The Fall Passage of Three Rare Western Warblers

In Santa Cruz County, California

Prepared by David L. Suddjian

Santa Cruz Bird Club Bird Records Keeper

September 11, 2000


 


Knowledge of the expected timing of migration is a key ingredient in finding and identifying birds. Together with information on a species’ geographic distribution, habitat requirements and relative abundance, the timing of migration provides the framework for when and where we should expect to find a species. Occurrences outside these expected parameters indicate especially noteworthy records and should also signal a need for extra caution in making identifications.

Here I present information on the timing of fall migration of three western migrants in Santa Cruz County, California: Nashville (Vermivora ruficappila), Black-throated Gray (Denroica nigrecens and MacGillivray’s (Oporornis tolmiei) warblers. Although they are not rare in the big picture, all three are rarely encountered during fall in Santa Cruz County. Even the most active birders usually find only small numbers each season. A summary of rare fall warblers in Santa Cruz County (http://santacruzbirdclub.org/recwarb.html) compares numbers of these three with those of other rare warblers.

Information was taken from the county record files for the years 1974-1999. I considered records from the period August 9-November 30 to illustrate the general timing and peaks of migration. Black-throated Gray and MacGillivray’s warblers both nest in the county, and some birds seen in early August likely pertain to local breeding birds rather than passage migrants. August records thought to fit into that category (mostly based on location) were not included. At the other end of the season, migration is known to extend beyond November 30 for a small percentage of individuals (especially Nashville and Black-throated Gray warblers). However, December records were excluded from this summary because they involve a significant proportion of over-wintering individuals.

Data was summarized using only the first known date for each record. Reports of more than one individual of a species at the same spot on the same date were treated as separate records. Reported coverage occurred in all of the 26 years considered here, but I used the 17 years with the most extensive coverage and reporting to derive average dates of first detection. These were 1974-1979, 1986-1988 and 1992-1999.

Nashville Warbler

The timing of the 87 fall records from 1974-1999 is shown on Figure 1. Records spanned August 30 – November 26, with 90% from September 13 – November 21. The average date of the first fall detection was September 23 (±11 days).
The next earliest record after August 30 was September 9. Records were scarce until late September. Migration peaked from September 24 through October 31, when 65% of all records occurred. Records were reported in November in 42% of the 26 years considered. Late passage migrants have been known to occur regularly in December and maybe even to early January. Three records graphed below were determined to be first known dates of over-wintering birds. These were first found November 13, 24 and 26.

Fall migrant Nashvilles were found primarily in deciduous riparian woodland, but also in coast live oak woodland, suburban areas and occasionally in other woodland and forest types. Fall records were very strongly oriented to the immediate coast and areas less than two miles inland, undoubtedly reflecting patterns of observer coverage to some degree.

Annual fall totals from 1992-1999 ranged from 3-11 birds, with an average of 6.5 per fall. 98% of all occurrences were of single birds, with two seen together just twice.

Figure 1. Timing of fall migrant Nashville Warblers in Santa Cruz County, CA. (n = 87).

 

Black-throated Gray Warbler

 
The timing of the 186 fall records from 1974-1999 is shown on Figure 2. Records spanned August 17 – November 26, with 90% from September 10 – November 12. The average date of the first fall detection was September 14 (±18 days).
August records were difficult to evaluate for this species, since it is a fairly widespread and numerous nesting species in the Santa Cruz Mountains. August records of birds believed to most likely pertain to passage migrants were reported in only 15% of the 26 years considered. After the August 17 record, the next earliest record was August 26. Migration increased steadily through September, and peaked from September 24 through October 31, when 70% of all records occurred. Records were reported in November in 46% of the years considered. Late passage migrants have been known to occur regularly in December and maybe even to early January. Six records graphed below were determined to be first known dates of over-wintering birds. These were first found September 23, October 14, November 19 (twice), 20 and 23.

Fall migrant Black-throated Grays were found primarily in deciduous riparian woodland, but also in coast live oak woodland, Monterey pine forest and planted woodlots, suburban areas and occasionally in other woodland and forest types. Fall records were very strongly oriented to the immediate coast and areas less than two miles inland, undoubtedly reflecting patterns of observer coverage to some degree.

Annual fall totals from 1992-1999 ranged from 5-33 birds, with an average of 16.4 per fall. Multiple birds were reported together on the same date on 24 occasions, mostly involving two birds, but also three birds together on six dates and four together on two dates.

Figure 2. Timing of fall migrant Black-throated Gray Warblers in Santa Cruz County, CA. (n = 186).
 

MacGillivray’s Warbler

The timing of the 52 fall records from 1974-1999 is shown on Figure 3. Records spanned August 10 – November 20, with 90% from August 21 – October 30. The average date of the first fall detection was September 10 (±20 days).
August records of birds believed to be migrants were reported in 26% of the 26 years considered. Migration peaked from September 9 through October 8 when 60% of all records occurred. Records were reported in November in just three years, two of which (November 15 and 20) were of birds known to have subsequently wintered.
Fall migrant MacGillivray’s were found primarily in deciduous riparian woodland or other areas with dense shrub cover, and occasionally in suburban areas. Fall records were very strongly oriented to the immediate coast and areas less than two miles inland, undoubtedly reflecting patterns of observer coverage to some degree.

Annual fall totals from 1992-1999 ranged from 1-9 birds, with an average of 4.0 per fall. 96% of all occurrences were of single birds at a given spot, with two seen together just twice.

Figure 3. Timing of fall migrant MacGillivray’s Warblers in Santa Cruz County, CA. (n = 52).