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Introduction Raptors Divers Waders General Ecology Environmental Problems
INTRODUCTION As with the m a m m a l s t o be addressed in the following chapter, m a n y species of birds, indeed probably m o s t of them, at one time or a n o t h e r at least visit rivers and streams. One a c c o u n t i n g lists 11 orders of birds t h a t are associated with riverine food webs and the ecology of rivers a n d streams; however, little solid i n f o r m a t i o n exists on the ecological impacts that birds have on the dynamics of these ecosystems, and m u c h t h a t we do know is in generalities. We will n o t discuss reproduction, nesting, and general aspects of the biology; there are far too m a n y species, a n d this information can be found in the n u m e r o u s field guides devoted to birds. We will limit our observations to the ecology of birds in relationship to r u n n i n g water ecosystems. The relevant groups are the raptors, w h i c h feed largely on fishes; diving birds, w h i c h feed on fishes, u n d e r w a t e r plants, and invertebrates; and a n u m b e r of wading birds, w h i c h feed on invertebrates and o t h e r prey in the shallows of rivers and streams. 305
PART III. The Biota of Rivers
RAPTORS The raptors (order Falconiformes) most closely associated with river and stream ecosystems in the United States are the bald eagle (Haliaetus leucocephalus), the golden eagle (Aquila chrysaetos), and the osprey (Pandion haliaetus carolinensis). Other fish-eagles perform similar ecological roles in other parts of the world. Viewers have thrilled to see an eagle or osprey (Fig. 20.1), soaring over the water, suddenly fold its wings to p l u m m e t toward the surface and, just before hitting the water, put on the brakes, thrust its feet into the water, and rise with a struggling fish in its claws. The osprey is especially adapted for this task; the soles of its feet have scaly pads that help to grasp fishes, and the front toe is double-jointed so that it can swing backward to m o r e efficiently grasp fishes from the water. Osprey are also k n o w n to train their y o u n g to catch fishes by d r o p p i n g a fish in the air and allowing t h e y o u n g to a t t e m p t catching the fish before it hits the water. More often t h a n not, the y o u n g have to retrieve the fish from the water, thus reinforcing the natural h u n t i n g instinct of snatching fishes from the water. Osprey nest near water and feed almost exclusively on fishes. Osprey were adversely impacted during the period w h e n DDT was widely used, b u t have recovered and are c o m m o n since elimination of the pesticide. Despite the beauty and thrill of seeing a raptor dive and catch a live fish from the water, this is n o t truly indicative of h o w most raptors, particularly eagles, obtain their food from rivers and streams, especially
An osprey, Pandion haliaetus carolinensis, in flight. (Photo by J.-L. Cartron.)
from the salmon-rich streams of Alaska and the Pacific Coast. In these streams, a more c o m m o n feeding pattern is to merely scavenge the spawned out carcasses of dead salmon (Fig. 20.2), which have completed their life cycle. These provide a rich harvest for the raptors, w h o can pick t h e m up with a m i n i m u m of effort. Everyone who has spent time along streams in areas where eagles, especially bald eagles, live is familiar with the large concentrations of eagles that frequent the riparian zone of streams having large populations of salmonids. They perch in large numbers in trees (Fig. 20.3) along the stream, waiting to spot a dead or dying fish. As m a n y as 3000-4000 birds have been counted along the Chilkat River in Alaska during the salmon run. They will dive and take live fishes, but scavenging is their main m e t h o d of feeding. Bald eagle concentrations are also found in areas of extensive waterfowl nesting, where they m a y catch flying ducks and scavenge weak or dead birds on the ground. One ecological impact of scavenging fish carcasses by eagles is that by removing the carcasses from the stream, they preclude nutrients and other chemicals from the decaying carcasses from entering the chemical mix of the water. However, given the huge numbers of fish bodies not removed by the birds, it is unlikely that this removal significantly alters the overall chemical regime of the water. Other raptors, including hawks, are also known to prey on fishes and waterfowl, but do not play significant roles in the dynamics of river and stream ecosystems.
FIGURE 20.2 Bald eagles scavenging on salmon carcasses. (Photo by W. Baker.)
PART III. The Biota of Rivers
FIGURE 20.3 Bald eagles, Haliaetus leucocephalus, perching in tree alongside salmonid spawning stream. (Photo by W. Baker.)
DIVERS Several orders of birds found associated with flowing waters fit into this general category: the loons (order Gaviformes), grebes (order Podicipitiformes), cormorants and pelicans (order Pelcaniformes), ducks and mergansers (order Anseriformes), and kingfishers (order Piciformes). The loons, cormorants, pelicans, mergansers, and kingfishers feed primarily on fishes, while the grebes feed mainly on invertebrates and amphibians, although they may also take fishes and plant matter. Diving ducks consume significant numbers of invertebrates, especially mollusks, and may also feed on submerged macrophytes. Loon (Fig. 20.4) populations have declined steeply, especially in the northeastern United States, and two reasons have been identified. First, the increase in recreational boating has increased the predation rate on loon eggs as the adults are frightened from their nests by passing boats. Second, the increase in acidification of lakes in this region has resulted in a decline of fish populations, the main diet of the loons. Although adults can readily fly to nearby lakes for feeding, the young cannot and so starve to death. These scenarios apply mainly to lake dwelling loon populations, but could apply to those nesting in the slower waters of river sloughs and backwaters. Kingfishers are familiar to everyone who has spent much time along rivers and streams. They can be seen perched on branches extending out over the water (Fig. 20.5), where they scan the surface for likely meals. Although they are known to dive relatively deeply when pursuing a fish,
FIGURE 20.4 The common loon, Gavia immer. (Photo by E. Swenson.)
FIGURE 20.5 The belted kingfisher, Ceryle alcyon. (Photo by J. Kelly.)
m o s t prey are taken at the water surface. Whereas fishes are the staple of their diets, kingfishers occasionally take o t h e r invertebrates as well. The y o u n g are t a u g h t to fish by the adults d r o p p i n g dead meals into the water to be retrieved by the fledglings.
PART III. The Biota of Rivers
It is largely unknown, as mentioned above, what impact the feeding of these birds has on food web dynamics in river and stream ecosystems. Instances are known where mergansers and cormorants have consumed large numbers of fishes, and this certainly could impact populations. Cormorants on the eastern coast of the United States have been regarded as competitors by commercial fishermen. Still, until stream ecologists begin to include birds in their studies, we can only speculate on their full role in the ecology of stream ecosystems. Conversely, ornithologists might widen their observations to include aquatic habitats in their analyses--a mutually beneficial partnership appears to be in order. There is one instance, however, where birds have a documented impact on other constituents of the stream ecosystem. A study of the Rio Frijoles in Panama revealed that little blue herons and green kingfishers preyed upon loricariid catfish in experimental open-topped enclosures located in shallow areas of the stream. These catfish graze algae from the surfaces of stones, and in the enclosures where the catfish were exposed to bird predation, algal growth exceeded that found where normal populations of the fishes were present in enclosures in deeper water. Because the fishes normally inhabit deeper water, thereby avoiding the predatory birds, algae proliferate in shallow areas. This represents an example of what ecologists call a "trophic cascade." The presence or absence of bird predation impacts the algae via the bird's effect on the intervening trophic level, herbivorous fishes.
WADERS Wading birds include the herons, egrets, bitterns, storks and ibises (order Ciconiiformes); the rails, coots, and waterhens (order Gruiformes); gulls (order Charadriiformes); and dippers and wagtails (order Passeriformes). For the most part, these birds are found along the periphery of rivers and streams, usually in backwaters where the current is not swift (Fig. 20.6). They take advantage of their long legs and long beaks to patiently stalk the shallow waters, spearing fishes and invertebrates. Some wading birds exhibit a type of commensal feeding whereby many smaller species, called "attendants," such as shorebirds and waterbirds, follow other species and take whatever comes their way after the water is stirred up by the "beaters." For example, Great and Snowy Egrets attend cormorants, and coots attend several species of ducks. Coots and waterhens feed largely on plant matter, but do ingest several types of invertebrates. Shorebirds exhibit a wide range of physical attributes which are related to where and how they feed. They vary in the length of their feet and bills, and this allows a type of "partitioning" of the food resources. Species with long legs and bills (e.g., the long-billed curlew, Numenius americanus) probe for insects and bottom-dwelling crustaceans in deeper water. The semipalmated plover (Charadrius semipalmatus) has short legs and bills and feeds on similar organisms, but in shallow water or land just above the water's edge. Several species exhibit a range of bill and leg lengths intermediate between these two examples.
FIGURE 20.6 Great blue heron,
Ardea herodias, wading in shallows. (Photo by W. Baker.)
Again, the impact of the feeding of these birds o n the total productivity of river and stream ecosystems is largely u n k n o w n . Herons have been shown to have a significant impact on b r o w n trout populations in Michigan's Au Sable River; mergansers and kingfishers also prey u p o n y o u n g trout. A great blue heron killed because it was feeding on tagged rainbow trout at an experimental facility in Colorado was found to have 26 tagged trout in its s t o m a c h m a significant n u m b e r given the small size of the stream and the total n u m b e r of fishes tagged. Gulls are notorious scavengers and are familiar sights gleaning the remains of fish carcasses left by bears feeding on spawning salmon. The American dipper (Fig. 20.7), or water-ouzel, (Cinclus mexicanus) is one of the most intriguing birds f o u n d closely associated with stream ecosystems. In fact, dippers are completely d e p e n d e n t on the productivity of the rivers and streams where they dwell. The small, gray birds can be f o u n d along the sparkling, swift-flowing headwaters of most m o u n tainous streams. The dipper perches o n rocks at the edge of the water,
PART III. The Biota of Rivers
The American dipper, Cinclus mexicanus. (Photo by R. Ryder.)
where it constantly dips its body up and d o w n every few secondsm looking m u c h like it is doing a series of knee-bends. Then, with a quick leap, it jumps into the water to feed on aquatic invertebrates, which it picks from the rocksmmayflies, stoneflies, caddisflies, worms, or any other likely prey. It can actually walk across the rocks of the streambed, and maintains its position on the b o t t o m by adjusting its wings so that the force of the current pushes it down. It has been k n o w n to "fly" as deep as 7 m in pursuit of food. As a predator of aquatic insects and other macroinvertebrates, it functions as a c o n s u m e r in stream food webs; it is also k n o w n to feed on fish eggs, but the degree of competition between dippers and trout has yet to be d o c u m e n t e d .
GENERAL ECOLOGY In general, birds have not been shown to play major roles in the food web dynamics and productivity in stream ecosystems. Instances have been shown, such as in the Panamanian streams, where predation by birds can impact the spatial distribution of the different c o m p o n e n t s of the aquatic food web. Nevertheless, we are woefully ignorant of the ecological relationships of birds in river and stream ecosystems. What we do know is largely inferred from observations, and this indicates that for the most part, birds function as top predators, preying on everything from invertebrates to fishes. In some cases, as with those species that feed on macrophytes and other plant material, they function as primary consumers. Much remains to be l e a r n e d m a n d quantifiedmin terms of birds and riverine ecosystems.
ENVIRONMENTAL PROBLEMS Fish-eating birds, including eagles, ospreys, cormorants, terns, and seagulls, can be exposed to contaminant concentrations high enough to impair growth and reproduction. The pesticide DDT, dioxin produced from burning chlorinated wastes and as a by-product of some pulp processing, and PCBs used in various industrial processes, share the property of biomagnification. With each step in the food chain, as predators consume many prey, because these chemicals tend not to be excreted, the contaminant load accumulates. Eventually, predators at the top of the food chain experience contaminant levels that cause reproductive failure or impair growth. In the Great Lakes region, eagles and ospreys nesting along lakeshores reproduce below replacement levels and show high PCB levels in their eggs and greater frequency of birth defects compared to birds nesting on inland lakes and rivers. In a surprising twist, these findings have led some biologists to argue for keeping dams on rivers to block fish movement from the Great Lakes into inland waters. According to the view that PCBs are the main culprit, dams protect inland populations of fish-eating birds, because fishes of rivers and inland lakes have lower PCB loads. Ironically, dams could be bad for the native biota of rivers, but good for eagles. An alternative explanation for higher reproductive success in inland waters is the possibility of greater food availability. Whichever argument ultimately wins out, one hopes that healthy rivers free of both dams and contaminants lie in our future. Such rivers make an important contribution to the well-being of fish-eating birds, including our national emblem.