A letter from Hi’ialakai, by Anon.
I am writing you from the deck of the research vessel, Hi’ialakai. We are at the dock resupplying. We are doing a project looking at what seabirds are eating. It can get messy. Here’s what I have had been doing:
One method involves extracting items from the digestive tract where what they are eating may be found in the oesophagus, crop, proventriculus, gizzard, or small intestine. Generally, the only items retained in the gizzard are hard parts such as bones, shells, exoskeletons, polychaete jaws, and squid beaks. Everything from the proventriculus to the oral cavity can often be sampled by lavage without harming the bird, whereas sampling the gizzard or intestine is only possible from dead, dissected specimens.
Once the food samples are obtained, I have to sort and identify them e.g. weight, linear dimensions, volume, taken. The size of the digested prey can often be estimated from measurements of undigested hard parts such as otoliths, bullae, bones, shells, and polychaete or squid beaks, but the accuracy depends greatly on the amount of digestion and wear of these items.
Shooting birds at sea is one way of obtaining dietary data. Shooting has, however, the obvious limitation of killing the birds, which raises ethical issues for me, because I love seabirds. Can you believe some people do this for research? Cruel. It is not even a good method. A substantial fraction (often 30% or more of birds shot at sea) do not contain any food items other than bony fragments in the gizzard. Therefore substantial numbers need to be shot to obtain an adequate sample. For these reasons, shooting should be unacceptable as a sampling tool. Also, because many seabirds feed socially, shooting a sample of birds at a single location may give a misleading indication of diet that may vary between aggregations. Consequently, shooting as a sampling tool can rarely achieve a representative picture of the spatial and temporal variation in diet.
Other sources of dead birds are oil spills, bycatches in fishing gear, and beached carcasses of oiled or wrecked birds, although birds from the last group have very often starved to death and yield few or biased data. Dead birds often arrive on beaches in a trickle, e.g. as a result of chronic oil pollution, or may hit a coastline en masse after an oiling incident, or a wreck following extended extreme weather. I think such large-scale events should be seized for diet studies whenever possible, because they often provide large samples across a range of species from the same time and location. Interestingly, after some oil spills specimens have even be sequestered for litigation purposes.
Items in the gizzard may be retained for a considerable time; sometimes until they are regurgitated. Squid beaks or polychaete jaws, for example, may be retained for a month or longer, so such retention needs to be taken into account when estimating dietary composition based on dissected dead birds and/or regurgitated samples. As the soft parts of squids or polychaetes are digested quickly, the beaks or jaws in the gizzard are often the only evidence of their presence in the diet.
Some seabirds, such as gulls (Laridae) and skuas (Stercoraridae), empty the gizzard from time to time by regurgitating indigestible material, whereas others, such as procellariiforms, rarely do so, so may retain hard parts of prey in the gizzard for many months. So such different regurgitating methods have to be taken into account.
Some birds, especially nocturnal petrels and shearwaters (Procellariidae and Hydrobatidae, respectively), when attracted to lights at night become disorientated and land on a ship’s deck or the ground. To lessen weight or as a panic response, they often vomit the contents of the upper digestive tract.
Other species such as gannets (Sulidae), cormorants (Phalacrocoracidae), gulls, and terns (Sternidae) at or near the nest or on their way to feed chicks often regurgitate food held in the proventriculus if disturbed. Chicks may also spontaneously regurgitate in response to disturbance, or can be easily stimulated to regurgitate. Such samples are often only partly digested material and readily identifiable in the field (e.g. from gannets, cormorants) or on return to the laboratory (gulls, kittiwakes, Rissa spp.). Another advantage is that this type of sampling can be repeated (using the same or different birds) through the breeding season. Hard body parts (otoliths, bones, etc.) are also often not worn by digestion (although there are different digestion rates among opaque and hyaline otoliths; so allowing reliable determinations of prey size.
Sampling this way can be especially valuable because it may be the only way to obtain dietary information from birds at sea and/or outside the breeding season. The problem with this technique is that it is entirely opportunistic. Nevertheless, such sampling can produce valuable information on the food types available at prey patches at sea.
Bird excrement is used in various ways to reconstruct diets. Hard parts from prey, such as bones, scales, eggs or otoliths of fish, parts of the exoskeletons of crustaceans, squid beaks and jaws and setae of nereid worms, calcite plates and spines of echinoderms, or shell hinges of molluscs may all survive digestion and are often excreted. If such parts are recognizable and still bear a relationship with original prey size, they may be used to identify prey and reconstruct prey size. Advantages of the method are that it is non-invasive and simple. Furthermore, large sample sizes can be processed and time-series built by repeated sampling schemes. Given that different methods often reveal different prey types, studying remains in excrement could reveal prey species previously unknown, e.g. Nereis jaws in sandwich tern (Sterna sandvicensis) excrement.
Studies of excrement, like many other indirect methods covered here, are unlikely to reveal all prey taken by the predator. Some prey are easily fully digested, and some birds also use other means to rid themselves of prey hard parts. Moreover, some parts survive better than others and some prey may be completely overlooked or greatly underestimated. For example, sandeel (Ammodytidae) otoliths appear in the faeces of great black-backed gulls (Larus marinus), but otoliths of gadoid fish too large to pass through the intestine are voided in pellets.
Another disadvantage is that excrement is unlikely to be collected at sea, unless a suitable platform on which they are deposited is available for sampling. Also, processing faecal samples can be unpleasant.
It’s messy work. But sitting on the deck of the boat after another round of sampling watching the birds soar, well, “special” is an understatement. I hope we can make sure they stay healthy long into the future.