One of the least studied groups of birds in North America is the goatsuckers (Family: Caprimulgidae). The members of the family, which have a world-wide distribution (approximately 90 species), are better known outside of North and South America as nightjars, due to the "night jarring" sound they make (Jackson 1994). The name "goatsucker" is the literal translation of the family name.
The primary reason for the poorly known status of goatsuckers is their nocturnal and crepuscular habits. North American goatsuckers, and most others, are active by night or in the dusk and dawn period, limiting our ability to observe and study these birds in their natural setting. Although nighthawks sometimes forage or migrate during the day, goatsuckers are rarely observed during daylight hours. Relative to diurnal birds, much less is known about goatsucker basic biology and natural history. Further study of these birds is certainly warranted, but the techniques commonly used to study diurnal birds are inappropriate for nocturnal birds. Similar problems have been encountered for other nocturnal birds such as owls (Family: Strigidae), albeit to a lesser degree. Therefore, to study goatsuckers, one requires either a set of field techniques modified from those used for diurnal birds or techniques designed specifically for nocturnal species.
Two goatsucker species occur in British Columbia. These are the Common Nighthawk (Chordeiles minor) and the Common Poorwill (Phalaenoptilus nuttallii) (Campbell et al. 1990; Brigham 1994). Like other goatsuckers, both species rely on their cryptic plumage to remain undetected while roosting during the day. In the daytime, roosting goatsuckers are seldom detected by the casual or untrained observer. Thus, common bird census techniques are likely to produce poor estimates of presence and abundance information for these birds. To census this group of birds accurately, census and capture techniques for goatsuckers must be suited for use during the crepuscular and nocturnal periods of the day.
The purpose of this manual is to recommend methodologies that can be used to survey Common Nighthawks and Common Poorwills in British Columbia. This manual provides a standardized sampling protocol for obtaining presence and relative abundance estimates in the field in British Columbia.
Common Nighthawks (hereafter referred to as nighthawks) are most commonly seen feeding high in the air during the dusk and dawn hours. Like most other goatsuckers, nighthawks feed on flying insects, but unlike most other caprimulgids (e.g., poorwills) insects are captured or "hawked" during continuous flight. Nighthawks are well-known for the non-vocal "booming" calls which are thought to be produced by air rushing through the primary feathers of males. These sounds are presumably used for courtship display and territorial advertisement. The typical vocalization given by both sexes during level flight is a nasal "peent", somewhat similar to some sounds made by European Starlings (Sturnus vulgaris).
Nighthawks breed during the summer months from the southern Yukon across most of southern Canada to the southern United States and parts of Mexico and Central America (Campbell et al. 1990). Wintering ranges extend as far south as Argentina (American Ornithologist's Union 1983). Nighthawks have been reported to occur throughout British Columbia during the summer months, although sightings are rare west of the coastal mountains north of Vancouver Island (Campbell et al. 1990).
In British Columbia, nighthawks usually arrive in late May or early June. Breeding occurs shortly after arrival, and a single clutch, almost always consisting of two eggs, is laid in early June. Nesting habitat is diverse, and includes logged or burned areas of the coastal forests, open ponderosa pine forest, the grassland habitat of the semi-arid interior, and sand and gravel habitats of marine and fluvial beaches. Less common habitats include many human-made habitats such as farmland and pasture lands, old gravel pits, and even gravel roof-tops in urban areas (Cannings et al. 1987, although see Brigham 1989; Campbell et al. 1990). Nighthawks are frequently observed to aggregate in large numbers prior to fall migration, which occurs in late August or early September. Typically, nighthawks are among the last migrants to arrive on the breeding grounds and the first to depart. More detailed information can be obtained in Campbell et al. (1990) and Cannings et al. (1987) and shortly, the Birds of North America Account for the species currently being prepared by S.D. Grindal.
Common Poorwills (hereafter referred to as poorwills) are most famous for their ability to enter deep torpor and potentially even hibernate. Torpor is a physiological state in which metabolism is substantially reduced and as a result, body temperature declines, often to only a few degrees above ambient temperature, presumably to reduce the energetic costs of thermoregulation (Brigham 1992; Csada and Brigham 1992, 1994b). Jaeger (1948, 1949) reported bouts of torpor that lasted for extended periods, and concluded that poorwills may be capable of hibernating, although this has yet to be substantiated (although see French 1993).
Poorwills get their common name from the interpretation of the sound of their calls, which are a distinctive "poor-will" or "poor-will-up." To the best of our knowledge, these calls are given only by males, especially during the breeding season, presumably to attract a mate and/or advertise territorial ownership (Csada and Brigham 1992).
Unlike nighthawks, but similar to most other goatsuckers, poorwills forage for flying insects by making short sallies or 'leaps' from the ground (Brigham and Barclay 1992). Although they can be active at all times of the night, there is evidence that poorwills increase foraging activity during nights when moonlight is present (Brigham and Barclay 1992; Bender 1994). Evidence from diet studies seems to corroborate this (Csada et al. 1992; Bayne and Brigham 1995). There are no quantitative data on the influence of lunar condition on vocal activity by poorwills, something that may have direct implications for surveying.
Within Canada, poorwills normally occur only in the southern interior of British Columbia, and in the Cypress Hills region and adjacent areas of southwestern Saskatchewan and southeastern Alberta. These two areas represent the northernmost limits of the species breeding range. The southern breeding range extends into southern California, Guanajuato, and central Mexico. Within British Columbia, most breeding poorwills are found in the Okanagan valley. They have been recorded as far north as the Nicola and Thompson valleys, and as far east as the Kettle valley (Cannings et al. 1987; Campbell et al. 1990). However, this is a species that often turns up in places not previously recorded when active searching occurs (Brigham 1994). Poorwill nesting habitat appears very similar to that of nighthawks, and these two species are sometimes found in close proximity. The preferred poorwill nesting habitats are dry, open, grassy or shrubby areas on hillsides, but forested parkland margins and logged sites are also used (K. Wang, personal communication).
Poorwills arrive in British Columbia between late April and late May (Csada and Brigham 1994a). Two clutches of two eggs each are usually attempted each summer, with the first laid in late May or early June, and the second typically laid in mid- to late-July (Csada and Brigham 1994a). Like nighthawks, "nests" are on the ground, usually in a small depression or hollow, and consist only of a bare patch of ground with no nesting material present. Following the breeding season, most poorwills leave for their wintering grounds by mid-September (Csada and Brigham 1992, 1994a). Currently, little is known about poorwill migratory routes or behaviour. A complete review of the current knowledge about the basic biology of this species can be found in Csada and Brigham (1992).
Current monitoring strategies for landbirds (e.g., Ralph et al. 1993; Canadian Wildlife Service 1994) have focused primarily on monitoring diurnal bird populations. However, the methods employed in these monitoring programs are not adequate for monitoring crepuscular and nocturnal bird species. Thus, the only way to account for nocturnal species in these monitoring strategies is to incorporate new survey methods. Except for the techniques developed for surveying boreal owl populations, little has been done to standardize methods of monitoring nocturnal bird species. Recently, the Canadian Wildlife Service (CWS) identified some shortcomings of the present Canadian Landbird Monitoring Strategy, and has made recommendations to incorporate new methods suitable for censusing nocturnal birds (CWS 1994). However, it is important to point out that at present there are no standard survey methods or monitoring protocols for goatsuckers. Table 1 outlines recommended methods for sampling goatsuckers in British Columbia.
Table 1. Recommended methods for sampling goatsuckers in British Columbia at three levels of intensity.
| Level of intensity | Recommended method(s) |
|
Presence/Not detected |
Point counts at night with audio playback |
| Relative abundance | Point counts at night with audio playback |
| Absolute abundance | No recommended methods |
The following standards are recommended to ensure comparison of data between surveys, and to mitigate several sources of common bias.
A minimum amount of habitat data must be collected for each survey type. The type and amount of data collected will depend on the scale of the survey, the nature of the focal species, and the objectives of the inventory. As most, provincially-funded wildlife inventory projects deal with terrestrially-based wildlife, the terrestrial Ecosystem Field Form developed jointly by MOF and MELP (1995) will be used. However, under certain circumstances, this may be inappropriate and other RIC-approved standards for ecosystem description may be used. For a generic but useful description of approaches to habitat data collection in association with wildlife inventory, consult the introductory manual, Species Inventory Fundamentals (No.1).
The best time of the year to census goatsuckers is early in the breeding season when birds are most vocal. For populations in British Columbia, nighthawks should be censused in mid- to late-June during the early nesting period. Poorwills should be censused in mid- to late-May just prior to egg-laying. After egg-laying, male poorwills often reduce or completely suspend all calling (Kalcounis et al. 1992; D. J. Bender, pers. obs.), and surveys based on listening for vocalizations are probably less accurate.
Do not survey goatsuckers during the following weather conditions:
Nighthawk and Poorwill surveys follow a survey design hierarchy which is structured similarly to all RIC standards for species inventory. Figure 1 clarifies certain terminology used within this manual (also found in the glossary), and illustrates the appropriate conceptual framework for a point count survey. A survey set up following this design will lend itself well to standard methods and RIC data forms.
Figure 1. RIC species inventory survey design hierarchy with examples.
The table below outlines the type of surveys that are used for inventorying Common Nighthawks and Common Poorwills for the various survey intensities. These survey methods have been recommended by wildlife biologists and approved by the Resources Inventory Committee.
Table 2. Types of inventory surveys, the data forms needed, and the level of intensity of the survey.
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| Point Counts |
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* PN = presence/not detected (possible); RA = relative abundance; AA = absolute abundance
