May 2, 2013 | 12
My photography skills – if I can call them that – are pretty atrocious. While on a break in Wales recently, I managed to photograph a sequence in which a Herring gull Larus argentatus (one of our most frequently encountered gulls) swallowed a Common sea star Asterias rubens. Yeah, that’s right, get into the habit of calling them sea stars – makes you feel so much better than stupid old ‘starfish’. Of course, gulls swallow sea stars on regular occasion and this image isn’t (so google reveals) at all remarkable. Google ‘gull eating starfish’ for yourself if you want to see far superior images (many of which are a bit gross in showing gulls swallowing unfeasibly big sea stars, some of which get stuck in the birds’ gullets and remain there for a while as an unsightly lump).
Large gulls are in fact important sea star predators, so much so that they might have influenced their evolution: this 2013 conference abstract describes how colour change observed during the ontogeny of the Pacific sea star Pisaster ochraceus might be an adaptational response to gull predation [adjacent image by Minette Layne]. However, the importance of gulls as sea star predators is apparently not well studied and there are some indications (I’m going from this 2009 poster by P. Monteforte and colleagues on Western gull L. occidentalis predation on Pisaster species) that gull predation is – in some places – having a negative impact on sea star populations. So, gull increase could conceivably result in sea star decline. Cue discussion of trophic cascades and so on: white-headed gulls certainly have many resources they can switch to when sea stars aren’t around, so it’s not as if they rely on these echinoderms as a prey base; they merely exploit them when available.
Here it’s worth saying that, while white-headed gulls of several species (Herring gulls in particular) are frequently said to be increasing exponentially in population size, this in fact only applies to urban populations: the overall populations of these birds are plummeting. Yes, even for the Herring gull, the UK population of which is currently at its lowest level since recording began in 1969/70 (see graph below, from the Joint Nature Conservation Committee’s Herring gull page). Believe it or don’t, it’s now a Red List species.
Back to sea stars… sometimes, gulls swallow the sea stars whole, as shown above, but on other occasions they shake them so that they break to bits (Wilkie 2010). As you may know, being broken into bits isn’t the end of things for a sea star, since it’s capable of autotomy at the base of each arm. Not only can the animal regrow the lost arm, the arm itself can survive and produce a new individual (hence explaining why the tearing of invasive sea stars in half to ‘kill’ them is probably not a good idea).
The feeding habits of Herring gulls remind me of the Tet Zoo article I wrote on this very topic back in 2009 (it’s here). On that occasion, study at a nesting colony revealed the swallowing by gulls of toy soldiers, plastic and metallic food waste, discarded medals, and a mobile phone (Camphuysen et al. 2008). And here’s something that concerns one of my favourite subjects: ‘unexpected’ ingestion of plants by a predator. It turns out that white-headed gulls of some species – you know, gulls… those big, predatory birds that eat fish, crustaceans, echinoderms and so on – eat enough plant material to act as locally important dispersers of seeds (in this case of madders [Rubia species]) (Nogales et al. 2001).
Gulls deserve more respect. They are not big white sky-rats.
For previous Tet Zoo articles on gulls and other seabirds, see…
Refs – -
Camphuysen, C.J., Boekhout, S., Gronert, A., Hunt, V., van Nus, T. & Ouwehand, J. 2008. Bizarre prey items: odd food choices in herring gulls and lesser black-backed gulls at Texel. Sula 21, 49-61.
Nogales, M., Medina, F. M., Quilis, V. & González-Rodríguez, M. 2001. Ecological and biogeographical implications of Yellow-Legged Gulls (Larus cachinnans Pallas) as seed dispersers of Rubia fruticosa Ait. (Rubiaceae) in the Canary Islands. Journal of Biogeography 28, 1137-1145.
Wilkie, I. C. 2010. Do gulls benefit from the starfish autotomy response? Marine Biodiversity Records 3, e12 doi:10.1017/S1755267209990480.