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Preliminary Analyses and Progress Reports

Trend Analyses

Spring

Trend_Spring2022 and Charts_Spring2022 provide the results of trend analysis on species monitored at Inglewood Bird Sanctuary through 2022. In Charts_Spring2022 the y-axis shows the annual abundance index estimate in captures per day. This only pertains to the 95%-migration window, so it should only be used to compare abundances between species within their respective migration windows. To compare overall abundances between species one should refer to the season totals.

2002-2022 (all-time)

AMRO, SWTH and YEWA continue to have significant negative all-time trends. GRCA went from significant to only near significant (p=0.06), likely a result of lacking analysis power due to the low number of overall captures for the species.

In general, the strongest (lowest p-values) and most negative trends are found among warblers and thrushes which is consistent with findings from the analyses for Fall.

2013-2022 (10-year)

LISP continues to have a significant positive 10-year trend and RWBL now also shows a significant positive 10-year trend.

In contrast to the all-time Spring trends there are no significant negative 10-year trends. With 19 captures in 2022 COYE almost doubled the amount of captures from the previous 9 years, resulting in a near significant positive trend for the species.

Fall

Trend_Fall2022 and Charts_Fall2022 provide the results of trend analysis on species monitored at Inglewood Bird Sanctuary through 2022. In Charts_Fall2022 the y-axis shows the annual abundance index estimate in captures per day. This only pertains to the 95%-migration window, so it should only be used to compare abundances between species within their respective migration windows. To compare overall abundances between species one should refer to the season totals.

1995-2022 (all-time)

TEWA has been added back to the list of warblers (OCWA, WIWA) with significant negative trends. WCSP, WTSP and BAOR continue to show significant negative trends.

It was surprising to only see 2 SWTH captures for the entire Fall season. Numbers for TRFL were also very low (11 captures) compared to historic averages. Both species moved closer to a significant negative all-time trend.

2013-2022 (10-year)

TEWA, OCWA (new) and WIWA make up the trio of warblers with significant negative 10-year trends. LEFL is a new addition to the significant negative 10-year trends. Notably all of these are insectivores.

Above average captures for CEDW (63) and HETH (3) in 2022 reduced their 10-year trends to be no longer significant.

Overall, the data suggests all-time declines for most species. This is likely be due to a combination of population declines and site selection by the species during migration. Declines continued to occur over the last ten years for most species but there may also be some species that are stabilizing or possibly recovering. Positive trends (though not significant) were most often found among sparrows in recent years. This could imply that the banding site became more attractive to them after the catastrophic 2013 flood.

Methodology

Trend analysis is based on total captures from 1995-2022 for fall migration and 2002-2022 for spring migration and was performed by contract biostatistician Mathias Fenton. In addition to trend analyses since inception Mathias has calculated trends over the last 10 years a time period frequently used by conservation regulators and managers.

95%-migration windows were determined for each season (spring/fall) based on all new capture records for the respective species. A 95%-migration window describes the period in a specific season within which 95% of all individuals of a species were caught historically. Trend analyses were restricted to these windows This technique reduces the amount of days with zero captures and avoids disproportional impact of unusually early or late captures on the analyses.

Daily captures were first standardized with respect to daily net hours to account for variation in net opening times. Net hour totals only included focal nets, which were defined as nets with at least one historic capture. After standardization the captures were normalized to represent the expected number of captures for each day under the condition that all focal nets were open for exactly six hours. For days where all focal nets remained closed, no capture data was provided to the subsequent model to reduce false negatives.

Normalized captures from all years were used in a single hierarchical model that modelled the daily captures as a function of year, day, and day2. The model was implemented in R using the library for Integrated Nested Laplace Approximation (INLA). The observation error distribution was specified as ‘negative binomial’ and auto-regressive process error terms were included for day and year. To obtain mean estimates for the trend associated with year as well as for the average daily abundance within a season (annual abundance index), the posterior probability distributions produced by the model were sampled 10,000 times. Confidence limits and significance levels for each estimate were calculated based on the distribution of samples. Because the model provides linear trend estimates for the logarithm of captures, percent population changes per year were obtained through back-transformation.

Trend Analysis Charts – Fall 2022