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. We base our survey methods on the survey protocol developed for the Ardea Herodias fannini (Pacific Great Blue Heron) species developed by Vennesland & Norman for the Heron Working Group in 2006.
Survey area: As of 2023, our survey area encompasses the entire Stanley Park heron colony. Some nests become obscured over time due to foliage growth as the season progresses, while others remain visible throughout the season. These nests may not have comparable outcomes. 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.
Definition of successful fledging: As per Vennesland & Norman (2006), chicks are assumed fledged when fledging behaviour is observed or when chicks reach 4-6 weeks of age and then their nests become obscured by foliage. We subtract any known chick deaths from specific nests that are reported to us. We are limited by the information we receive about predation and mortality of chicks.
Nests that remain visible through the entire survey season may have a more accurate assessment of fledging numbers than nests that become partially obscured. However, the assessment of fledging based on the above criteria is established in literature and used in other surveys of the Pacific GBHE in our region. When we use standardized criteria, we avoid underestimating fledging counts and ensure our numbers are comparable to other surveys.
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.