Fauna Series No. 4

Fauna of the National Parks — No. 4 Ecology of the Coyote in the Yellowstone

CHAPTER XI:
BIRDS IN RELATION TO COYOTES

TRUMPETER SWAN

The relationship of coyotes to trumpeter swans (Cygnus buccinator) is of special importance because of the relatively small numbers of these swans still surviving. In the United States their breeding range is restricted to the Yellowstone National Park, Jackson Hole, and Red Rock Lakes regions. In Canada they are more widely distributed and less rare.

Status of swan in Yellowstone National Park.—The best information available on the status of the swan in Yellowstone National Park, taken from the 1937 and 1938 reports, is included in the following table:

Recent counts have been more thorough than the earlier ones, but even these last census figures may be incomplete.

In 1937, 40 adult Yellowstone swans produced 29 cygnets, while at Red Rock Lakes 38 adult swans produced 51 cygnets. This apparently lower productivity in Yellowstone is perhaps due to the presence of a larger number of nonbreeding birds in the park, birds probably too young to breed, or unmated for some other reason; also the Red Rock Lakes Refuge contains a larger area of suitable nesting habitat than does the park.

The swans begin nesting as soon as the ice goes out in late April and early May. Hatched in June, between 30 and 35 days after the eggs are laid, the cygnets usually do not begin to fly until late September.

In 1937 there were 7 pairs of swans which produced 29 cygnets. So far as known all but one cygnet of those that hatched survived through the summer.

The small increase in 1938 was due mainly to the small number of swans nesting and to small broods hatched. Three adult females found dead in early spring were probably potential breeding birds. Oberhansley and Barrows in their 1938 Yellowstone trumpeter swan studies (submitted in 1939) record 12 eggs that failed to hatch due to sterility, human disturbance, and unknown causes. The four cygnets surviving when the count was made in August were produced by four pairs. A fifth pair abandoned its nest apparently due to human intrusion.

At Beach Springs two unhatched eggs were found in the nest after the swans had left. At Swan Lake four of the six eggs laid failed to hatch. The eggs at Swan Lake remained in the nest for some time after the birds had left before they were collected and during this time, a period of 8 days, had not been molested.

At some of the lakes where swans have been raised, coyotes are concentrated. At Trumpeter Lake where seven cygnets were raised in 1936 and again in 1937, coyotes and coyote signs were frequently noted at the lake. Nineteen of thirty coyote droppings found at the lake contained only grasshoppers, and the others, except for one, contained elk, pocket gopher, and field mouse. The only evidence of waterfowl predation consisted of some remnants of a green-winged teal found on the bank and in one dropping. The swans here spent considerable time resting on the banks and occasionally walked with their brood a couple of hundred yards to an adjoining lake.

Usually the nests are located on islands in the water but occasionally they are on the shore. In 1937 the nest at Beach Springs and the one at Geode Lake were on shore. In the vicinity of these lakes coyotes are common. Since the swan on a nest is very conspicuous, the coyotes frequently must have seen these swans on the nests. While one bird is on the nest the mate is much of the time nearby, often only a few feet away. It is possible that two swans are a little more than a coyote cares to tackle, especially when mice and pocket gophers are available in abundance for food.

The swan may also enjoy some measure of safety from the watchfulness of its associates. At Trumpeter Lake, and some of the other lakes, many blackbirds, mainly red-winged and yellow-headed, nest in some numbers. The presence of many of these birds on the margins of the lakes makes it unlikely that a coyote could remain long in the vicinity without attracting their attention and their alarm notes would warn the swans. Other birds, such as ducks and grebes would also help keep the lake inhabitants in formed of the approach of a coyote. The swans, when feeding, are often followed by grebes, ducks, and geese who benefit from the stirring up the swan gives the under-water vegetation.

The swans themselves are alert but do not show much fear of other animals. On May 28, 1938, Frank Oberhansley and I watched two adult swans at Geode Lake from 4:05 p. m. to 5 p. m. They were idling about 4 feet apart on the low shore about 7 feet from the water. Their necks rested gracefully over their backs and their bills were pushed under the feathers inside the wings. The eyes of one of the birds seemed completely covered by the feathers most of the time, but the other had its eyes exposed. Once or twice during the first half hour that we watched they stretched their necks to look around but for more than 10 minutes at a time they seemed to sleep soundly. At 4:45 the swan whose eyes showed stretched its neck upward and a moment later the other also lifted its head into the air. They both looked inland; neither stood up. Presently a black bear passed below us about 25 feet from the swans. The bear did not pay any attention to the birds; it may not have seen them. While the bear was passing, the swans commenced to preen themselves and presently tucked their heads away for another nap. Two geese grazed on grass near them, always alert, and surely useful in announcing the approach of strangers.

On June 5 Oberhansley saw the two swans at Geode Lake resting on the shore where we had seen them on May 28. While he watched, a coyote passed near them, where we had seen the bear pass. The swans paid very little attention to the coyote, who in turn paid little attention to them. In trotting past it barely glanced in their direction.

In the fall of 1938 the Bureau of Biological Survey transported four swan cygnets from the Red Rock Lakes Migratory Waterfowl Refuge to some warm springs on the Elk Refuge in Jackson Hole. One of the birds disappeared soon after being released and no further trace of it was found. It may have flown away. The other three swans survived the winter even though it appeared in March that food in the water was becoming scarce. It is interesting to find these young birds, transported to a new environment, surviving in spite of the fact that coyotes were common. Coyotes were frequently seen in the vicinity of the area occupied by the birds.

A number of observations indicate that factors not at all related to predation tend to lower the swan population. In the spring of 1926 a dead mature swan (manuscript of Trumpeter Swan Report, Yellowstone National Park, Summer 1936) was found at Swan Lake, but the cause of death was not determined. The carcass was intact, so the bird seemed to have died from disease. Another dead swan was found on Daly Lake, between Livingston and Gardiner, early in the spring. The cause of death was unknown.

During the winter of 1935—36 Ranger Frank Anderson saw a dead swan floating down the Yellowstone River. A few years ago, O. J. Murie found two dead swans in a pond near Moran, Wyo., in early spring, which had died of disease or starvation.

During the spring of 1938 three dead swans were found in Yellowstone Park. All were adult females, probably birds which had been regular breeders in the park. The circumstances surrounding the deaths of these swans are as follows:

On April 12, Tom Phillips, a workman at the Buffalo Ranch, found two coyotes feeding on an adult swan on Slough Creek. Part of the back had been eaten and the neck was severed. The bird seemed to be in fair condition. It weighed 18-1/2 pounds, and I estimated that 1-1/2 pounds had been eaten. The intestines, proventriculus, and gizzard were empty and only a small quantity of sand was found in the latter. It was thought by Mr. Phillips that this swan was killed by coyotes, but considering the lack of food in the digestive tract it is more likely that the bird was not in normal condition and may have died before the coyotes found it. Archie Hull, in charge of Red Rock Lakes Migratory Waterfowl Refuge, told me that he had lost several swans from lead poisoning, caused by lead shot in waters frequented by duck hunters, and that he had found birds dying from this cause before losing their fat. Although no shot was present in the gizzard of the swan under discussion, there have been cases in which all the lead had been absorbed before death ensued.

On May 5 Ranger Walter Gammill found a dead swan on the shore of a shallow pond a few hundred yards east of Trumpeter Lake. I had seen the swan on this pond on April 25 and 28 and again on May 1 when it was resting on the shore 5 or 10 yards from the water. The shore of the pond where the swan was found is low and level, offering no cover for a stalking coyote. The head and one leg was missing and a little of one breast had been eaten. This was an adult female. The proventriculus was empty and the gizzard contained only about a level teaspoonful of grit. The intestines were also almost empty. The bird was so emaciated that there was no fat on the skin. The bird had unquestionably been sick.

On May 10 the linemen in the park reported that a dead swan was lying in Trumpeter Lake. Assistant Park Naturalist Oberhansley retrieved the swan and when we examined it we found no physical injury except a small festered spot on the back. This swan was also emaciated, weighing only 14 pounds, 14 ounces. The proventriculus was swollen and congested with green food. A number of much worn lead shot were found in the gizzard. The condition of the proventriculus and the presence of lead shot indicates that the bird, an adult female, died from lead poisoning.

On April 28 and May 1,1938, a pair of swans were seen building a nest on Trumpeter Lake where the nest had been located the previous year. A mound of dead reeds was built about a foot above the water. After May 1, although the swans remained at the lake during the summer, no further nest building was noted. Shillinger and Cottam, (1937, p. 400) suggest that lead poisoning might upset the breeding activities of waterfowl. They write:

Even though a sublethal dose of lead is taken, experimental evidence indicates that the poison so upsets the normal physiological processes that interference with reproduction may result. It is well known that lead acts as an ahortifacient in mammalian females and there is evidence that leads us to believe that it may induce sterility in birds.

There is a possibility, in view of the death of at least one swan and possibly two others from lead poisoning, in Yellowstone, that a sublethal dose of lead may also have interfered with the breeding of the pair at Trumpeter Lake.

The present status of the trumpeter swan is being given much attention. The 1937 census in the Yellowstone and Red Rock Lakes areas revealed 168 birds, and in 1938 there were 151. How many additional birds this region will support is unknown but it is not unlikely that the swan population is approaching the saturation point in this restricted area. Two limiting controls inherent in the region may be lack of nesting sites and a shortage of winter food. The nesting waters used by swans must have suitable nesting sites and also an adequate proper food supply. No study has been made of the wintering areas so far as I know, but it would seem that in the wintering waters there is a definite limit to the swan food supply. These same waters are occupied by numerous ducks and geese in winter, probably congregating there to the carrying capacity of available food supply.

J. A. Munro, Chief Federal Migratory Bird Officer for British Columbia, reports (Pough, 1939) on the winter food of the estimated 500 trumpeter swans in British Columbia:

The number of cygnets usually equals or exceeds the number of adults and from this it can he inferred that the summer loss through natural enemies is not large. Nevertheless there is, periodically, a heavy loss from starvation brought about by adverse weather conditions.

Probably the most serious direct mortality factor affecting the swans today is lead poisoning, to which these birds are very susceptible. Some of the victims have been found but no doubt many others have not been observed. Lead poisoning may operate not only in reducing the swans on the present range but also to prevent a spread into other areas.

Munro is quoted as follows concerning losses from lead poisoning:

At that time (1918) definite information was available regarding the wintering ground of one band which at its maximum contained 22 birds. The area involved was established as a Federal Bird Sanctuary with a warden service which is still maintained. Subsequently the greater part of this particular band died from lead poisoning; the number returning grew smaller each year and the flock finally disappeared.

These birds were probably so unfortunate as to spend part of their time on lakes whose bottoms contained much lead shot.

Another factor detrimental to the spread of swans is the accidental shooting of the birds when they get out into unprotected waters.

It was rather unexpected to find that the coyote in Yellowstone exerts no appreciable pressure on the trumpeter swan population. However the long necks of the swans give them an advantage in seeing any intruders. Furthermore, the swan is no doubt an adversary to be respected, for it is known that a swan can administer a powerful blow with its wings. At any rate, whatever may be the deterrents, the coyotes can find plenty of food during the summer without taking risks of being bruised. The data available at the present time indicate that the coyote does not represent an important mortality factor for the trumpeter swan.

In Yellowstone National Park and in all nesting areas precautions should be taken to prevent the birds from being molested during the summer season when the birds are nesting and raising their young. Roads, trails, fishermen, and other disturbing factors should not be permitted where swans are nesting and raising their families. Disturbance, at least in one known case, when the eggs had been laid, resulted in failure of eggs to hatch.

To insure the survival of the trumpeter swan in the United States a wider distribution should be encouraged. A step in this direction was taken by the Biological Survey when they moved four young swans into southern Jackson Hole, where three of them remained and wintered successfully. Probably more extended efforts of this kind would be desirable.

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Last Modified: Thurs, Dec 20 2001 10:00:00 pm PDT
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