Over the last week I’ve gotten something of an around-the-world tour of seabird conservation- some of the papers have been reviews, which means that they combine information from as many studies as they can and provide a broad view of trends, while others were targeted at specific species. No matter which way you look at things, seabirds are facing some serious challenges- habitat loss, invasive species, changing food resources– but not every seabird species is in the same boat, so I think it’s worth looking at both sets of literature. (And I’d like to point out that there is an entire journal, Waterbirds, devoted to these species, so there are lots of people investigating these issues.)
These royal and sandwich terns nest in colonies on the edge of vegetation near the beach- once the chicks hatch, they form a creche on the sand where adults guard them
Given current rates of sea level rise, it’s probably not surprising that nesting seabirds are finding themselves in more and more restricted locations. Certainly here in Louisiana continued availability of nesting sites for pelicans and certain tern species is partially due to human efforts to restore island and marsh locations (Visser and Peterson 1994)- in my own study sites I have seen marsh areas expanded by adding a shell berm behind an island and then filling in the enclosed area with sediment and I have seen tons of sand piped onto eroding beaches. Because certain species prefer bare sand and shell areas for nesting (for example, black skimmers here in the Gulf or little terns in the Mediterranean), these birds automatically have limited nesting options because habitat that is bare this year could start filling in with vegetation the next. This is a natural process at the mouth of any river as sediment is deposited and then colonized by vegetation, but we’ve put some extra pressures on these systems. Researchers in the Po Delta of Italy suggested that accelerated loss of nesting habitat due to sea level rise and human use of the same areas means we should be continually creating new little islets for birds to use (Fasola and Canova 1996). In the Chesapeake Bay area, improved prevention of storm breaches at some islands has limited the creation of new tidal shoal areas for nesting (Brinker et al. 2007). And habitat loss isn’t connected to just nesting opportunities– Kittlitz’s murrelets in Alaska forage in areas directly around glaciers, perhaps because the mixing of horizontal water layers concentrates food resources (Kissling et al. 2007), so disappearing glaciers means less foraging habitat.
I’ve mentioned the challenges posed by invasive species many times- with seabirds we are often looking at birds that nest on the ground or in burrows or crevices, so invasive predatory mammals represent a particular threat. When breakwaters connected an island off the coast of Chile with the mainland, a variety of land mammals, including rats and rabbits, made the journey across (Simeone and Bernal 2000)- when feral dogs took up residence, they killed at least 35 adult Humboldt penguins. A study on Reunion Island suggested that the introduction of cats had had massive impacts on the endemic Barau’s petrel- they calculated that 10 cats could kill up to 900 petrels in a year and 58% of recorded kills were adult birds, which makes it an even bigger issue (Faulquier et al. 2009). Why does the high number of dead adults make it worse? Seabirds tend to be long-lived species that are slow to mature and have small clutches (maybe 1-2 eggs)- like us, they are k-selected species. Since chicks fall victim to a variety of deaths, including predation, starvation, and weather-related mortality, a pair of breeding adults typically needs to have several different breeding seasons before they succeed in raising two young which can replace them. Another few seasons are needed to raise enough young to expand the breeding population. When adults experience high mortality, there aren’t enough new breeders to make up for the losses. Of course, when predators are numerous enough, it doesn’t matter if they don’t focus on adult birds- a study in British Columbia found that introduced rats living near a colony of nesting ancient murrelets were feeding almost exclusively on eggs and chicks, with probably some vegetation thrown in (Hobson, Drever, and Kaiser 1999). Not surprisingly, the population of murrelets had dropped more than 90% in less than 4 decades.
When it comes to finding adequate food for themselves and their chicks, many seabirds face a world that is changing. Increased water temperatures connected to the North Atlantic Oscillation are linked to more herring and less capelin in the North Atlantic- more herring seems to have a positive effect on several guillemot species, but is not a good situation for kittiwakes who depend more on capelin (Sandvik et al. 2005). Since sea temperatures are predicted to increase in the future, we may also see that herring populations expand at the expense of capelin, and that scenario, with different fish species, has also played out in the Gulf of Alaska (Osterblom et al. 2008). Why are dietary substitutions a concern? If the fish being substituted do not meet the dietary requirements of the birds or their growing chicks, adult survival and nesting success may be compromised (in the same way that eating only lettuce because hornworms got your tomatoes, eggplants, and peppers, is probably not going to give you all of the nutrients from vegetables that you need). And seabirds are also impacted by our fishing activities. In previous posts I’ve talked about sustainable seafood (see All at Sea), so I’m not going to get into all of that here, but our fishing gear, such as long-lines and gillnets, can catch or trap and drown birds, and overfishing of some stocks had has a negative impact on chick survival, for example with Atlantic puffin and herring stocks in Norway (Tasker et al. 2000). At the same time, it’s important to point out that many bird species have also figured out that our fisheries are a good source of food, since unwanted catch is thrown back- at least 23 species were documented using bycatch off the coast of Patagonia by Yorio & Caille (1999), so it’s not all bad.
Seabirds face other threats, too, such as hunting or egg-collecting, but the big ones revolve around habitat loss, predation from invasive species, and changes in fish stocks. As you can imagine, researchers, in addition to documenting these issues, have been investigating possible solutions and options for supporting seabird populations around the world. For my next post, I’ll dive into that literature.
Works cited:
Brinker DF, McCann JM, Williams B, Watts BD. 2007. Colonial-nesting seabirds in the Chesapeake Bay region: where have we been and where are we going? Waterbirds 30:93–104.
Fasola M, Canova L. 1996. Conservation of gull and tern colony sites in northeastern Italy, an internationally important bird area. Colon. Waterbirds 19:59–67.
Faulquier L, Fontaine R, Vidal E, Salamolard M, Le Corre M. 2009. Feral cats Felis catus threaten the endangered endemic Barau’s petrel Pterodroma baraui at Reunion Island (Western Indian Ocean). Waterbirds 32:330–336.
Hobson KA, Drever MC, Kaiser GW. 1999. Norway rats as predators of burrow-nesting seabirds: insights from stable isotope analysis. J. Wildl. Manag. 63:14–25.
Kissling ML, Reid M, Lukacs PM, Gende SM, Lewis SB. 2007. Understanding abundance patterns of a declining seabird: implications for monitoring. Ecol. Appl. 17:2164–2174.
Osterblom H, Olsson O, Blenckner T, Furness RW. 2008. Junk-food in marine ecosystems. Oikos 117:967–977.
Sandvik H, Erikstad KE, Barrett RT, Yoccoz NG. 2005. The effect of climate on adult survival in five species of North Atlantic seabirds. J. Anim. Ecol. 74:817–831.
Simeone A, Bernal M. 2000. Effects of habitat modification on breeding seabirds: a case study in central Chile. Waterbirds 23:449–456.
Tasker ML, Camphuysen CJ (Kees), Cooper J, Garthe S, Montevecchi WA, Blaber SJM. 2000. The impact of fishing on marine birds. J. Mar. Sci. 57:531–547.
Visser JM, Peterson GW. 1994. Breeding populations and colony site dynamics of seabirds nesting in Louisiana. Colon. Waterbirds 17:146–152.
Yorio P, Caille G. 1999. Seabird interactions with coastal fisheries in northern Patagonia: use of discards and incidental captures in nets. Waterbirds 22:207–216.