Home Range

RESEARCH APPROACH

In the Sierra Foothills, California Black Rails are found in freshwater wetland patches. While many of these wetlands are fed year-round by natural springs, the majority of sites are maintained by irrigation water, a valuable commodity shared between people and wildlife. Understanding how Black Rails use space in their wetland habitats can help us estimate the amount of water necessary to sustain their populations. By examining the intensity of space use we can draw conclusions about an animal’s home range (the area used by the animal to carry out basic functions) and its core area (where it spends the majority of its time). Information about home range and core area allows us to determine the optimal wetland patch size for Black Rail populations.

Uncovering the behavior of a rarely seen bird requires the use of radiotelemetry. We fit Black and Virginia Rails with radiotags much like a person wearing a backpack. Each radiotag transmits a unique radio frequency that can be identified from outside the wetland with a handheld receiver and antenna thereby reducing our wetland intrusion. We estimate the location of a bird by recording the bearing of the strongest radio signal at three sampling stations such that the intersection of all three bearings creates a triangle. The center of the triangle created using these directional lines is the location, or fix, of the bird. We collect daily fixes for each radiotagged bird and input the data in a computer program to statistically analyze the size and shape of the home range and core area of each individual.

A proper triangulation is created when the bearing at any given sampling station points in a direction between the other two sampling stations. Ideally, each sampling station is outside the boundary of the wetland in order to minimze disturbance. The star represents the fix on the bird.

WHAT WE HAVE LEARNED

Black Rails in the Sierra Foothills have a much smaller home range (0.35 + 0.7 ha) and core area (0.08 + 0.02 ha) than those in the San Francisco Bay Area. As expected, Virginia Rails have a larger home range (0.53 + 0.11 ha) and a larger core area (0.12 + 0.03 ha) on average than Black Rails. Black Rail core areas, represented by thick lines in the below imagery, remain separate while home ranges, represented by the thinner lines, show more overlap. Virginia Rails follow the same trend with home range overlap but no core area overlap. While Black Rails tend to reside in shallower water depths, Black Rail and Virginia Rail home ranges and core areas are not mutually exclusive as shown in the map. This raises questions about interactions between species and partitioning of resources such as food, a topic that can be addressed with stable isotopes research.