Long-term monitoring data reveals precipitous declines in local thrush populations inhabiting the Chocorua Lake Basin (NH)

By Harriet P. Hofheinz and Henry C. Stevens

Executive Summary

Established in 1968, the Chocorua Lake Conservancy continues to conserve the natural resources of the Chocorua Lake Basin in the lower White Mountains of New Hampshire, an ideal location for breeding birds to feed and nest. In the 1980s, the originator and organizer of this survey, Harriet P. Hofheinz, became concerned about the decline of Neotropical migratory songbirds, particularly in the lake basin area. To investigate if their populations were indeed declining, she commenced a breeding bird survey explicitly for the three thrush species that consistently breed in the area: the Hermit Thrush (Catharus guttafus), the Veery (Catharus fuscescens), and the Wood thrush (Hylocichla mustelina). If at the end of the survey, her concerns were born out, one could assume that other Neotropical songbirds in the vicinity were also being negatively affected.

The Breeding Thrush Survey (BTS) project started in 1993. Each year, volunteer teams were organized, trained on the bird calls, and then sent into the field to collect data on the three species. From 1993 through 2018, the BTS, modeled after Audubon’s Breeding Bird Survey, aimed to document a snapshot of the thrush population inhabiting the Chocorua Lake Basin during the height of their breeding season.

Each June, volunteers surveyed five routes covering Chocorua conservation lands at sunrise and at dusk, recording all Hermit, Veery and Wood Thrush heard or seen. And indeed over these past 25 years, statistical analysis shows significant declines in Veeries and Wood Thrushes, with the Wood Thrush experiencing a sharper decline than Veeries. The Hermit Thrushes have also shown decline over time, though the negative trend was considered not significant. Data on Wood Thrushes in recent years may indicate potential exterpation in the Chocorua Lake Basin. These results highlight both an urgent need to focus conservation efforts on reducing further declines in this family of songbirds and other Neotropical species, and the utility of citizen science work.

Background

Hermit Thrushes, Veeries, and Wood Thrushes are insectivorous songbirds that prefer moist, deciduous woodland in which to breed and rear their young. Wood Thrushes have the most compact breeding range of the three species, extending from the East coast only to the Midwestern US. Veeries breed from NE to Washington State, extending north into southern Canada, British Columbia and Newfoundland, and the breeding range of the Hermit Thrush expands beyond that. Wood Thrushes and Veery winter in the Central American tropics, while the Hermit Thrushes spend their winters in southern parts of the US and the Caribbean.

Hermit Thrushes and Veeries have comparatively larger breeding ranges and habitat tolerances than Wood Thrushes, earning them each an IUCN conservation status of Least Concern. Because of Wood Thrushes’ smaller breeding range, possibly more vulnerable breeding grounds, and loss of wintering habitat, these birds have suered more than their conspecifics, and are currently listed as Near Threatened according to IUCN. All three species over the time of this study have experienced population declines across their range.

Methods

To document thrush populations in the Chocorua Lake Basin, surveyors conducted three to five-minute point counts on five predetermined routes along trails spread throughout the Basin (Figure 1) covering approximately five miles in total. The survey focused only on the Hermit Thrush, the Veery and the Wood Thrush, although occasionally a Swainson’s Thrush (Catharus ustulatus) was heard, but not counted. On each route, point-counts were spaced 300 meters apart and each route was surveyed twice a day. Morning surveys were between approximately 04:30 and 07:30, and evening surveys between 18:00 and 21:00. To account for bad weather, surveys were conducted on at least two days on the second or third weekend of June. Therefore, each route was surveyed multiple times per year. In compiling the dataset, however, only the survey date with the highest number of individual birds recorded was selected to represent that year.

To analyze the dataset in Table 1, we used basic linear regression analyses in R for each species. The F-statistic parameter is simply the measure used by the statistical analysis itself and is used to determine the final p-value. Adjusted R-squared values explain the proportion of variance for the number of total thrushes for each species (dependent variable) that is explained by time (independent variable) measured in years. In other words, higher R - squared values indicate that changes in number of thrushes are mostly due to time and not merely by chance. Df simply lists the total degrees of freedom used in each analysis. Resulting p-values below 0.05 indicate a significant, negative trend in population over time.

Figure 1. The Map of the Chocorua Lake Basin depicts the five different survey routes used to document the three Thrush species.

Results

The Regression analyses showed that Veeries and Wood Thrushes have experienced significant, negative declines over the 25-year period from 1993 to 2018 (Table 1). Hermit Thrushes also experienced negative declines, though the trend was not significant according to a 95% confidence interval. The total number of individual Hermit Thrushes recorded varied more between years than either the Veery or Wood Thrushes (Figure 2).

Table 1. Results of regression analyses for each of the 3 species surveyed, Hermit Thrush (HETH), Veery (VEER), and Wood Thrush (WOTH). An asterisk indicates a significant p-value.

Species F-statistic Adjusted R-squared Df p-value

HETH. 3.903 0.104 24 0.05981 VEER 8.43 0.2291. 24 *0.00779 WOTH. 38.88 0.6024 24 *0.00001

Figure 2: Total Individual count of 3 Thrush species

1993

Figure 2. The total number of individual Hermit Thrushes (HETH), Veeries (VEER), and Wood Thrushes (WOTH) recorded each year from 1993 to 2018, For each year, there are 3 corresponding data points. HETH counts are blue, VEER are green and WOTH are red. Dotted lines represent linear trend lines for each species.

Concluding Notes

As anticipated by the originator of this survey, local thrush populations have decreased in the Chocorua Lake Basin. The Wood Thrush population suffered particularly precipitous declines, with 2012 and 2014 yielding zero individuals, and subsequent years holding no more than a handful. Given the protected status of the forests within the basin, we speculate that the main drivers of thrush declines may be occurring in their wintering grounds in Central America or somewhere along the flyway during migration. Factors negatively affecting thrush populations are likely the same ones affecting Neotropical migratory songbirds across North America—climate change, habitat alterations/ loss, and the associated continental decline in insects, which are crucial not only for adult survival, but for successful rearing of young as well.

This study revealed concerning population trends for our New England thrush species and most likely mirrors other breeding Neotropical species. It also highlights the utility of citizen science data in identifying natural phenomena. Such citizen science and local monitoring schemes are often some of the longest-term datasets on wildlife available in a region, and therefore are invaluable in targeting conservation issues and maintaining an accurate understanding of ecosystem health. As human-induced disturbances continue to exacerbate already worsening environmental conditions and conservation funding remains stagnant, we may rely increasingly more on citizen science data to improve habitat and wildlife management strategies and track how different species are responding to these challenges and changes.