SHOREBIRDS and seabirds belonging to the order Charadriiformes are among the most conspicuous animals inhabiting our coastal environment. This extremely diverse order of birds is second only to passerines in terms of number of families and species. With a global distribution that ranges from the Arctic to the Antarctic, they can be found in almost any type of terrestrial habitat as well as in the sea. Consequently, they exhibit remarkably varied breeding strategies and migration systems.
Although the monophyly of Charadriiformes is well established, the relationship of the families within this order has been controversial. According to the most recent evidence, the order consists of three major groups: Charadrii (which includes plovers, oystercatchers, avocets, stilts and thick-knees), Scolopaci (which includes sandpipers, snipes, jacanas and painted-snipes) and Lari (which includes skuas, skimmers, gulls and terns). The first two groups constitute what is commonly known as shorebirds, while the last consists of seabirds.
This article describes a few relevant ecological aspects of shorebirds and seabirds, with emphasis on the species of our coastal areas and estuaries and their main conservation challenges both in the global and local contexts. Shorebirds, also called waders, are small- to medium-sized birds that inhabit all continents and are closely associated with open environments, especially wetlands. Most of them spend at least part of their lives on tidal flats at estuarine or marine shorelines, while some use inland habitats such as grasslands, rivers, lakes and lagoons. As a result of their cosmopolitan distribution and habitat associations, these birds exhibit diverse morphological adaptations, including marked variations in body size and bill morphologies. These morphological variations allow shorebirds to exploit diverse trophic resources while coexisting in large numbers at key sites along their flyways. Flyways represent routes, used by several species, that encompass the full range of breeding, stopping, staging and non-breeding areas occupied by a population during the annual cycle.
There are about 215 recognised shorebird species in the world, distributed unevenly among 14 families. The largest number of species is hosted by Charadriidae and Scolopacidae. These two families show quite marked morphological and behavioural differences. Members of Charadriidae (i.e. plovers and allies) have relatively large heads with short bills and large eyes that facilitate visual foraging, for which they use a “run-stop-peck” strategy, considered the ancestral condition in shorebirds.
On the other hand, Scolopacidae members (i.e. sandpipers and allies) have long bills relative to their body size that are varied in shape (for example, upturned in godwits, straight in knots, decurved in curlews, wedge-shaped in turnstones). Most members of this family feed on buried prey using tactile strategies that, in many cases, rely on mechanoreceptors located at the tip of the bill; others use their brush-like tongues to feed on biofilm.
Another notable difference between Scolopacidae and Charadriidae is related to their breeding areas and migration strategies. Most sandpipers are northern-latitude breeders that in many cases undertake amazing migrations to the southern extreme of the globe. Most plovers, however, breed on temperate and tropical latitudes and are less prone to migrating long distances.
One of the most fascinating aspects of shorebirds is their ability to migrate. Over 60 per cent of shorebird species undertake short- to long-distance migrations. Several members make some of the most extreme migratory flights in the world, exceeding 30,000 kilometres a year. Migration annually takes shorebirds from breeding to non-breeding grounds where they may spend as much as eight to ten months in a year.
From an evolutionary point of view, migration strategies arise if the benefits of moving seasonally are greater than the benefits of staying in one region. However, to take advantage of the world’s seasonality, birds must perform energetically demanding flights. Different species have developed different migration strategies, undertaking journeys of varying lengths and duration. Most species travel in many short periods of migratory flight interspersed with short periods of refuelling (this strategy is called “hop”). Others may do long-distance flights interspersed with resting and fuelling periods (“skip”). Finally, in the most extreme cases, some shorebirds may travel in one very long migratory flight (“jump”). Following the latter strategy, some of the species that inhabit the Indian shores and estuaries make very long non-stop endurance flights—about 8,000 kilometres in six days in the case of red knots and more than 10,000 kilometres in seven days in the case of godwits. To accomplish these long-distance migratory flights, shorebirds rely on energy and metabolic water that is mainly stored subcutaneously in the form of lipids but also as proteins in flight muscles.
Just before starting the trip, migratory shorebirds deposit huge amounts of triglycerides and at the same time hypertrophy their flight muscles without any noticeable increase in exercise. In this way, they are capable of doubling their mass before migration, for which they spend considerable time foraging while increasing their consumption rate in a period of hyperphagia.
Seabirds are defined as those birds living in and making their living from the marine environment, which includes coastal areas, estuaries, wetlands and oceanic islands. Within Charadriiformes, seabirds are grouped in the suborder Lari that comprises about 127 species. In Indian coasts, this group is typically represented by gulls and terns. From a morphological perspective, Lari constitutes a heterogeneous group. These birds range from small species—170–220 grams in weight and with an 84–86 centimetre wingspan; to large species, 810–1,335 grams in weight and with a 128–142 centimetre wingspan. Some members feed on only a few types of prey, so they are considered specialists, while others can use a wide range of resources, thus being considered generalists. Furthermore, they are trans-equatorial migrants that travel large distances across several oceanographic systems.
Seabirds are equally at home on land, in the air and on water, changing from one surface to the other, often daily. Such flexibility requires unique morphological and physiological adaptations to an environment that has also exerted selective force on their behaviour, ecology and demography. Their bills, feet and body shapes show innumerable adaptations to various lifestyles, allowing seabirds to swim and dive using webbed feet or even their wings to propel themselves in the water.
Although all seabirds use their bills to capture and handle food, they exhibit adaptations for different types of feeding. An emblematic example is that of skimmers, which are represented in our coasts by the Indian skimmer. They have a specialised bill in which the lower mandible is compressed laterally and is longer than the upper mandible. This provides a greater surface area that, along with their flight mechanics, allows skimmers to catch fish by efficiently skimming the water surface with their lower mandible as they search for prey with tactile cues. Adaptations for feeding in marine environments are not just restricted to bills. By feeding in the sea, these birds also must deal with high physiological loads of salt. To accomplish this, they rely on salt glands (i.e. organs for excreting excess salts) that are found in shallow depressions around the orbit above the eye, which help them to cope with a diet overloaded with salt. In addition, like shorebirds, seabirds can cope with fattening periods as a physiological adaptation to migration.
Seabirds are at the higher trophic levels of the marine food web. When breeding, they must return to their nest after every fishing trip to carry food for their chicks, a pattern referred to by ecologists as central-place foraging. During the non-breeding season, social foraging is widespread, and it usually involves several species. Foraging can occur during the day or night, and presumably these different foraging habits have evolved in response to the behaviour of their prey. The seabird’s diet consists mainly of fish, crustaceans, and/or molluscs. However, some species have incorporated anthropogenic items in their trophic spectrum. To obtain food, different foraging strategies, generally linked to morphological and/or physiological features, are used. The feeding method adopted by gulls include picking up items from the ground, surface dipping, jump-plunging, and other forms of dipping. For terns, plunge-diving, diving-to-surface, dipping and hawking (to catch insects) are well-documented feeding techniques. Although most seabirds catch live prey, scavenging is a feeding method employed by a small proportion of seabirds, especially gulls and skuas. Many scavenging species have increased dramatically in number, and this has often been attributed to the feeding opportunities presented over many decades by fishery waste.
Kleptoparasitism (i.e., parasitic interaction in which one animal steals food from another) is another special technique for acquiring prey. Specialist kleptoparasites exhibit adaptations that are absent in opportunist species, such as the ability to detect and attack hosts, carry food concealed in the proventriculus, sustain prolonged aerial chases, and adapt their breeding cycle to match that of their host.
The last few centuries have been marked by a rapid loss of biodiversity, habitat loss and degradation being one of the primarily identified causes. This biodiversity crisis not only involves extinctions but also a decrease in the number of individuals in local populations, which can result in changes in the composition of communities and the function of ecosystems. Accordingly, both shorebird and seabird species have been experiencing drastic population declines. Decreasing population trends are caused by a variety of threats, with climate change being among the most important on a global scale. The effects of this threat are particularly relevant to species inhabiting coastal habitats such as estuaries and those breeding in high latitudes, where climate change has already significantly modified the ecosystem. Some of the most important land-based threats include the presence of invasive alien species, problematic native species (e.g., those that have become super-abundant), human disturbance, changes in land use and land cover, habitat deterioration, commercial and residential development, and hunting. Finally, on the sea, some of the main problems are associated with bycatch (in a gillnet, trawl and other fisheries), pollution (oil spills, chemical contaminants, plastic and marine debris), noise (busy shipping lanes, seismic surveys, and sonar), and prey depletion caused by overfishing, energy production, and mining.
Stress in Paradise
Indian coasts and estuaries have been subjected to novel threats and increasing levels of pollution in the last few decades. Global data indicate that although estuarine and coastal systems provide vital habitats for many plants and animals, as well as a wide variety of goods and services for millions of people, they are some of the most heavily used and threatened natural systems in the world.
The proximity of coasts and estuaries to urban areas, industrial parks and ports has led to contamination by coliforms, hydrocarbon derivatives, pesticides and heavy metals, to mention some of the major threats. In recent years, problems associated with the expansion of invasive alien species have also become relevant. The legal protection of the ecosystem, as well as the recognition of its importance by international and local communities, shows signs of hope for Indian coasts and estuaries to remain a healthy ecosystem in the long term and thus to continue supporting viable shorebird and seabird populations.
Dr Vaithianathan Kannan is a wildlife biologist who works with the Sathyamangalam Tiger Conservation Foundation Tamil Nadu Trust, Erode, Tamil Nadu.
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