Knowledge Gained from Video-Monitoring Grassland Passerine Nests

Pamela J. Pietz, Diane A. Granfors, and Christine A. Ribic

Abstract. In the mid-1990s, researchers began to adapt miniature cameras to video-record activities at cryptic passerine nests in grasslands. In the subsequent decade, use of these video surveillance systems spread dramatically, leading to major strides in our knowledge of nest predation and nesting ecology of many species. Studies using video nest surveillance have helped overturn or substantiate many long-standing assumptions and provided insights on a wide range of topics. For example, researchers using video data have (1) identified an extensive and highly dynamic predator community in grasslands that varies both temporally (e.g., by time of day, nest age, season, year) and spatially (e.g., by habitat, edge, latitude); (2) shown that sign at nests is unreliable for assigning predator types and sometimes nest fates; (3) contributed to the understanding of the risks and rewards of nest defense; and (4) provided information on basic breeding biology (e.g., fledging ages, patterns of incubation and brooding, and male/female roles in parental care). Using examples from grasslands, we highlight accumulated knowledge about activities at the nest documented with video surveillance; we also discuss the implications of this knowledge for our understanding of avian ecology. Like all tools, video nest surveillance has potential limitations, and users must take precautions to minimize possible sources of bias in data collection and interpretation.

Key Words: avian behavior, breeding ecology, camera, grassland, nest monitor, nest predators, passerine, video surveillance.

In the 1990s, the plight of grassland birds received increased attention (Johnson and Schwartz 1993, Knopf 1994, Johnson and Igl 1995), as researchers began to recognize that grassland species were showing "steeper, more consistent, and more geographically wide-spread declines than any other behavioral or ecological guild" of North American birds (Knopf 1994:251). Many grassland passerine populations had been declining for decades (Peterjohn and Sauer 1993, Herkert 1995, Igl and Johnson 1997), and it was thought that high rates of nest predation could be contributing to these declines (Basore et al. 1986, Martin 1993). At that time, there were few data on the identity of nest predators of grassland passerines. Predator sign at grassland duck nests had been studied intensively (Sargeant et al. 1993, 1998); however, at passerine nests, assignment of nest fates and identity of predators were usually based on assumptions (Best 1978, Wray et al. 1982, Vickery et al. 1992). Often, when a passerine nest was revisited, only an empty bowl remained, with few or no clues as to what had happened (Hussell 1974, Major and Gowing 1994).

Determining fates of grassland bird nests by direct observation generally is not feasible. Nests of many species of grassland birds are well hidden in vegetation, making it difficult or impossible to view nest contents from a distance, and are in open terrain, making unobtrusive observation a challenge. Predator communities often include both nocturnal and diurnal nest predators, which would require 24-hr surveillance. Identifying fates and predators of active grassland passerine nests could not be adequately addressed using artificial nests, still cameras, or conspicuous equipment (Pietz and Granfors 2000a). The need for a new tool was evident.

In 1996, Pietz and Granfors (2000a) began testing a video surveillance system (hereafter camera system) specifically designed to monitor grassland passerine nests. This first system used a black-and-white camera, about 4 x 4 cm on each side, with infrared (940–950 nm) light-emitting diodes (LEDs) to cryptically illuminate the nest area at night (Fig. 1.1a). Cameras had to be close to the nests (typically <30 cm) to record activity at the nests and the fate of nest contents without vegetation obstructing the view (Fig. 1.1b). Cameras, in waterproof housings, were made as small as possible to minimize disturbance to the nesting birds and to avoid attracting other animals. The camera angle and placement were adjusted at the nest with the aid of a handheld video monitor (Fig. 1.1c). The camera was connected by cable to a time-lapse videocassette recorder (VCR) and battery (Fig. 1.1d) about 40–50 m away. VCRs were set to record continuously and capture about 4 images/sec because early trials showed that some predation events took only a fraction of a second. At this recording speed, videotapes had to be changed (Fig. 1.1e) daily. The person changing the tape connected a handheld video monitor to the VCR (Fig. 1.1d) to determine (with reasonable certainty) if the nest was still active, thus eliminating the...