Irruptive Land Birds
on the Santa Cruz County
Christmas Bird Count, 1957-2000
Prepared by David L. Suddjian
Santa Cruz Bird Club
Bird Records Keeper

16 January 2001
Introduction
 
“Irruptive” land birds – those species that may be numerous one year, but sparse the next – add flair to each winter season. Ten such species occur regularly in the Santa Cruz area: Red-breasted Nuthatch (Sitta canadensis), Golden-crowned Kinglet (Regulus satrapa) Hermit Thrush (Catharus guttatus), American Robin (Turdus migratorius), Varied Thrush (Ixoreus naevius), Cedar Waxwing (Bombycilla cedrorum), Red Crossbill (Loxia curvirostra), Pine Siskin (Carduelus pinus), American Goldfinch (C. tristis) and Evening Grosbeak (Coccothraustes vespertinus). Some, such as the crossbill and grosbeak, are real “boom or bust” species, present in irruptive numbers one year but absent the next. Others, such as the robin, are always present and may be common, but over the years their numbers exhibit dramatic rises and ebbs.

The Christmas Bird Count (CBC) is the only consistent, large-scale effort that measures the local status of these species each winter. The Santa Cruz County CBC has been conducted annually since 1957. In this note I examine the annual early winter abundance of these irruptive species over the count’s 44-year history. I also compare the occurrence of four of these species (Red-breasted Nuthatch, American Robin, Red Crossbill and Pine Siskin) on the Santa Cruz County CBC with their occurrence on the neighboring Año Nuevo, Palo Alto, Moss Landing and Monterey Peninsula CBCs for the 16-year period from 1983-84 to 1999-2000.

Data was taken from the results of the 57th through 100th CBCs, published by the National Audubon Society (Field Notes vol. 12-25, American Birds vol. 26-47, Field Notes vol. 48-51 and American Birds un-numbered vol. published 1998-2000).  Unpublished results of the Santa Cruz County CBC from 1975, 1976, 1996 and 1997 were obtained from the records of the Santa Cruz Bird Club. Data for the Año Nuevo CBC in 1988-1989 was not published and was not included here.
 

The Count Circles
 
Santa Cruz County’s 15-mile diameter count circle is centered at Henry Cowell Redwoods State Park. The circle encompasses coastal areas from Davenport to New Brighton State Beach, and extends inland to include Bonny Doon, Boulder Creek and Scotts Valley. The circle includes much of Ben Lomond Mountain, the San Lorenzo Valley and the entire main stem of Soquel Creek. Elevations range from sea level to 2,800 feet. Habitats are diverse within the circle, but a large portion is occupied by varied conifer and broadleaf evergreen forests, providing good resources for irruptive land birds. Principal conifers include coast redwood (Sequoia sempervirens), Douglas fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), knobcone pine (P. attenuata), and plantings of Monterey pine (P. radiata). Broadleaf forest trees include tanoak (Lithocarpus densiflora), coast live oak (Quercus agrifolia), interior live oak (Q. wislizenii), madrone (Arbutus menziesii), California bay (Umbellularia california), California buckeye (Aesculus californica) and bigleaf maple (Acer macrophyllum ). Other habitats include riparian woodland (red alder [Alnus rubra], Black cottonwood [Populus balsamifera], willow [Salix spp.], box elder [Acer negundo], big leaf maple, western sycamore[Platanus racemosa]), grassland, chamise (Adenostoma fasciculatum) and manzanita (Arctostaphylos spp.) chaparral, coastal scrub, eucalyptus stands, agricultural fields, orchards, wetlands, littoral habitats, rural residential, suburban and urban development and city parks.

The Año Nuevo count circle is centered 20.75 miles northwest of the center of the Santa Cruz County circle. It also contains substantial areas of conifer and mixed evergreen forest. The Palo Alto count circle is centered 26.75 miles north-northwest of the Santa Cruz County circle. It is entirely inland and has less conifer forest, but does have extensive oak woodland, mixed broadleaf evergreen forest and a well-developed urban forest. The Moss Landing count circle is centered 19.5 miles east-southeast of the Santa Cruz County circle. Its conifer forest is limited to the northeast portion of the circle, but it also has significant areas that have been planted with Monterey pines and other conifers. The Monterey Peninsula count circle is centered 35.25 miles south-southeast of the Santa Cruz County circle. It lies across Monterey Bay from Santa Cruz and includes large areas of native Monterey pine forest.
 

Data Analysis and Changes In Observer Effort and Coverage

The statistic “total party hours” (i.e., the sum total of diurnal hours spent in the field by all counting groups) provides the most useful measure for standardizing the data for the species considered here, normalizing the data to account for changes in observer effort over the years. The graphs for each species shown below portray data that has been standardized by total party hours. Of course, not all peaks in the annual rise and fall of a species’ numbers are interpreted as irruptions. The somewhat arbitrary definition of an “irruption” used here is a count >150% of the standardized average for a species. Some major irruptions far exceed that measure.

Observer effort changed greatly over the Santa Cruz County CBC’s history (Figure 1). The number of observers and hours spent in the field have increased from the count’s early years to the present. Along with increased effort came more extensive and thorough coverage of the count circle, better knowledge of where to find birds and improved identification skills (referred to below as “birding knowledge”). These factors have undoubtedly affected the results of the count in ways which cannot be simply adjusted for, as may be done for changes in field effort.
 
 

Figure 1. Observer effort increased over the history of the Santa Cruz County CBC.
 
 

Effort, coverage and birding knowledge made significant leaps on the Santa Cruz County CBC in the mid 1970’s and again in the late 1980’s. For the species considered here, improvements in coverage and birding knowledge probably elevated counts of most species in most years, especially species which favor particular habitats or resources (e.g., Red-breasted Nuthatch and Evening Grosbeak) and those which may be easily overlooked (e.g., Golden-crowned Kinglet). Thus, in the count’s later years all species were found more consistently and at least low levels of abundance were recorded most in most, as even minor incursions were more readily detected. Substantial population irruptions and ebbs are still quite evident in the data set, although their perceived magnitude was likely affected by improved coverage and knowledge.

 

I do not know the detailed history of the four neighboring counts as well, but I suspect it parallels that of the Santa Cruz County CBC with regard to improved area coverage and birding knowledge. Levels of overall field effort were substantial for all these counts during the period of comparison (Figure 2).  Año Nuevo had the lowest TPH (avg 140 hours). Effort at Monterey Peninsula (avg 191 hours) was generally similar to Santa Cruz County (avg 192 hours). Palo Alto and Moss Landing had higher levels of effort (avg 235 hours for both).
 
 

Figure 2. Field effort on the neighboring CBCs for 1983-1984 to 1999-2000.

 

All CBCs are conducted from mid-December to early January. The Santa Cruz County CBC was conducted between 27 December and January 2 during 1957-1970, and from December 14-22 from 1971-2000. From 1983-1984 to 1999-2000 the Año Nuevo count was conducted early in the first week of January, Palo Alto during the third week of December, Moss Landing on January 1 and Monterey Peninsula in the last week of December.

 

Part II

 

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