Survey Methods
Rooftop surveys (which gave us visual access to the sample nests) were conducted every two weeks. Sample nests were chosen for their observability from the rooftop all season, even after leaves filled out in the tree canopy. We started by counting every nest we could, then ruled out any nests that were hidden as the foliage grew. Many of our results are derived from data collected from rooftop surveys, assuming that the sample is representative of the whole colony.
Ground surveys were conducted every two weeks until April, when we reduced the frequency to once a month due to foliage growth, which greatly reduced visibility. These ground surveys were the best opportunity to count all nests throughout the colony , including the ones not visible from the rooftop.
(Photo: Heron Cam/Vancouver Park Board)
Survey Limitations
We note the survey limitations we face each year, which give necessary context when interpreting the figures reported in this report.
Sample nests were chosen for their visibility through the season, from proximity to rooftop vantage and lack of foliage concealment. Foliage growth greatly limited our ability to choose samples randomly, despite random sampling being the ideal for this study. We chose to sample all observable nests in the interest of having the largest possible sample size (and therefore more data gathered), despite possible factors that affect the selected nests’ representativity of the whole colony. For example, the sample nests could be more susceptible to predation because of their higher visibility. These samples nests are often the same each year due to their position, however, some of these nests fall or are dissembled while new ones are built elsewhere.
Fledgling counts are assumed between surveys. After chicks reached 10 weeks, we considered them as fledged if they were gone between one survey and the next. This introduces some probability that chicks counted as fledged may not have survived (e.g. fell from the nest, predated by eagles). We partly mitigated this by recording the number of dead, fallen chicks collected by Park Board staff during the fledging period.
While these limitations are present in our study, this colony continues to provide some of the best collected datasets of Pacific GBHE due to easy access. We are able to run these surveys with high frequency through the season, and the results provide some indication of regional trends, especially when compared with results from other colonies along the Georgia Strait.