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|Index Plots for Burrow Nesting Colonial Waterbirds|
Bermuda Petrel, Manx Shearwater, Black-vented Shearwater, Fork-tailed Storm-Petrel, Leach's Storm-Petrel, Ashy Storm-Petrel, Black Storm-Petrel, Ancient Murrelet, Cassin's Auklet, Rhinoceros Auklet, Atlantic Puffin, Horned Puffin, Tufted Puffin
Definition of burrow - Any hole in the ground.
There are a few factors which must be considered before beginning to design your monitoring program for burrow nesters.
Species Identification - Many burrow-nester colonies contain more than one species; the most diverse containing 5-6 species. Where index plots are subjectively placed, an effort should be made to minimize diversity, especially of species similar in size. When transects are used, multiple species often cannot be avoided. In such cases, it will be necessary to assign burrows to species groups based on entrance sizes. The following divisions are suggested:
Occupancy - Many burrows have tunnels so long that observers cannot reach the nest chamber with their arm, or they are curved which also may restrict access. Furthermore, it is frequently impossible to see the nest chamber from a burrow entrance with a bright light. Typically, excavating entry ports to tunnels will not be used for routine monitoring. Remote camera set ups are probably the best tools, particularly for puffin and Rhinoceros Auklet burrows, to determine the contents of burrows quickly. Nevertheless, for storm-petrels, Ancient Murrelets, and Cassin's Auklets reaching into each burrow is the recommended method. Indirect evidence of occupancy should be used for puffins and Rhinoceros Auklets when cameras are not available, and Cassin's auklets when burrows are too deep to reach the chambers. Indirect evidence of an occupied burrow would be the presence of droppings, feathers, or egg fragments near the burrow entrance.
Placement of transects - We need to consider the influence annual monitoring has on vegetation succession at colony sites. Choosing random plots along transect lines instead of going back to the same burrows each year will have less influence on the vegetation and should be considered.
It is important to note that accuracy and precision are both problems for methods developed to count burrow nesters and methods will be improved as studies are completed to determine the best methods for counting burrow nesters. We need to better understand the relationship between the number of burrows and the number of birds, and how much error is associated with these numbers. In the meantime, the following protocols are recommended for estimating population trends using population indices.
Burrow-nesting seabirds are hidden from view at their nest sites and colony attendance is either highly variable among days and hours (Tufted Puffin) or it occurs only at night (other species). As a result, it is not feasible to monitor population trends by counting birds. Instead, the target population is the number of burrow entrances in a series of index plots.
It is seldom possible for observers to get to all burrows on an island without technical climbing gear, because burrow-nesters often use steep slopes or bluff edges. Therefore, the sampling "universe" would usually be the portion of the colony that is accessible to observers.
Circular Plots: If the objective is to monitor trends, but not necessarily to estimate the overall population size, a series of permanent circular plots would be appropriate. Circular plots are easier to mark (one stake) and layout (a line of fixed diameter) than transects or other quadrates. Nevertheless, the configuration of a colony (e.g., a narrow coastal strip) may lend itself better to quadrates than circular plots in some cases. Like the transects referred to above, the diameter of the plots would be based on the species. Plots should be located subjectively to include areas of high, moderate and low density because the object is to be sure that both increases and declines are noted (i.e., choosing just high density plots could result in not seeing increases which are showing up in less saturated areas). For trend monitoring, at least 10 plots should be monitored in small colonies, and up to 20 plots are needed in large colonies. Plots should be photographed and mapped in such way to facilitate relocation. GPS coordinates for the center point also should be recorded. Center stakes need to be tall enough to be seen at a distance even when vegetation is tall and they should be of a material that will last (plastic, iron).
Important Considerations when setting up plots: Long-term population trend monitoring depends on standardizing as much as possible. It is critical to mark plots in such a way that they can be resurveyed in an identical manner. GPS coordinates could be used to get observers into the vicinity, but permanent obvious plot markers are essential. For circular plots or quadrates, locations should be selected within density strata to insure that areas with high, medium, and low density are included. Once colonies are delineated and crude abundance codes have been assigned to general areas, plots could subjectively located. The steps are as follows:
For index plots, the total number of burrows in all plots would be summed for the annual statistic. In addition, the mean occupancy rate would be calculated. Inter-year comparison would be made with non-parametric paired sample tests (probably the non-parametric Friedmann test for n > 3). Long-term trends would be characterized using regression.
Two parameters are of interest: 1) The number of burrow entrances in the index plots, and 2) the proportion of burrows that are occupied in the index plots. The purpose of trend monitoring is to detect changes in burrow-nester populations, and burrow-entrances are only an indication of all possible nest sites. Occupancy rate must be estimated to relate burrows to birds. A burrow is considered occupied if an egg or a chick is present. This statistic will vary among plots. It is essential to record the units being used.
Things that could bias your counts:
Observing burrows is much easier before vegetation becomes rank, but evidence of occupancy (see below) is not as prevalent as later checks. Most species of burrow-nesting seabirds are prone to abandon nests if disturbed early in incubation, but then tolerate disturbance much better after mid-incubation. Ideally, burrows should be counted in plots as early as possible, but not until incubation is well underway (for those species where observers will reach into nest chambers, and then rechecked after chicks hatch).
Advantages: Index plots to determine population indices likely provide the most cost- effective approach for monitoring burrow nesting species and do not require the same level of effort as methods to estimate total population . Monitoring for population trends in seabirds is becoming an objective of conservation biologists concerned with marine ecosystems worldwide. Seabirds are seen as relatively inexpensive indicators of change in this complex ecosystem. It is unlikely that "presence or absence" or "relative abundance" data will provide the kind of information most useful in the ecosystem approach to conservation. In Alaska, burrow-nesting seabirds have been identified as important indicators (U.S. Fish and Wildlife Service 1992). As such, trends will be used to track the response of target populations to natural and man-caused events. These data will be used with information gathered on other components of the ecosystem (e.g., forage fish, plankton, marine mammals, oceanography) to try to understand processes, an integral part of ecosystem management.
A double-observer approach can be used to determine detection probabilities. The first observer would walk the transect line or completely cover the study plot, marking each nest with an object such as a colored washer. If multiple species are present at a site, different colors should be used for the burrows of each species. Different colors should also be used to distinguish between occupied and unoccupied burrows. After the first observer has finished their survey, a second observer independently marks each burrow using a different set of colored washers. This process could be repeated by additional observers if necessary. After the surveys are completed, an additional person (or possibly more if the burrows are difficult to find in the terrain) would visit the transect/plot and indicate how many burrows were located by each observer, and how many were missed (or mis-classified) by one of the observers, and these data can be used to calculate detection probabilities.
Once you've used this technique to gather data, decide which approach to use to analyze your data.
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