Mute Swans (Cygnus olor), endemic to Eurasia, were introduced to North America city parks, zoos, avicultural collections and estates in the late 1800s and early 1900s. The international releases and accidental escape of these birds and their progeny resulted in a rapidly expanding free-falling feral population along the northeastern Atlantic Coast of the United States. The 1999 US Atlantic Flyway mid-summer survey counted 12,541 mute swans, most of which occurred along coastal wetlands from Massachusetts to Maryland.
The first breeding record of feral Mute Swans in southern Ontario was in 1958. Following this, Mute Swans began colonizing the lower Great lakes in the Mid-1960s and 1970s and the population has been expanding ever since.
In this paper, we estimate the rate of increase of Mute Swans in the lower Great Lakes, and suggest that appropriate measures must be implemented soon if the population is to be controlled.
We used three independent data sources to estimate the rate of increase of Mute Swans around the Great Lakes (see map 1).
1. Long Point Aerial Surveys (LP). Long Point, on the North Shore of Lake Erie, has some of the most productive wetlands in the Great Lakes basin. Aerial surveys to count waterfowl on these wetlands have been conducted during the spring and fall in 16 years between 1971 and 2000.
2. Mid-winter Index (MWI). Ground-based surveys have been conducted annually in January since 1980, along the north shore of Lake Ontario to count wintering waterfowl.
3. Christmas Bird Counts(CBC) are run annually during a 3-week period between mid-december and early January. Teams of volunteers search for as many species of birds as possible, and count all of the individuals of each species they encounter. We used data from all counts in the lower Great lakes basin that reported Mute Swans at least once since 1980.
All counts were analyzed using Poisson regression with a log-link function, corrected for overdispersion, to estimate the rate of increase. Models were fitted assuming exponential growth, as well as second-order year terms to test for changes in the rate of population growth. For analysis of CBC data, sites were included as covariates, to allow for the fact that some counts were not run every year. Effort (measured as log(party-hours)) was tested as a covariate, but had little effect on the estimates, probably because swans are conspicuous and readily counted irrespective of the number of observers.
Long Point Aerial Surveys:
The average annual population growth rate was 16% (SE 3%) based on peak spring counts, and 12% (SE 2%) based on peak fall counts, despite a marked drop in the populations after spring 1993 (possibly related to the unofficial control efforts on nearby marshes). Prior to that drop, the average rate of population growth on both surveys was approximately 30% per year (SE 5%).
Mid-Winter Index of Mute Swans on Lake Ontario, 1980-2000:
Mean annual population growth rate from 1980-2000:
Assuming exponential population growth (blue line), was 10% per year (SE 1%). However, there was some evidence (p>0.01) that the growth rate has been increasing over time. Assuming a second-order curve, the estimated growth rate increased from about 3% per year in the early 1980's to about 16% per year in the late 1990s.
Christmas Bird Counts of Mute Swans, 1980-2000:
Christmas Bird Counts:
The estimated average population growth rate for counts on the Ontario side of Lake Erie, Ontario, and St. Clair was 15% per year (SE 1%), while estimated growth on the U.S. side was 21% per year based on exponential growth curves (red lines). However there was some evidence that the growth rate has been slowing over time, especially on the US side (P < 0.01). Assuming a second-order growth curve (blue lines), the estimated growth rate decreased from about 20% to 10% per year in Ontario, and from 40% to 11% per year in the U.S.
All 3 data sets indicate very rapid population growth rates, of between 10% and 30% per year. There was some disagreement among data sets as to whether the growth rate is increasing (MWI), stable (LPAS) or decreasing (CBC). Nevertheless, even the lowest estimates indicate populations are increasing by at least 10% per year. An increase of 10% per year leads to population doubling every 7 years, while 30% increase leads to doubling in 3 years.
This rapid growth is likely a consequence of the establishment of populations in new habitats, climatically similar to the native range of mute swans in Europe, with few natural predators, little inhibition from native breeding waterfowl (which they can outcompete), and minimal disturbance from humans (in Ontario, they have protected under the migratory bird act since the early 1970s.)
Without human intervention to control numbers, it seems likely Mute Swans will continue to increase rapidly until they reach carrying capacity.
The estimated wintering population of mute swans in the Netherlands was 13,000 in the late 1980s, corresponding to a density of 0.31 swans/km squared. If the carrying capacity for Mute Swans around the Great Lakes is similar, this would correspond to a population of 50,000 in Southern Ontario. With an increase of 10% per year, the population could reach 50,000 birds in 40-50 years.
Is this a problem?
Any non-native species has the potential to disrupt ecosystems. Among waterfowl, there are numerous examples of introduced species establishing large populations and becoming serious conservation problems through displacing, out-competing, or hybridizing with native species.
Several aspects of their ecology suggests that Mute Swans could be a particularly serious problem. They are extremely aggressive and occupy and defend large parcels of wetland habitat during the breeding season. They not only can displace native waterfowl from breeding and staging habitats, but have also been reported to kill adult and juvenile ducks, geese, and other waterbirds that enter their territories. Mute Swans consume or uproot large amounts of aquatic vegetation, degrading marshes and potentially reducing food supplies for other waterfowl. Further, Mute Swans can hybridize with both Trumpeter (Olor buccinator) and Tundra Swans (Olor columbianus). Because of their aggressive disposition they have also been reported to injure humans, especially near their nests.
Thus, despite their attractive appearence, Mute Swans are likely to pose a serious threat to native species of waterfowl and the integrity of the ecosystem if their populations are allowed to continue growing.
- The most effective control measures for long-lived species like Mute Swans are to reduce adult survival, most easily through hunting and/or culling. In corroboration of this, shooting adults has been used effectively to control swans (e.g., in Vermont), while methods to reduce productivity (e.g., egg addling) have had a limited effectiveness (e.g., in Rhode Island).
- Mute Swans should be removed from the list of species protected in Canada under the Migratory Bird Conservation Act, thus allowing sport hunters to assist in control.
- In view of the rapid rate of increase, control measures should be implemented as soon as possible, before the populations become too large. In view of their high potential growth rate, it could be necessary to remove at least 30-40% of the population every year to lead to a reduction.