Declining bird populations, especially those that breed in North American grasslands, have stimulated extensive research on factors that affect nest failure and reduced reproductive success. Until now, this research has been hampered by the difficulties inherent in observing nest activities. Video Surveillance of Nesting Birds highlights the use of miniature video cameras and recording equipment yielding new important and some unanticipated insights into breeding bird biology, including previously undocumented observations of hatching, incubation, fledging, diurnal and nocturnal activity patterns, predator identification, predator-prey interactions, and cause-specific rates of nest loss. This seminal contribution to bird reproductive biology uses tools capable of generating astonishing results with the potential for fresh insights into bird conservation, management, and theory.
About the Author
Christine A. Ribic is Associate Professor of Wildlife Ecology at the University of Wisconsin, Unit Leader, US Geological Survey, Wisconsin Cooperative Wildlife Research Unit. Frank R. Thompson, III is a Research Wildlife Biologist in the Sustainable Management of Central Hardwood Ecosystems and Landscapes Unit of the Northern Research Station of the USDA at the University of Missouri-ColumbiaPamela A. Pietz is a Research Wildlife Biologist in the Northern Prairie Wildlife Research Center of the USGS in Jamestown, North Dakota.
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Video Surveillance of Nesting Birds
By Christine A. Ribic, Frank R. Thompson III, Pamela J. Pietz
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2012 Cooper Ornithological Society
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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 need to physically revisit the nest. The camera was left in place until the nest failed or succeeded (i.e., fledged young). Camera systems were deployed as far apart as possible within and among study sites to reduce the chance that individual predators with large home ranges [e.g., fox (Vulpes spp.), coyote (Canis latrans)] would encounter more than one nest with a camera.
From the mid-1990s through the early 2000s, these or similar camera systems were used in a variety of grassland bird studies (Winter et al. 2000, Renfrew and Ribic 2003, Klug 2005, Grant et al. 2006). The purpose of this paper is to use this body of work and the papers in this volume to provide an overview of the contributions these camera systems have made to the understanding of grassland bird ecology. We include updated test results for some of the questions explored with smaller data sets by Pietz and Granfors (2000a). With these sources of information, we address the following topics: fates of nests, eggs, and nestlings; predator identification and predator ecology; standard methods of data collection and analyses; predator behavior and predator–prey interactions; and parental and nestling behaviors. We close with caveats related to the use of cameras at nests and the interpretation of data collected with camera systems.
FATES OF NESTS AND NEST CONTENTS
Studies using video nest surveillance (hereafter camera studies) confirmed that predation was the leading cause of nest failure for grassland passerines (Pietz and Granfors 2000a, Klug 2005, Renfrew et al. 2005, Ribic et al., chapter 10, this volume). In addition, video data revealed that some successful nests (i.e., at least one young fledged) lost part of their contents to predators (i.e., partial predation) (Pietz and Granfors 2005). Results from studies in North Dakota and Minnesota showed that predation not only accounted for most nest losses (Table 1.1) but also was the leading cause of mortality among nestlings (Table 1.2).
Camera studies revealed that partial predation sometimes led to nest abandonment by the parents [e.g., in Northern Bobwhite (Colinus virginianus); Ellis-Felege et al., chapter 13, this volume]. Abandonment also occurred at some passerine nests subjected to cowbird parasitism and removal of host eggs (Hill and Sealy 1994, Romig and Crawford 1995). Video data allow researchers to link proximate events (e.g., egg removal) with nest fates; however, classifying such nests may then become ambiguous using current terminology. For instance, in the examples above, should the cause of nest failure be considered predation or parental abandonment?
Parental abandonment also may be caused by deployment of cameras near nests, particularly during the egg stage (Pietz and Granfors 2000a). Nest abandonment that occurred <1 day after camera deployment was assumed to be induced by the nesting birds' intolerance for the presence of the camera, the disturbance caused while setting up the camera system, or both. In a sample of passerine nests monitored during 1996–2001, 31 of 37 abandonments occurred within 1 day of camera deployment and, thus, were considered to be camera induced (Table 1.1). In the 1996–2001 sample, nearly 22% of 137 nests were abandoned within 1 day when the camera system was deployed during egg laying or incubation; only one such abandonment occurred (<2%) among 51 nests when the camera system was deployed during or after hatch. Nest failures attributed to cameras are discussed in the "Caveats" section.
In addition to predation, video surveillance revealed factors leading to nest failure or loss of eggs or nestlings that may have been misclassified as predation in the absence of video data (Pietz and Granfors 2000a). For example, two Clay-colored Sparrow (Spizella pallida) nests in small shrubs gradually tipped over as the nestlings grew, and the nestlings suddenly fell out. Unless the nestlings were still present (e.g., on the ground) when the observer returned to check the nest, the observer would have found only an empty, disheveled nest that appeared to have been torn from the shrub by a predator.
Video data also showed that some nestlings left the nest prematurely, seemingly on their own accord (here we define "prematurely" as earlier than expected based on fledging ages from undisturbed nests). For example, at a camera-monitored Savannah Sparrow (Passerculus sandwichensis) nest in Minnesota, a small plains gartersnake (Thamnophis radix) attempted to remove 7-d-old nestlings but failed. One nestling left the nest during the snake's visit and the remaining four nestlings departed within the next 1.5 hr. Video data from undisturbed nests showed that Savannah Sparrow nestlings usually do not fledge until they are 9–10 days old (Pietz et al., chapter 4, this volume).
Many cases of "forced fledging" (sensu Pietz and Granfors 2000a) took place while a predator was still at the nest. In such cases, the young were clearly motivated to leave the nest by the presence of the predator, but classifications of nest and nestling fates remain ambiguous. At one Savannah Sparrow nest in North Dakota, a 7-d-old nestling fled the nest while a white-tailed deer (Odocoileus virginianus) was eating its nest mates (Pietz and Granfors 2000b). Technically, the young bird that left the nest would have been considered a fledgling. In this case, however, the fate of the "fledgling" was known because the deer caught it while it was still in camera view; it survived <10 sec outside the nest (Pietz and Granfors 2000b). Forced fledging occurred at nearly 20% of our nests that were visited by predators and accounted for about 10% of young that were classified as fledged (Table 1.2; Pietz et al., chapter 4, this volume).
People checking nests also can cause premature or forced fledging. In one case, three Clay-colored Sparrow nestlings stayed still while an observer was at the nest, but they all left the nest less than a minute after the person departed (table 3 in Pietz and Granfors 2000a). How forced fledging affects survival of those individuals is seldom known. Certainly, if nestlings are sufficiently ambulatory, forced fledging may be advantageous for nestling survival (Lima 2009).
Camera studies have revealed that the determination of nest fates is not always as clear-cut as depicted in the literature. As more studies collect nest data using video, researchers may need to set new standards for terminology and for classifying nest and nestling fates.
PREDATOR IDENTIFICATION AND ECOLOGY
Researchers have investigated many factors that potentially affect nest predation. In this extensive literature, there are studies that draw opposite conclusions regarding the effects of just about every factor tested—including nest concealment, nest stage, habitat edge, and landscape characteristics (e.g., references in Pietz and Granfors 2000a, Jones and Dieni 2007). One likely explanation for these conflicting results is that the predator communities differed among studies. Before we can understand the ecological factors and underlying mechanisms that govern nest predation, we must first know who the predators are (Lahti 2009; Weidinger 2009, 2010; Benson et al. 2010; Thompson and Ribic, chapter 2, this volume). Video surveillance at nests has helped researchers to do this.
Camera studies have revealed a surprising diversity of predators at grassland passerine nests. For example, in the North Dakota and Minnesota studies (1996-2001), there were 16 different predators identified to the level of genus or species, including 11 mammals, four birds, and one snake (Table 1.3). Similar levels of diversity were found in other grassland studies (Table 1.4; Davis et al., chapter 14, this volume). In addition to confirming culpability by species assumed to be nest predators, camera studies have documented unsuspected nest predators, such as jumping mice (Zapus spp.) and white-tailed deer (Pietz and Granfors 2000a, 2000b), as well as cattle (Bos taurus) (Nack and Ribic 2005).
Video data have allowed researchers to start exploring how nest predator communities vary at multiple spatial scales. Grassland camera studies across several states, from Wisconsin to Montana and south to Nebraska, have illuminated some regional similarities and differences in predator communities (Table 1.4). Unsurprisingly, raccoons (Procyon lotor) were documented more often at eastern study sites (e.g., Renfrew and Ribic 2003), where the mix of row-crop agriculture [particularly corn (Zea mays)] and woodlands provides quality habitat for raccoons (Dijak and Thompson 2000). Some differences in predator communities reflect latitudinal ranges of taxa. For example, in the more northerly grasslands (Montana, North Dakota, and Minnesota), snakes accounted for less than 5% of nest predation events in which predators were identified, and these all were by gartersnakes (Thamnophis spp.) (Table 1.4). Farther south, however, the number of snake species and the proportion of snake predations increased markedly. For instance, in Nebraska and Iowa, snake species accounted for more than one-third of nest predations (Table 1.4). The disparity in prevalence of snake predation between cool and warm climates has been documented beyond grasslands (King and DeGraaf 2006). At smaller spatial scales, researchers are just beginning to investigate how predator communities differ among different grassland habitats (Ribic et al., chapter 10, this volume). Understanding how predator communities vary spatially can be used to help guide grassland bird conservation efforts (Thompson and Ribic, chapter 2, this volume).
Predator communities also can vary temporally, such as across seasons and years. On an extremely long temporal scale, distributions of some snake species and other nest predators that are currently limited by temperature (e.g., fire ants) may change as a result of warming associated with climate change. At the opposite extreme, video surveillance has allowed researchers to examine predation at much finer temporal scales by pinpointing the exact time that predation events occur. This information has prompted new ways of looking at predation ecology.
Knowing the time of predation allows researchers to explore differences between nocturnal and diurnal nest predators. For example, predators hunting during the day have more visual cues available to them, whereas nocturnal predators probably rely more on scent. This led us (the authors) to expect that diurnal predators would find nests with open bowls more easily than nests with covered bowls, but that nest type would be less likely to matter to nocturnal predators. To test this idea, we determined the time when a predator first removed (or destroyed) an egg or nestling from a nest. We called this the "initial predation" (sensu Pietz and Granfors 2000a) and, because it likely reflected conditions under which the predator found the nest, we used it as a measure of predation risk. We calculated separate rates of initial predation for day and night, using nest data from our North Dakota and Minnesota studies (1996–2001). As predicted, open nests tended to be more vulnerable than covered nests during the day, whereas at night predation risks (i.e., initial-predation rates) for the two nest types were similar (Fig. 1.2). The same result was found in an earlier analysis using just 1996-1997 data (Pietz and Granfors 2000a). In that paper, daily predation rates also were reported for nearly 300 nests that were monitored without video surveillance (i.e., non-camera nests); no difference was detected between open and covered non-camera nests ([chi square]1 = 0.00, P = 0.98), suggesting that predation risk associated with nest cover may only be detectable if diurnal and nocturnal predation can be separated.
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Table of Contents
PrefaceForewordPart 1. Synthesis/OverviewChapter 1: Knowledge gained from video-monitoring grassland passerine nestsPamela J. Pietz, Diane A. Granfors and Christine A. RibicChapter 2: Conservation implications when the nest predators are knownFrank R. Thompson, III and Christine A. RibicChapter 3: Gamebirds and nest cameras: present and futureSusan N. Ellis-Felege and John P. CarrollPart 2: Breeding BehaviorChapter 4: Hatching and fledging information from grassland passerine nestsPamela J. Pietz, Diane A. Granfors and Todd A. Grant Chapter 5: Attendance patterns and survival of Western Meadowlark nestsLarkin A. Powell, Matthew D. Giovanni, Scott Groepper, Mitchell L. Reineke and Walter H. SchachtChapter 6: Sprague's Pipit incubation behaviorStephen K. Davis and Teslin G. HolmesChapter 7: Patterns of incubation behavior in Northern BobwhitesJonathan S. Burnam, Gretchen Turner, Susan N. Ellis-Felege, William E. Palmer, D. Clay Sisson and John P. CarrollChapter 8: The influence of weather on shorebird incubationPaul A. Smith, Sarah A. Dauncey, H. Grant Gilchrist and Mark R. ForbesChapter 9: Nocturnal activity of nesting shrubland and grassland passerinesChristy M. Slay, Kevin S. Ellison, Christine A. Ribic, Kimberly G. Smith and Carolyn M. SchmitzPart 3: Behavioral Responses to Predation/Predator IdentificationChapter 10: Bird productivity and nest predation in agricultural grasslandsChristine A. Ribic, Michael J. Guzy, Travis J. Anderson, David W. Sample and Jamie L. NackChapter 11: Predator identity can explain nest predation patternsJennifer L. Reidy and Frank Thompson, IIIChapter 12: Nest defense: grassland bird responses to snakesKevin Ellison and Christine A. RibicChapter 13: Partial depredations on Northern Bobwhite nestsSusan N. Ellis-Felege, Anne Miller, Jonathan S. Burnam, Shane D. Wellendorf, D. Clay Sisson, William E. Palmer and John P. CarrollChapter 14: Identification of Spragues's Pipit nest predatorsStephen K. Davis, Stephanie L. Jones, Kimberly M. Dohms and Teslin G. HolmesPart 4: TechnologyChapter 15: Development of camera technology for monitoring nestsW. Andrew Cox, M. Shane Pruett, Thomas J. Benson, Scott J. Chiavacci and Frank R. Thompson III