Author: Timothy D. Kurt
Institution: University of Wisconsin, Madison
Date: June 2005
Abstract
The present experiment investigated relationships among dominance status, sex, risk, and feeding behavior using house sparrows, Passer domesticus. Feeding behavior was observed in wild male and female house sparrows at high- and low-risk food sites (i.e., those with and without a model snake, respectively). At high-risk sites, birds avoided the snake by feeding on the opposite side of the food source. Small-badge (presumed subordinate) males far outnumbered other males at high-risk feeding sites, whereas medium-badge (presumed intermediate ranked) males far outnumbered other males at the low-risk sites. For low-ranking males, this strategy might maximize food acquisition and reduce the cost of competition. Overall, the present findings suggest that animals' feeding behaviors and responses to risk differ depending on social rank.
Introduction
Several studies in birds indicate that the level of risk associated with a feeding situation differentially affects feeding behavior in dominant and subordinate animals. For example, in willow tits (Parus montanus; Ekman and Askenmo 1984; Hogstad 1988; Koivula 1994), black-capped chickadees (Poecile atricapillus, Desrochers 1989), and white-throated sparrows (Zonotrichia albicollis, Schneider 1984; Piper 1990) dominant birds forage closer to shrub or tree cover, or in safer areas of tree-canopy than subordinates. Furthermore, subordinate birds seem to avoid competition with more dominant animals by using lower risk food sources only in the absence of dominants (Ekman and Askenmo 1984; Koivula 1994; Desrochers 1989; Slotow and Rothstein 1995; Krams 1998). Subordinate animals may use these strategies to avoid competition with dominants and to increase their access to food sources (Halley 2001).
The present experiment explored relationships among dominance status, sex, predation risk, and feeding behavior in house sparrows, Passer domesticus. House sparrows form large, mixed-sex flocks during the non-breeding season. Adult male house sparrows possess a distinctive bib of black plumage, often referred to as a "badge of status," the size of which positively correlates with ability to win dominance encounters against females and other smaller badge males (Møller 1987; Solberg 1997; Liker 2001; Gonzalez 2001 and 2002). Unlike males, females do not possess a status-signaling badge. Studies of male-female interactions have shown that females tend to be subordinate to the most dominant males, and to possess intermediate positions in the dominance hierarchy (Liker 2001). Female house sparrows may be more likely to leave a disturbed food site than males (Breitwisch and Hudak 1989), suggesting possible differences in the response of males and females to risk.
In the present study differential feeding behavior in non-breeding male and female house sparrows was examined at high- and low-risk feeding sites. We predicted that subordinate animals (small-badge males) would be more likely to feed under high-risk conditions than would dominant animals (large-badge males). We expected that females would be equally distributed at low- and high- risk sites, based on the assumption that they tend to be of intermediate rank.
Few studies have examined the relationships between dominance, feeding, and risk-taking behavior in wild birds, though these are important factors affecting social behavior, reproductive success and survival. The information that would be gained from these studies could contribute to conservation efforts and our basic understanding of social interactions among group-living animals. The "badge of status" that male house sparrows display allows us to compare the relative dominance rank of many individuals while minimizing disruptions to the birds. This study demonstrates a novel way of measuring badge sizes of male birds without handling the animals in any way.
Methods & Materials
We observed non-breeding male (n = 102) and female (n = 100) house sparrows in naturally occurring flocks at 12 different sites within a 1.2 km2 area of the University of Wisconsin Madison campus in October of 2002. Daily observations of each site in the 2 weeks prior to testing showed that each had a steady number of birds. These birds remained close to the site at all times that observations were made. There is little scientific information available regarding house sparrow winter ranges and individual birds' movements between flocks. While a small number of sparrows in this study may have moved between flocks, this number appears negligible considering the steady group sizes and large number of total birds observed throughout the study. In addition, all of the sites were separated by at least 400 m. We divided the sites into groups of three and began testing the first three sites on October 6th, 2002. We began testing at the second three sites on October 13th, the third three on October 20th and the last three on October 27th.
For two weeks prior to testing, we placed food at each site. This allowed the animals to become acclimated to the time of the observation period, the presence of the researchers, the setting up of equipment, etc. For the first week, we spread three cups of mixed popcorn, thistle seed, cracked corn and wild finch seed into a 30 cm2 square area on the ground at approximately 09:00 each day. We aligned the back edge of the square parallel to and about 46 cm in front of some sort of cover, such as bushes, a wall, or trees. We chose this distance because other studies have shown that distances of just one meter from cover are risky enough to cause some birds to avoid feeding (Schneider 1984). During the first week, food was placed daily regardless of the amount left from previous days. During the second week, food was present at each site for three hours daily. We reduced the period in an effort to train the birds to come at a more specific time, between 09:00 and 12:00 h, the hours during which we would be testing. Burlap squares that fit under the 30-cm2 square food spot facilitated removal of the food. In addition, we placed green bamboo stakes at each corner of the food square one week before testing. Two stakes of 15 cm and 30cm were also placed sequentially from the front and back edges of the square. These served as depth cues to aid in estimating male badge size during later data collection from video (see below).
We estimated the number of birds daily during the two weeks prior to testing by counting the number of birds within a 20-meter radius of the site. The flocks were then split evenly by size so that each condition, high and low risk (details below), would have approximately equal numbers of birds.
During testing, observations were made at all sites for one hr on three non-consecutive days (total of three hr per site). Food was provided only during the one hr testing period on test days, but during the entire three hr period on all other days. We tested one site during each of the three hr between 09:00 and 12:00, and randomly assigned each site a testing time, which remained the same each test day. The time of testing was counterbalanced across conditions (high versus low risk, described below).
To collect data, a video camera (Sony, model CCD-TRV58) was extended to full height on a tripod, focused at maximum zoom and placed at a pre-marked spot 13.75 m away. All films were recorded at the same magnification. At high-risk sites, a 1.8 m inflatable snake (Biocontrol Network, http://www.biconet.com, model # TB2661) was placed 30 cm away from one side of the food with its head towards the front of the food square. Video recordings began when the food was placed on the ground and continued for at least 15 min after the first bird arrived at the food source. If no birds were observed, taping was stopped at 45 min, and the researcher moved to the next site.
The videotaped observations were examined using an analog to digital media converter (Dazzle Videowave, 225 Charcot Ave., San Jose, CA) on a PC computer. It is possible that over the three days of testing, many of the same birds were recorded on different days. Therefore, to avoid the possibility of non-independent data points, only one day of video data from each site was used in analysis. The specific day to be used was randomly chosen by an independent coworker; unless birds were only present on one day, that day was automatically included in analysis.
Since some flocks had a large number of birds, and tracking their movements over the entire observation period was extremely difficult, we divided the video into 16 (for minute zero 15) three s bins, and observations were made for each bin. The time in s of the first bird to be seen on camera was recorded as minute zero, and three s observations were taken at one min intervals until min 15.
During each three s bin the following measurements were recorded: (1) the total time spent by each bird at the food site (or the continued presence of birds that had already there), (2) the sex of each bird present not already recorded, and (3) the badge size of every male not already recorded (this was possible for 80% of males).
Relative badge size was determined by measuring the medial height and width of the badge in millimeters directly from the video screen. Since it is not clear whether height or width of the badge is more important to dominance signaling, we multiplied height x width to get an estimate of the badge's total area. Most badges were measured when the birds were facing the camera head-on. For any birds in profile (or any angle not facing the camera head-on), badge size width was measured from the midline of the bird to the lateral edge of the badge and then doubled. We corrected badge size measurements for differences resulting from birds nearer or farther from the camera by measuring a known area as it appeared at a reference point on the video-screen and then generated correction factors to compensate for the increase or decrease in size caused by the corresponding distance from the camera when recorded. For males positioned 30 cm in front of the food square, badge area was multiplied by 0.950, badge sizes for those positioned 15 cm in front of the square were multiplied by 0.975, and for those birds positioned at the front edge of the food square, relative badge size was left unchanged. We multiplied the badges of males 15 cm behind the front edge by 1.025, and badges of those at the back of the square by 1.050. Finally, the badges of males 15 and 30 cm behind the back of the square were multiplied by 1.075 and 1.100, respectively, and any that were at the tree, bush or wall at the very back of the site were multiplied by 1.125. These methods were validated by pilot experiments measuring objects of known width and height in the lab.
Results
Figure 1: Percent time spent on or away from the side of the feeding area with a snake, in the high-risk condition, or the corresponding side, the low-risk condition.
One low-risk site did not yield any data and was dropped along with the matching high risk site (i.e., the one with the same approximate flock size), reducing the sample size from 12 to 10. Measurements for flock sizes recorded during the first day of testing showed that the flocks were evenly divided between the two conditions (t8 = 0.14, P = 0.89; High risk site: mean = 39, SD = 26.17; Low risk site: mean = 41.4, SD = 28.40).
We analyzed the first bird to arrive and remain at each site for 20 s and found that the birds at low risk sites were equally likely to be observed on either side of the food source (Figure 1). In contrast, at the high risk sites, the equivalent birds were far more likely to be observed in the half of the feeding area away from the snake (Figure 1). A Mann-Whitney-U test revealed no relationship between the latency for the first bird to arrive at high or low risk sites (n = 10, U = 1.149, P = 0.31). No significant differences were observed between the total numbers of birds or in the number of males and females appearing at high and low risk sites (X2=0.306, n = 202, P > 0.05). Additionally, sex did not appear to play a role in determining which bird would arrive first at either the high or low risk sites, or in the amount of time a bird spent at a high or low risk site, however given the small sample sizes for these measures, this could not be examined statistically.
Figure 2: Badge sizes of males at each site.
It was possible to estimate badge size for 81 out of 102 males. When plotted by size, these badges fell into three distinct groups. Our badge size categories are based on this grouping pattern: "small" is defined as a relative size of 1-33 mm2, "medium" as a badge of 34-66 mm2, and "large" as a badge of 67-99 mm2. These sizes are relative only. The majority of birds at low risk sites possessed medium size badges (X2 = 31.27, n = 81, P < 0.001; Figure 2). In contrast, males with small badges far outnumbered males with either medium or large badges at high risk sites (Figure 2). Examining the sites individually yielded similar results. Finally, the amount of time each bird spent at the food source did not correlate to badge size of males at either the high risk sites (n = 36, r = 0.0090, P = 0.5820) or the low risk sites (n = 45, r =0.0110, P = 0.4960).
Discussion
The differences in number of birds with low and intermediate rankings at low- and high- risk sites might reflect intense competition between birds in these groups, a possibility supported by studies showing that males with similar sized badges fight more among themselves than do males with more extreme differences in badge sizes (Møller 1987). As a result of this competition, subordinate males may be forced to feed under riskier conditions than their larger-badge counterparts. This interpretation is consistent with multiple studies in birds demonstrating that subordinates tend to feed under riskier conditions than more dominant animals (Schneider 1984; Lahti 1997; Koivula 1994), and that this is a result of exclusion from safer foraging sites by competition from dominants (Ekman and Askenmo 1984; Desrochers 1989; Koivula 1994; Krams 1998; Slotow and Rothstein 1995).
In addition to possessing a high rank, large-badge males tend to be older and likely more experienced than intermediate or small-badge males (Veiga 1993). It is possible that these males perceive and evaluate potential threats differently from less-dominant males as a consequence of age and experience. In the laboratory, one could measure dominance rankings by pairing birds and observing the outcome of aggressive interactions. This would allow more controlled studies, but would not be ecologically valid. The advantages of field studies include studying natural populations in an ecologically relevant context.
No differences were observed in the number of females feeding at sites of high or low risk. This is consistent with our prediction that that females would be equally distributed at low- and high- risk sites, based on the assumption that they tend to be of intermediate rank. However in the present study the dominance status of individual females was not known. Additional studies in which female dominance status is known are necessary to examine more precisely possible sex differences in dominance effects on response to risk.
Capturing wild birds and measuring badge sizes directly could eliminate some errors that might result from digital analysis. In addition, placing identification bands on the birds would allow more precise tracking of individuals and give valuable insight into winter ranges and the movement of individuals between feeding groups. We found this was not feasible with the large number of birds involved in this study. As a result, we did not obtain badge size measurements for all of the birds at each site, only those that entered view of the camera. It is possible that certain sites might have had more males of a certain badge size than others, possibly due to differences in risk associated with each site. We tried to minimize this effect by picking sites that had similar amounts of pedestrian and vehicle traffic. We believe it is likely that all sites had similar proportions of the different badge size males.
The present data indicate that social rank is one of multiple factors, including the risk of a situation, internal variables and other environmental factors that interact to regulate behavioral decisions. Further investigation involving varying levels of risk and reward might illuminate the decisions and trade-offs that animals of different dominance ranks must make when feeding.
Acknowledgements
The authors gratefully acknowledge Donald P. Teague and Emily Sallows for assistance with various aspects of this project, Nicholas Keuler for statistical advice, and the Wisconsin/Hilldale Undergraduate/Faculty Research Award granted to TDK and LVR.
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