoop down to eat the goopy innards.

I often take a casual look around the paved areas at work (the Duke University Marine Lab) to see what shells the local seagulls have dropped on the ground. The shell fragments are often quite large and easily identifiable, but I can also identify quite small fragments, since I spend an awful lot of time looking at shells. Probably the most common fragments are from the clam Mercenaria mercenaria, but fragments of whelks (Busycon spp.) and moon snails (Polinices duplicatus*) are also very common. (Remember that some of the snail shells contained hermit crabs at the time of seagull predation.) A smattering of other mollusc species can also be identified.

In addition to the parking lot, seagulls also like to drop shells on the paved bridge leading to Pivers Island and on the DUML cement dock. I have also occasionally seen seagulls drop shells on hard-packed sand/muddy sand at low tide when I'm out in the field.

*I think that's the right name—I'll have to check

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Some of the shell fragments are actually whole or nearly-whole valves. (image ©Yasmin von Dassow)

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Close-up of of shell fragment assemblage. A large snail shell fragment (likely Busycon contrarium) is visible in the lower right quadrant. (image ©Yasmin von Dassow)

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Shell fragments left by seagulls on the DUML dock. The vast majority of fragments are from the clam Mercenaria mercenaria. (image ©Yasmin von Dassow)

Questions I've been pondering:

  1. Do seagulls eat different prey at different times of year? If so, can the difference be detected by examining shell pieces left behind in the parking lot? Is any difference the result of prey behavior (for example, maybe some snails are harder to get at certain times of year because they burrow)?
  2. Is parking lot shell fragment diversity an accurate measure of prey diversity?
  3. Do the different species of shells show consistent breakage patterns?
  4. How do seagulls know to drop the shells on pavement but not on parked cars?
  5. Is it harder to break dropped shells on hard-packed sand than on pavement? If there were no human-made pavement around, would the seagulls eat fewer shelled organisms?
  6. If the shell fragments weren't constantly getting run over by cars and/or swept away, would a giant pile of seagull "trash" accumulate over time? Would there be a visible seagull midden?
  7. What's going on on the flat rooftops of the DUML library and Bookhout Research Lab that I can't see? Maybe the seagulls like those spots the best of all.

A note about experiments:

I absolutely do not endorse any behavioral manipulation or harassment of seagulls. Some of the questions above can be answered solely through observation, and shell fragments can and should be handled only when the seagulls are not feeding on them.

biology birds crabs ecology marine organismal-biology predation shells snails

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