Mute Swans in Maryland: A Statewide Management Plan
Maryland Department of Natural Resources
Wildlife and Heritage Service
April 14, 2003

POPULATION ANALYSIS

Impacts of Reducing Survival and Recruitment on Population Growth Rate

The DNR constructed a mathematical model of the Maryland mute swan population. The model used average values for survival and productivity. Most values came from studies of mute swans in Maryland. The model does not take into account events such as weather that might affect population change in a particular year. The model also assumes that population growth is independent of the size of the population. In other words, the values for survival and productivity do not change as the population grows. The model was used as a tool to compare the relative effects of different management strategies on population growth.

Most wildlife population management falls into two main categories: (1) affecting reproductive output or recruitment and (2) affecting the survival rate of adult birds. The model allows a comparison of how changes to the reproductive output or survival rates influence the growth rate of the population. A common means of decreasing waterfowl reproductive output is through egg and nest destruction. Addling eggs reduces the proportion of nests that successfully produce cygnets (i.e., hatching success). The model was run at different levels of hatching success to simulate various levels of egg addling effort. The simulations indicated that it is necessary to reduce hatching success by 80% just to stabilize the population (Figure 3).

Figure 3: Projected Population Growth Under Different Management Scenarios

In contrast, when annual adult survival rates in the model were reduced, it took just a 20% reduction to result in a population that will slowly decline over time. While egg removal/destruction can reduce production of cygnets, merely destroying eggs does not reduce a population as quickly as removing immature or breeding adult swans. The comparisons show that the mute swan population is much more sensitive to changes in adult survival than to changes in hatching success (Figure 3). These findings are very similar to other modeling exercises for long-lived waterfowl species of geese and swans.