Golden-winged warbler (Vermivora chrysoptera) COSEWIC assessment and status report: chapter 7

Population sizes and trends

Search effort

Researchers at the Cornell Lab of Ornithology in Ithaca, New York are carrying out a Golden-winged Warbler Atlas Project (GOWAP) that aims to determine the range of acceptable habitats and area requirements (if any) of Golden-winged Warblers, especially in relation to natural vs. human-created habitat types; map the frequency of hybridization with Blue-winged Warblers throughout the range of Golden-winged Warblers; and define the parameters that constitute a “safe haven” site for Golden-winged Warblers in the regions of coexistence with Blue-winged Warblers

The Breeding Bird Survey (BBS) has approximately 4,400 active routes throughout Canada and the United States with roughly 2,900 routes being surveyed annually. Each route is 39.2 km long with 3-minute point counts every 0.8 km (Sauer et al. 2005). BBS data represent the most comprehensive data set in North America for analysis of bird population trends.

The entire Golden-winged Warbler breeding range falls within the BBS coverage area. However, several caveats in using these data should be acknowledged: 1) in areas of hybridization between Blue-winged and Golden-winged Warblers, hybrids and phenotypic Blue-winged Warblers may also sing the Golden-winged Warbler song and visual confirmation is not required (Sauer et al. 2005); 2) later generation hybrids often show only subtle signs of introgression meaning that their classification would be, incorrectly, to the parental species; and 3) BBS routes are limited to areas with roads and experienced observers, leading to inadequate sampling of the species in some areas.

The Ontario Breeding Bird Atlas (OBBA) also conducts surveys to determine prevalence of Golden-winged Warblers in the province, using coverage of 10-km squares as well as random point counts. Data currently available from the second OBBA come from a search effort about 85% of that of the first atlas so comparisons made between the two should accurately reflect changes in abundance of the Golden-winged Warbler (M. Cadman, pers. comm.). However, the first OBBA (1981-1985) treated the Golden-winged/Blue-winged Warbler complex differently than the second (2001-2005). As with the BBS, “heard only” records were used for the first atlas, while now only records that actually involve a sighting of the singing bird are included. This makes comparisons between the two atlases more difficult (M. Cadman, pers. comm.) although the data are likely still indicative of population trends.

 

Abundance

Estimates of population size are “best guesses” and are based primarily on BBS data which have limitations (discussed above) – actual numbers of Golden-winged Warblers in Canada are yet to be confirmed and this can only be accomplished by intensive surveys in a wide variety of terrains and locations.

The global population size is estimated at 105,000 to 270,000 breeding pairs (K. Rosenberg, pers. comm.; Partners in Flight (PIF) data; Sauer et al. 2005).  Forty percent of the population is thought to breed in Minnesota, with 69% of all the birds being found in Minnesota, Michigan and Wisconsin. Canada currently supports an estimated 18.5% of the breeding population (N = 19,425 - 49, 950), with 18.2% of the global population being found in Ontario (N = 19,110 - 49,140; P. Blancher, pers. comm.; BBS data, Sauer et al. 2005).  

 

Fluctuations and trends

Declines have been taking place for over 30 years (Gill 1997; Sauer et al. 2005) with the current rate of decline being estimated at 3.4%/year (P = 0.00001, 241 routes) in the United States, with declines as steep as 8.5% /year (P = 0.000, 127 routes) being seen in USFWS Region 5 (see Table 2 for list of States; Sauer et al. 2005). The species is currently experiencing an overall (i.e. U.S. and Canada combined) decline of 2.4%/year (P = 0.001, 271 routes), making the Golden-winged Warbler one of the most vulnerable and steeply declining of all North American passerines.

In Canada, BBS data between 1968 and 2002 indicate a significant annual increase of 8.9% (p < .05, 47 routes), but there has been a dramatic 79% decrease in the last ten years (-14.4%/year, P < .05, 37 routes, 1993-2002; CWS 2005).

An Ontario Breeding Bird Atlas is currently in the final year of data collection (i.e. 2005) which will provide an up-to-date picture of what the trends are for this species in Ontario. In the first Ontario BBA (1981-1985) Golden-winged Warblers were located in 469 10-km squares, whereas data collected for the second atlas thus far (2001-2004) reports them in 278 squares. Their range has thought to have contracted in 200 squares (i.e. were found in these squares in the first atlas, but were not located in the same squares during the second atlas), and has expanded in 118 squares. There is an overlap between the first and second atlases in 160 squares (34%).

In contrast, the Blue-winged Warbler occurred in 139 squares in the first atlas, and has been recorded in 196 squares in the second atlas thus far. In addition, while the Blue-winged Warbler has contracted its range in 47 squares, it has expanded in 123 squares. These data suggest that Golden-winged Warblers are decreasing in many areas of Ontario, while Blue-winged Warblers and their hybrids are increasing (BBA data supplied by M. Cadman, pers. comm.).

Banding data from the Long Point Bird Observatory (LPBO, Port Rowan, ON) show that the number of captured Golden-winged Warblers increased until the mid-1980s then decreased (Table 1). The number of Blue-winged Warblers banded at LPBO has also been increasing over the past thirty years, and at a much greater rate than Golden-winged Warblers. Indeed, there has been a change in the ratio of Golden-winged Warblers to Blue-winged Warblers from 2.4:1 to 0.14:1 over a period of thirty years (LPBO data; J. McCracken, pers. comm.; Table 1).

 

Table 1. Banding data from Long Point Bird Observatory (LPBO) for Golden-winged (GWWA) and Blue-winged Warblers (BWWA) from 1960-2004 (LPBO data provided by J.D. McCracken).
9-year period Number Banded Ratio
  GWWA BWWA (GWWA:BWWA)
1960-1968 12 5 2.4:1
1969-1977 12 8 1.5:1
1978-1986 35 65 0.54:1
1987-1995 61 182 0.34:1
1996-2004 25 173 0.14:1

In summary, Golden-winged Warbler numbers are declining overall. Declines are less pronounced in Canada than they are in the U.S., but the rate of decline is now increasing in many areas of the country, especially in Ontario. Some areas may still be experiencing increases associated with their range expansion (e.g. Saskatchewan and Manitoba) but this is not necessarily indicative of them having greater success in these areas than in others and is likely associated with, at least in part, their allopatric status.

 

Rescue effect

The ongoing range expansion of Golden-winged Warblers into Canada means that populations will likely continue to increase in some areas. However, whether these populations can sustain the species in Canada will depend on both habitat availability and arrival of Blue-winged Warblers. The Golden-winged Warbler may persist in refugia at higher elevations and latitudes, and perhaps in swamps where contact with Blue-winged Warblers is limited (Gill 2004).

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