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BirdSource Feature

Population Trends in Evening Grosbeak

Christmas Bird Count data show changes in
Evening Grosbeak numbers between 1960 and 1998.

The Evening Grosbeak (Coccothraustes vespertinus)photo of male Evening Grosbeak(detail) by William Paff slide# 647.1 breeds throughout the northern forests of Canada and Northeastern United States, as well as at higher elevations in the West into central Mexico. During certain winters, they move to more southerly latitudes in the East, or to lower elevations in the West. These winter irruptions typically begin in early October, and flocks of Evening Grosbeak are conspicuous where they regularly aggregate at bird-feeding stations.

The National Audubon Society's 100-year-old Christmas Bird Count (CBC) is an excellent tool for tracking such irruptions as well as for monitoring long term bird population trends. The CBC is particularly important in tracking birds that breed in remote regions but that winter in areas that are more accessible. Since much of the breeding range of the Evening Grosbeak is inaccessible, winter surveys like the CBC provide the best indicator of the health of the species.

For this analysis, we measured Evening Grosbeak abundance obtained from CBCs between 1959 and 1998. Not all the counts held during this period were analyzed. Instead, only counts that occurred during at least half of the 40 years of the analysis period were included. This way, we reduced the bias resulting from new counts that started during the later years of the analysis period. To minimize bias due to varying observer effort, the total number of birds observed during any CBC was divided by the total party hours for that count, thereby allowing a standardized birds/party hour (BPH) comparison. For each year, the average BPH was calculated from all CBCs that were above 30 latitude. The annual average BPH was graphed, and a linear regression was calculated in order to look for indications of continuous increase or decrease in abundance of grosbeaks over the time period in question. This same method was also used for within-state or -province analysis.

Graph 1: The mean number of birds/party hour between 1959-1998. The Y axis is the mean birds/party hour, and the X axis is year.

The results from the CBC clearly show that the Evening Grosbeak is an irruptive species across much of North America. What is apparent is that Evening Grosbeak exhibits a biennial pattern of irruption (Graph 1); years where they were plentiful were followed by years when low numbers were reported. When Evening Grosbeak did irrupt, they invaded different regions (Figure 1). This is best exhibited by the following animated map (Figure 1), which shows hot spots of Evening Grosbeak abundance. Note how the regions where the highest BPH values (gold coloration) vary from year to year.

Figure1: Animated map of Evening Grosbeak distribution between 1988 and 1998.

time period

59-98

60-79

80-98

60-69

70-79

80-89

90-98

Evening Grosbeak

0.0177

0.2323

*

-0.5988

***

0.0005

-0.0278

-0.0114

-0.747

***

Table 1: Linear Regression R2 values. The (-) preceding an R2 square value indicates a negative trend. Significance Levels are- * = p<0.05, ** = p<0.01, ***= p<.0001.

Graph 1 also shows that Evening Grosbeak numbers were stable or increased until 1980 when their numbers began to decline significantly. Table 1 shows the results of the regression analysis. The decline of Evening Grosbeak numbers between 1980 and 1998 was significant (p<0.001) which indicates that there is less than one chance in one thousand that the decline in Evening Grosbeak numbers was due to random fluctuations in abundance. Table 1 also indicates that the rate of decline has increased between 1990 and 1998. Figure 2 shows which state or province exhibited the most significant declines in Evening Grosbeak between 1980 and 1998. What is clear from Figure 2 is that the Northeast and Great Lakes region show the steepest declines in Evening Grosbeak numbers. On the other hand, Evening Grosbeak numbers appear stable in the Rocky Mountain region.

Figure 2: Map showing where Evening Grosbeak numbers are exhibiting the most serious declines between 1980 and 1998.

The cause of the decline in Evening Grosbeak numbers is unknown, but there are several possibilities. The most obvious is that Evening Grosbeaks may simply not be moving as far south during the winter due to the hemispheric trend in warmer winter temperatures. The declines might also be related to food availability. Hardwood tree seeds, a favorite natural source of food of Evening Grosbeaks, may be less common due to broad-scale changes in forestry practices in Canada. Finally, Evening Grosbeak numbers in the East may be stabilizing after their colonization of the north woods east of the Great Lakes. Prior to the late nineteenth century, the Evening Grosbeak did not occur east of the Great Lakes; since then they have expanded rapidly across Ontario, Michigan, Quebec, the Canadian Maritimes, and New England.

Whatever the reasons for the decline in Evening Grosbeak numbers, it is through long term continent-wide citizen-based monitoring projects like the CBC and Project FeederWatch that the health of wild bird populations can be assessed. This monitoring can both sound an alarm before the situation becomes dire as well as lead to a greater understanding of overall environmental changes that may be the cause of dramatic changes in bird populations. Another BirdSource project, the Irruptive Bird Survey, is tracking the abundance of winter food across the continent, in order to determine whether irruptive migrants vary their wintering areas by according to natural food abundance. Thank you to all who have contributed to these important projects. We encourage you to continue to participate.

— Steve Kelling

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