Seabird Paper - 2009
This topic submitted by Tyler Bruns at 12:25 PM on 5/15/09.
Every now and then we catch a Boa Constrictor, Drake Bay, Costa Rica!
Tyler Bruns
Dr. Cummins
TME 513
5/14/09
The Seabirds of San Salvador
In our upcoming trip to the Bahamas, there will be a lot of emphasis placed on coral reefs, angelfish, sharks, rays, algae, and many other creatures that reside underwater. However, there is another important set of animals that is easy to miss if we are only looking down into the sea below. Seabirds represent an important, and necessary, link in the chain of marine ecosystems. There are fourteen individual species of these marine birds that breed on the island of San Salvador. What characteristics they have in common, what distinguishes them from one another, their role in ecosystems present on the Bahamian island of San Salvador, and our relationship with these birds, both positive and negative, will all be explored in this paper.
Before one can understand the seabirds of San Salvador, one must first know what makes a bird different from any other organism, as well as their origins. Birds, also known as Aves, belong to the phylum Chordata. This means that all birds have a Òhollow dorsal nerve cord, gill slits, and a notochord at some stage in their development,Ó (Chordata, 2004), because that is what separates Chordates from other Animalia phyla. All birds are also warm blooded, lay eggs, have a four-chambered heart, wings, feathers, hollow bones, and a beak. These features are what separate birds from every other type of organism on our planet. Where did birds come from though? For many years, scientists had no idea how birds came to exist on this planet, but in the last fifty years, their origin came to light. ÒIt is thought that birds evolved from reptiles sometime during the Jurassic period 190-136 million years ago,Ó (Aves, 2004). Signs of this reptilian ancestry are apparent in their scaly legs and in their feathers, which are just reptilian scales that have been modified for other uses such as flight. This means that the closest living relatives to modern day birds are snakes, turtles, and alligators!
However, this description only helps to tell us more about birds in general. What about seabirds? How are they different and what makes them so special? ÒSeabirds are defined as birds that rely on the sea for their food and only return to land to nest,Ó (Angel, 2007). The seabirds feed through four different techniques: pursuit diving, plunging, surface feeding, and stealing from other bird species. The pursuit technique has the bird swim on the waterÕs surface, dive below the surface of the water, and then finally chase down its prey. Species that utilize plunging move from the air straight into the water, catching their prey and quickly returning to the air. Surface feeding usually involves catching small fish that come to the surface or through the use of filters to eat the plankton present in the water. Finally, stealing is simply that, the theft of prey from other bird species. To accommodate these niche-feeding styles, seabirds have developed several special adaptations including: webbed feet, salt glands, waterproof plumage, and in some cases, (penguins are a good example) the loss of flight in favor of better swimming abilities. Seabirds often undergo migrations to and from their breeding grounds where they form colonies with other birds of the same species. ÒSince seabirds are vulnerable to predation when nesting, they often nest in remote islands or sea cliffs,Ó (Angel, 2007), making San Salvador an excellent site for observing seabirds. As we explore San Salvador, it is important for us to keep an open eye for the following bird species!
On the island of San Salvador, there are fourteen individual species of seabirds that nest there which include: White-tailed Tropicbirds, Magnificent Frigatebirds, Brown Boobies, Red-footed Boobies, Double-crested Cormorants, Laughing Gulls, Brown Noddies, Gull-billed Terns, Royal Terns, Roseate Terns, Bridled Terns, Sooty Terns, Least Terns, and the AudubonÕs Shearwaters, (Hayes, 2003). Of these fourteen species, each have their own special characteristics that separate them from the other species, and they all interact with the marine ecosystems in their own unique ways. That makes the knowledge about the lives of these seabirds just as important as the coral reefs and the organisms that live in them. However, in order to adequately describe each of these species of birds is too broad a topic to cover in just one paper, so the focus will be placed on only five of those species: the White-tailed Tropicbird, Magnificent Frigatebird, Brown Booby, Sooty Tern, and AudubonÕs Shearwater. These five species were picked to give the most diverse and broad representation of the numerous bird species that exist on this island.
The White-tailed Tropicbird, Phaethon lepturus, will be the first of five bird species that will be discussed. This species was aptly named because these birds Òrarely leave subtropical or tropical regions,Ó (Lee and Walsh-Mcgehee, 1998). Their plumage is mostly white with black streaks that go through their eyes and cut across areas of their wings. They also have red to orange colored beaks. Unlike several other bird species, their plumage does not change during breeding seasons and are thusly denoted as sexually monomorphic. They are the smallest of three other species of Phaethon with a wingspan of only around three feet and a weight of 350g. ThatÕs less than one pound! As previously mentioned, seabirds rely on the sea for their livelihoods, but White-tailed Tropicbirds take this to the extreme. ÒThey are one of the most pelagic bird species in existence, rarely are they seen near land unless they are mating,Ó (Lee and Walsh-Mcgehee, 1998). While at sea, they feed on squid and flying fish using the plunge diving technique. They are not adept at pursuing their prey once they are underwater and as such must rely on catching their prey quickly after reaching the water. When breeding season comes around, these birds leave the isolation of the sea and return to their nesting sites on secluded tropical islands. After a male has courted a female and the two become a pair, they lay only one egg per clutch with usually only one clutch per season. This one egg per clutch is the norm among seabirds, due to the fact that seabirds are usually fairly long lived. Though no reliable data on the lifespan of this species has been collected, Òa similar species has been noted with living over twenty years,Ó (Klimkiewicz and Futcher, 1989). Since the birds usually return to their old nests year after year, it is not uncommon for them to mate with the same spouse as the previous year, but they are not monogamistic. After breeding season, the birds migrate all over the tropics and remain fairly isolated until the next breeding season.
Next is the Magnificent Frigatebird, Fregata magnificens. This bird species is easily distinguished by its large size, black to dark brown plumage, and deeply forked tail. With a wingspan of up to eight feet and a weight of four pounds, they can be quite intimidating compared to other seabirds. This size comes in handy when they steal food from other seabirds, a trait that they are notorious for. ÒFrigatebirds have a reputation of being pirates, reflected in their colloquial name Man-of-War Bird,Ó (Diamond and Schreiber, 2002). This statement can be misleading however, since these birds catch most of their food on their own. They feed off of fish and squid that come near the waters surface by snatching them up with their long beaks without getting a single feather wet. It is good that they have perfected this technique because, unlike other seabirds, they do not have a waterproof plumage. Without this waterproof plumage, they suck up water like a sponge and find it very hard to fly should they become wet. Like White-tailed Tropicbirds, Magnificent Frigatebirds return to secluded islands, especially in the Caribbean, to breed. These birds are more colonially oriented than the tropicbirds however, with most nests resting only a few feet away from one another. During this time it is easy to recognize male birds that blow up their large red gular sacs as a way to attract females. After mating, the females lay only one egg per clutch with only one clutch per year at maximum. Once again, this fact probably has a strong correlation with their long life spans. ÒNo data for this species has been collected, but the Great Frigatebird is noted as having a life span of over thirty years,Ó (Clapp and Hackman, 1969).
Thirdly is the Brown Booby, Sula leucogaster. Brown wings and body with a white underbelly is the common plumage for the Brown Boobies, but their beaks and feet can vary from yellow to greenish blue to gray in color. Compared to other Boobies, the Brown Booby is medium in size with a wingspan that can reach five feet and a weight that varies from two to four pounds. This Booby feeds mainly on flying fish and squid using the same plunge diving technique that the White-tailed Tropicbird utilizes. During the breeding season, ÒBrown Boobies congregate in small colonies of tens to hundreds of pairs,Ó (Schreiber and Norton, 2002). However, Brown Boobies lay two eggs per clutch, which is unlike the previous two species, which lay only one egg per clutch. Only one chick is usually raised per clutch though, because Òthe first to hatch usually out competes its sibling during feedings and frequently pushes it out of the nest,Ó (Schreiber and Norton, 2002). This ensures that the strongest sibling is the one that survives to reproduce. In reference to life spans, the Brown Booby has a very lengthy one, estimated to be Òat least thirty years and possibly up to fifty years,Ó (Simmons, 1967). Unfortunately, the areas where Brown Boobies can nest have been reduced to secluded islands as human influences and the introduction of new predators have limited where they can breed. Not only that, but these factors have also significantly reduced their overall numbers. It is thought that the current population is Òonly 10% of historic levels,Ó (Schreiber and Norton, 2002).
There are six different species of terns known to breed on San Salvador as well as the Brown Noddies (that are often classified as terns). The Sooty Tern, Sterna fuscata, was chosen for deeper analysis since it has the largest presence in San Salvador with around five hundred pairs known to breed there, (Hayes, 2003). These terns are fairly small birds with only three-foot wingspans and half a pound of mass, but are actually regarded as medium sized as far as terns are concerned. As far as plumage, Sooty Terns are mostly black with a white patch on their forehead and a whitish gray belly and neck. They do alter their plumage slightly between breeding and non-breeding stages however. These birds are well known for staying aloft most of their lives since they are far from land unless they are breeding. Such behavior is somewhat strange however since they have poorly waterproofed plumage, which forces them to avoid landing on the water. They prefer to snatch up their prey, which are small fish and squids, while flying over the surface of the water. When they head back to land during the breeding season, they form some of Òthe largest, densest seabird colonies in the world, at times numbering more than 1 million birds,Ó (Schreiber et al., 2002). Like most seabirds, they prefer to breed in isolated islands where human disturbances and predators are minimal. Per clutch, there are usually two to three eggs, but it is debated whether or not those eggs all come from the same mother. The oldest known breeders were around thirty-four years old, (Cossee 1995), which makes the average life span most likely somewhere just short of that.
Last, but certainly not least, is AudubonÕs Shearwater, also known as Puffinus lherminieri. With an average wingspan of just over two feet and a weight of a little over a third of a pound, this bird is quite small compared to other seabirds. Its plumage is mostly brown on top and a white underneath, making it fairly plain as well. These birds are pelagic and thusly rarely return to land unless to breed. This is possible by their ability to feed in several ways including plunge diving, pursuit diving, and surface feeding. It appears that this bird is a jack-of-all trades! When they do return to isolated islands to breed, only one egg per clutch is produced. Often these birds are the most active at the breeding grounds during the night, since they are far out at sea during the day. When they are at the breeding grounds, they are either engaging in courtship or caring for their young. These birds are long lived like many other seabirds as well. This bird is colonial, though the colonies are not exceedingly large. Finally, this bird does not undergo great migrations common to other shearwaters and some other seabirds, usually preferring to stay somewhat close to their breeding grounds. (AudubonÕs Shearwater, 2008).
Although seabirds are a necessary and important link in marine ecosystems, they are currently in great danger. ÒApproximately 20 percent of the world's seabird species are threatened by extinction,Ó (Hayes, 2003). Twenty percent! In fact, four of the five species that were described earlier have been targeted by Dr. William Hayes for increased protection due to their threatened status in the West Indies. The only one not needing special protection is the Sooty Tern, which is the most abundant seabird in the Bahamas, (Kushlan and Steinkamp, 2007). There have been several extinctions of seabirds in the last two centuries as well including the Great Auk and the Spectacled Cormorant, and many more are still at risk including the Eskimo Curlew (OÕHarra, 2005). The reasons for this sharp decline in seabird populations are as numerous as they are concerning: pollution, loss of habitat, over hunting, introduction of predators, introduction of exotic/invasive species, reduction in food sources, egg collecting, and colonial disturbances. One or two of these elements occurring infrequently would not be able to cause such a population decline in seabirds, but the combination of all these problems and the fact that few are intermittent or infrequent, produces a deadly cocktail for seabird populations. These problems are also magnified by several characteristics of seabirds including Ònesting in high density at few sites, small clutch sizes, prolonged nest occupancy because of slow chick growth, and slow maturation,Ó (Hayes 2003).
What are possible solutions? What is being done about this? The simple answer to the first question is to just reverse what has already been done to hurt the seabird populations, however, this is always easier said then done. There are plenty of governments, industries, and organizations that are largely apathetic to the seabirds' plight due to ignorance, greed, oneÕs own need to survive, or some combination of these things. In addition, there is the fact that accidents happen sometimes without any malicious intentions involved, such as oil spills. The silver lining to this cloud is that people are realizing just how important these seabird populations are and are simultaneously trying to conserve and manage them properly. In fact, several organizations and governments are already taking action. Now, once endangered or threatened species have made great rebounds to healthy population levels, such as the Brown Pelican. Even the United States is already in the process of creating a conservation plan for marine birds within their borders, (OÕHarra, 2005). The fight is far from over, but the gears of change are turning, and with some help, we can lead these bird populations back to health and stability.
The final question is, ÒWhy should we care?Ó As stewards of this world, it is our duty to preserve the lives of those species unable to defend themselves from these hardships that are often created by our influences. In a more practical sense, we need them for food, locating fish, maintaining balanced ecosystems, and as bioindicators for the environment. ÒMarine birds are useful as bioindicators of environmental pollution in estuarine and marine environments because they are often at the top of the food chain, ubiquitous, and many are abundant and common, making collecting possible,Ó (Burger and Gochfeld, 2004). Whether it is our philosophical duty or our practical need for them, seabirds, and in fact all marine organisms, are not something that we can ignore nor destroy without severely jeopardizing our own ability to survive.Work Cited
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Tropical Marine Ecology of the Bahamas and Florida Keys
Tropical Ecosystems of Costa Rica
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