Cues used by brood parasites and predators to locate nests Paula Grieef Department of Zoology, University of Manitoba Winnipeg, Manitoba, Canada R3T 2N2 E-mail:

Introduction

Both predators and cowbirds must locate nests and choose a suitable one from among those found. They may use similar cues, as brood parasites can be considered a type of predator (Wiley 1982). Very little is known about how predators locate nests (Smith et al. 1974, Collias and Collias 1984). Few studies have looked at the predator/prey interaction from the point of view of the predator. Much of the literature describes tactics used to avoid being eaten or what occurs after food has been found or procured (but see Bell 1991). There is a large body of literature on optimal foraging that discusses what happens once the prey has been found (reviews in Krebs and McCleery 1984, Pyke 1984), indicating that predators should choose the most profitable prey given their current condition.

Andersson (1981) described three general methods of searching by predators: (1) continuous movement, (2) sit and wait, and (3) pause and travel. Different animals use different techniques. Many avian predators often use the sit and wait and the pause and travel techniques. Birds have been documented perching and scanning, hovering and soaring in search of food (Carlson 1985, Viitala et al. 1995). Avian predators that hunt on the ground and mammalian predators are believed to find prey using directional, systematic searching or random search movements (Zach and Smith 1981, Benhamou 1992).

Three methods of searching, in this case for nests to parasitize, have also been documented for Brown-headed Cowbirds: (1) watching hosts at their nests, (2) silently searching for nests, and (3) flushing hosts off nests (Norman and Robertson 1975, Wiley 1988). The most common method is watching hosts (Friedmann 1929, Hann 1941, Payne 1973).

Two behaviours predators and cowbirds could cue in on are activity and aggression of nest owners, which may indicate that there is an active nest in the area and draw the predator or parasite to the nest. Some other cues that might be used, include song rates of adults and host aggression to indicate the bird's quality (Smith 1981, Arcese and Smith 1988), activity around the nest to indicate nesting stage, nest placement (e.g., height, supporting vegetation, concealment), and nest type (Thompson and Gottfried 1981, Fleischer 1986, Orians et al. 1989, Briskie et al. 1990, Colwell 1992). These cues may lead predators or cowbirds to the location of nests, may help them select nests and perhaps even provide information on the fitness of the adults (Smith 1981). For example, the more time a male can devote to singing, the more fit it may be (see Greig-Smith 1980, 1982; Reid and Sealy 1986).

In this study, I examined experimentally five cues that might be used by both predators and parasites to locate nests: nest concealment, nest height, vegetation supporting the nest, host activity, and host aggression.

Predation has been shown to be an important factor in nest-site selection (Murphy 1983) and that better-concealed nests suffer less predation (McLean et al. 1986, Sugden 1987, Brown and Fredrickson 1989). Cowbirds, therefore, should parasitize better-concealed nests because they will also benefit from this reduced level of predation (see Gates and Gysel 1978).

Predation by visual predators apparently is related to concealment (Knapton 1978, Wray and Whitmore 1979) with better-concealed nests being preyed upon less frequently, assuming that predators do not use the activity of nest owners to locate nests (Collias and Collias 1984). Olfactory predators should not be affected by the degree of concealment of a nest (Clark and Nudds 1991, Holway 1991). I predicted that concealment is not related to nesting success of Clay-colored Sparrows because the Franklin's ground squirrel (Spermophilus franklinii), an olfactory predator, is the main predator.

Nest height and supporting vegetation may be used to locate nests. If nests are at a set height or in a particular species of plant, this would allow predators and parasites to form search images and "know" where to look. Briskie et al. (1990) showed that higher nests were parasitized more frequently. Filliater et al. (1994) discussed five hypotheses that explain how both nest height and vegetation could provide cues for predators when looking for nests. These hypotheses involve nest inaccessibility, nest height (high, mid-height and low nests) and vegetation (common and rare plants). They indicate that there is a wide variety of possible explanations for where nests are placed depending on the type of predators in the area. I predicted that both nest height and supporting vegetation would be related to nesting outcome.

Nest activity may be used by cowbirds to locate nests (Friedmann 1929, Hann 1941, Buech 1982, Wiley 1988). I tested this by placing out old nests (see Thompson and Gottfried 1981). To be able to use host activity to find nests, activity must be centred near or at the nest and occur when parasitism is appropriate. These activities may include nest building, mate guarding, nest visits and egg laying. Several studies have shown that when old nests were placed out, even with eggs, cowbirds did not lay eggs in them (e.g., Laskey 1950, Thompson and Gottfried 1976). Thus, Lowther (1979) and Thompson and Gottfried (1981) attempted to simulate host activity by adding one egg per day to each nest, but still recorded only a low frequency of parasitism (see also Wiley 1988). This suggests that host activity is indeed necessary. I investigated this by simulating five levels of increasing host activity; empty nests, nests with full clutches, nests in which one egg was placed per day, model hosts and song. I predicted that by simulating increasing amounts of host activity, the frequency of parasitism should increase.

Predators may also use nest activity to locate nests (review in Collias and Collias 1984). The activity of adults may direct the predator to the nest site or may cue it to an active nest nearby. Here, too, I predicted that by simulating increasing amounts of activity of nest owners at artificial nests, the frequency of predation should increase.

Host aggression has usually been thought to discourage both predators and parasites (Buitron 1983, Smith et al. 1984, Martin 1992). Some hosts can distinguish cowbirds from other potential nest threats (Nice 1937, Robertson and Norman 1976, Neudorf and Sealy 1992). Enemy recognition can be tested experimentally by placing models of different enemies near nests and quantifying the birds' responses to them. Some species react more aggressively to a cowbird model, thus providing a potential cue for cowbirds to locate active nests (Robertson and Norman 1977, Smith 1981). For the aggressive behaviours to act as a cue nesting birds must respond to a threat at a distance that would enable the cowbird to use the behaviour to locate the nest. If the birds do not react until the cowbird is very close (e.g., 0.5 m or less), then the nest probably has already been found and aggressive behaviours would not act as a cue (Neudorf and Sealy 1992). If the level of aggression increases as the nest is approached, then the cue may function (Duckworth 1991). I predicted that hosts will react to a nest threat at distances greater than 0.5 m.

Predators also may use aggression to locate nests. Some studies have found that more aggressive pairs were depredated more frequently (Searcy 1979, Röell and Bossema 1982), whereas other studies have found the opposite (Greig-Smith 1980, Blancher and Robertson 1982). I predicted that if predators use aggression to locate nests, nest owners should respond aggressively towards a model at different distances from their nests, perhaps becoming more aggressive as the model is placed closer to the nest and the threat increases.

In the present study, I examined the importance of host activity and aggression to Brown-headed Cowbirds and predators in locating nests. I also determined if nest concealment, supporting vegetation and nest height were used as cues to locate nests. The study species involved is the Clay-colored Sparrow, an accepter species (Hill and Sealy 1994).

Methods

Study Site

This study was conducted at the University of Manitoba Field Station, Delta Marsh (50°10'N,98°22'W), Manitoba, during the springs of 1993 and 1994, in an area known as the Oxbow Woods. Situated along the southern edge of Delta Marsh, this woodlot is surrounded by old-field succession dominated by snowberry and wild rose (Rosa sp.) (see Evans 1972, Gamble 1980, Hill 1992, Hill and Sealy 1994). I searched for sparrow nests daily from mid-May to the end of June. I searched the habitat thoroughly, checking every tuft of grass and snowberry bush. I flagged and numbered each nest approximately 2 m away, and inspected each nest daily for signs of predation, i.e. broken eggs, eggshells, tipped nests, missing eggs (see Major 1991, Sealy 1994) and parasitism, i.e. presence of a cowbird egg.

Nest Concealment

I quantified the degree of concealment at each nest by assigning cover values on a scale from 0-5, that corresponded to decreasing visibility of the nest (0=100% visible, 1=80% visible, etc.). Estimates were made at eight compass directions and one from above the nest. I then calculated an average concealment value. All measurements were taken one meter from the nest and at nest height and the observer's eye level (see Holway 1991). These measurements simulated the cowbird's or avian predator's vantage point (observer's eye level; see Gochfeld 1979) and a mammalian predator's vantage point (nest height). I estimated the cover value on the day the nest was tested (see below) or on the day the last Clay-colored Sparrow egg was laid. This ensured that the nest was active (Friedmann 1929, McGeen 1971). I then correlated concealment values with the nest outcome (predation, parasitism or success). I considered a nest successful if the clutch was still intact three days after the last egg was laid. The three-day cutoff was chosen because Clay-colored Sparrows are sometimes parasitized even after the clutch is complete. However, most parasitism occurs during the egg-laying period (Hill 1992). I ended the experiment before fledgling success was known because I wanted a comparable time frame for both parasitism and predation.

Nest Height and Supporting Vegetation

I recorded the height of the nest rim from the ground. For analysis, I broke the heights into increments of 100 mm, the approximate height of a single nest. The dominant plant species in which the nest was placed was recorded to determine if there was any relationship to nest outcome.

Host Activity

Old Clay-colored Sparrow nests (collected by D.P. Hill in 1991, and by me in 1993) were place randomly in Clay-colored Sparrow nesting habitat from 3 to 18 June in 1993 and 6 to 21 June in 1994. Nests were placed in sites with known (controlled) concealment values (Lowther 1979) for eight days, which simulates one day as an empty nest, four days of egg laying and three days of incubation (four-egg clutches are the modal clutch size for Clay-colored Sparrows at Delta (Hill 1992)). I placed the nests 3 m apart along a transect, 1 to 2 m alternately to the left or right of a rope stretched along the transect. I flagged each nest location along the rope so that I could relocate the nests. I chose a distance of 3 m between nests because Clay-colored Sparrow territories are small (natural nests are sometimes within 5 m of each other (Knapton 1978)). At this distance, nests were 4 to 6 m apart because they were placed on alternate sides of the rope 1 to 2 m from it. Nests received one of five treatments: (1) no eggs, (2) full clutch (four eggs), (3) one egg per day, (4) one egg per day plus model Clay-colored Sparrow, or (5) one egg per day plus model plus song (Thompson and Gottfried 1981). Each treatment, consisting of 20-35 nests, simulated laying and different amounts of host activity to determine if host activity influenced predation and parasitism frequencies (see Wiley 1988). Comparisons between treatments for the three nest outcomes to determine if host activity affected nesting outcome.

The models were placed 0.5 m from the nest for 30 minutes every morning from 0630-1000 (Central Standard Time) for 7 days (Wiley 1988). Cowbirds are active in nesting areas in the morning and then move off these areas to feeding areas during the late morning or early afternoon (Rothstein et al. 1984). The models were Clay-colored Sparrows, freeze-dried and mounted on poles, and placed facing the nest. Songs of Clay-colored Sparrows were recorded in 1993 and transferred to a loop tape at a rate of 9 songs/minute, a rate that fell within the natural song rate (Bent 1968). Song was played back for 30 minutes on a cassette recorder placed at the base of the pole. The model treatment and model plus song treatment were randomly assigned to nests and to a different 30-minute period every day so that each nest received the treatment over each 30-minute period during the experiment. Two runs of nest placements were conducted to increase sample size and these were combined because there was no difference between them (1993: Fisher exact test, p = 0.917; 1994: Fisher exact test, p = 0.533).

Artificial eggs were made of plaster-of-Paris and painted to resemble Clay-colored Sparrow eggs (Rothstein 1975). The eggs were slightly larger and heavier than real Clay-colored Sparrow eggs (2.2 g vs. 1.6 g; 17.5 mm x 13.4 mm vs. 17.1 mm x 12.7 mm; measurements from Walkinshaw 1944 and Bent 1968). The slightly greater mass and size should not affect the study because predators were able to remove the eggs (pers. obs.) and there is some natural variation in egg size (Bent 1968). Cowbirds should not be affected because they are known to parasitize species with eggs that are larger than those of Clay-colored Sparrows (e.g., Fleischer 1986). I placed nests in active territories as the study was conducted during the breeding season of Clay-colored Sparrows. Thus, there was an increased level of activity near the experimental nests that was in addition to the simulated levels of activity. This may have been a problem but the activity probably was similar for each artificial nest. This problem was not addressed in other studies using artificial nests in active territories (see Lowther 1979, Thompson and Gottfried 1981). For nest concealment, nest height, supporting vegetation, and host activity, Fisher exact tests (2-tailed) were used for desired comparisons of each cue and nest outcome (Conover 1980, Zar 1984).

Host Aggression

Enemy recognition

To test whether Clay-colored Sparrows recognized different nest threats, I used the data on responses by sparrows to three models (female Brown-headed Cowbirds, Franklin's ground squirrel and Common Grackle Quiscalus quiscula) placed 0.5 m from active nests. I added a fourth model, a Fox Sparrow (Passerella iliaca), to serve as a control. The sparrow is a good control because it is similar in shape and size to female cowbirds but neither parasitizes nor preys upon Clay-colored Sparrow nests. It is found on the study area only during migration and therefore should rarely interact with Clay-colored Sparrows. Thus, I expected the Fox Sparrow to elicit low levels of aggression (see Hobson and Sealy 1989, Neudorf and Sealy 1992, Bazin and Sealy 1993). These models allowed determination of responses to different enemies. In this series of tests, I included only tests where birds responded because I was interested in the aggressive responses of the birds and not whether or not they responded by avoiding the nest.

The tests were conducted during the egg-laying stage and data were tape-recorded and later transcribed. The responses of Clay-colored Sparrows were recorded following the methods of Smith et al. (1984), as modified by Hobson and Sealy (1989). Responses were: (a) time spent < 2 m, 2 m to 5 m or > 5 m from the model, (b) vocalizations (chip, quiet chip), (c) hidden in the vegetation, (d) attacks, (e) feeding, (f) incubating, (g) perching, (h) out of area, and (i) singing. I scored categories a, c, e, f and h as the number of 10-second intervals in which they occurred while I scored all other categories as the actual number of times they occurred in the 5-minute test. The time Clay-colored Sparrows took to react to the model was also recorded as an indication of host attentiveness. The responses of both male and female were combined as the sexes could not be distinguished in this unmarked population. Each test was run for 5 minutes with 15-minute rest periods between each model presentation to reduce carry-over aggression. Observations were made from a blind 5 to 10 m from the nest. The models used in the distance testing were taxidermically mounted in upright, non-threatening postures. Models were placed facing the nest. Nests were tested from 0600 to 1900 (CST), each nest was tested only once at all three distances. The test was started when a bird returned to within 5 m of the model. If no bird showed up within 30 minutes, the test was ended and this was considered no response.

Distance testing

To test the nest-cue hypothesis, I placed a female cowbird model 0.5 m, 2.5 m and 4.5 m from nests to determine at what distance Clay-colored Sparrows reacted to a cowbird. The farthest distance was selected on the basis of territory size, which sometimes placed nests as close as 5 m apart (Knapton 1978). This test permitted determination of the possibility that a cowbird could use host aggression to locate nests. To determine if aggressive behaviour could be used as a cue by predators, model testing at the same three distances was done using an avian predator (Common Grackle) and a mammalian predator (Franklin's ground squirrel). The grackle was chosen as the avian predator because it breeds in the study area.

I chose the Franklin's ground squirrel as the mammalian predator because it is found on the study area in substantial numbers and is known to prey on bird nests (Sowls 1948, Knapton 1978, Sargeant et al. 1987). To act as a control, I tested four nests with a model Fox Sparrow placed at the three distances.

For the distance-model testing, Friedman analysis of variance was used due to the nonparametric nature of the data. For the enemy recognition testing, I used the Kruskal-Wallis test. If a significant difference was found between the distances or models, I then used nonparametric multiple comparisons to determine where the difference was (Conover 1980, Conover and Iman 1981; also see Neudorf 1991).

Results

I found 112 Clay-colored Sparrow nests, of which 13 were parasitized (11.6%), 12 were preyed upon (10.7%), and 87 were successful (77.7%). The outcomes of the nests did not differ between the two years (Fisher exact test, p = 0.712).

Nest Concealment

Most nests (>67%) were highly concealed (concealment values between 4 and 5). For all concealment values taken at both eye-level and nest-level, there was no significant difference for the three possible outcomes (eye-level, Fisher exact test, p = 0.391; nest-level, Fisher exact test, p = 0.642). Concealment values measured from above were also not significant for the three possible outcomes (Fisher exact test, p = 0.149).

Nest Height and Supporting Vegetation

Nest height did not affect nest outcome (Fisher exact test, p = 0.203). Forty-one percent of the nests were built at heights from 101 to 200 mm. Snowberry was the dominant plant with 77% of nests built in this species. Nest outcome was not related to supporting vegetation (Fisher exact test, p = 0.826).

Host Activity

Only one of the five treatments affected nest outcome. The fifth treatment (one egg/day plus model plus song) was significantly different from the other treatments (Fisher exact test, p = 0.009), because no nests in this treatment were depredated. None of the nests from any treatment was parasitized, approximately 33% of nests were depredated and 66.6% of nests were successful for the first four treatments. Distribution of nests according to concealment values for both eye-level and nest-level were similar to active nests. Most nests fell in the 80-100% concealment category. Concealment also did not effect the outcome for any treatment at eye-level. However, for concealment values at nest-level, 80%-100% concealment had a significant effect on the outcome for treatment five (Fisher exact test, p = 0.001). Again this was due to the lack of predation on nests in this treatment.

Host Aggression

Enemy recognition

The 0.5-m distances from the model testing described below were used to test for enemy recognition. There were two significant responses, distance <2 m and distance >5 m (Table 1). Clay-colored Sparrows spent more time closer to the cowbird model than any of the other three models and they also spent more time farther from the Fox Sparrow model than the others. The alarm call, 'chip' (Walkinshaw 1944) frequency was not quite significant but increased in response to the cowbird and sparrow models but not for either predator model.

Table 1. Summary of responses of Clay-colored Sparrows to four models presented at 0.5 m from the nest, and results of Kruskal-Wallis test and associated multiple comparisons.
	Type of model presented
Responsea	BHCOb (17)c	COGR (10)	FGSQ (10)	FOSP (4)	p-valued
< 2 m	31.8 ± 4.81	9.6 ± 4.22	9.7 ± 4.02	0.3 ± 0.32	0.0014
> 5 m	1.4 ± 0.81	3.1 ± 2.51	3.3 ± 3.21	11.8 ± 4.82	0.0171
Chip	8.1 ± 2.7	1.0 ± 0.7	2.7 ± 2.7	5.0 ± 4.7	0.0562
Responses are given as mean ± s.e. a Categories of distance, incubation, in vegetation and leaves area were measured as the number of 10-sec intervals that birds were engaged in the behaviours. All other behaviours were measured as the actual number of times they occurred within a trial. b BHCO=Brown-headed Cowbird, COGR=Common Grackle, FGSQ=Franklin's ground squirrel, FOSP=Fox Sparrow. c Combined sample sizes for male and female are given in parentheses. d Results of the Friedman test for comparisons among the four models.1,2 Results of multiple comparisons for determining differences between models. Means with different superscripts differed significantly (p<0.05).

Distance testing

Seventeen nests were tested with a model female cowbird placed at three distances from the nest. Of the recorded responses to the model, only four were significant: distance <2 m, distance 2-5 m, chips, and perch changes (Table 2). The rate of chip calling increased as the model was placed closer to the nest. The time females incubated was greatest when the model was placed the farthest from the nest, although this difference was not quite significant (Table 2). None of the more aggressive behaviours, such as flybys and chips, was significant because each was rare. The time adults took to respond and the number of adults that responded also were not significant for all three distances.

Table 2. Summary of responses of Clay-colored Sparrows to cowbird models presented at three distances from the nest, and results of Friedman test and associated multiple comparisons.
	Distance (m)
Responsea	0.5 (16)b	2.5 (16)	4.5 (17)	p-valuec
< 2 m	33.5 ± 4.31	22.4 ± 4.02	6.8 ± 3.13	0.0001
2-5 m	5.6 ± 1.81	14.6 ± 2.82	29.5 ± 2.63	0.0001
Chip	9.8 ± 2.91	7.3 ± 2.81,2	1.0 ± 0.72	0.0166
Close passes	0.5 ± 0.4	0.0	0.0	0.1302
Incubation	3.4 ± 2.3	2.5 ± 1.9	13.0 ± 3.6	0.0529
Perch changes	30.8 ± 5.91	21.2 ± 5.02	12.2 ± 3.32	0.0018
Responses are given as mean ± s.e. a Categories of distance, incubation, in vegetation and leaves area were measured as the number of 10-sec intervals that birds were engaged in the behaviours. All other behaviours were measured as the actual number of times they occurred within a trial. b Combined sample sizes for male and female are given in parentheses. c Results of the Friedman test for comparisons among the three distances. 1,2,3 Results of multiple comparisons for determining differences between distances. Means with different superscripts differed significantly (p<0.05).

Clay-colored Sparrows did not react aggressively to the model Franklin's ground squirrel as only one behaviour recorded was significant, the quiet chip (Table 3). Bent (1969) described a "tsip" call that is similar to the quiet chip. Incubation and perch changes showed distinct but not significant trends. Incubation increased as the model distance from the nest increased, whereas perch changes decreased.

Table 3. Summary of responses of Clay-colored Sparrows to Franklin's ground squirrel models presented at three distances from the nest, and results of Friedman test and associated multiple comparisons.
	Distance (m)
Response	0.5 (9)	2.5 (8)	4.5 (8)	p-value
Quiet chip	1.9 ± 0.61	2.6 ± 1.01	5.3 ± 1.72	0.0443
Incubation	0.0	2.0 ± 2.0	6.0 ± 4.1	0.1567
Perch changes	21.0 ± 5.8	13.9 ± 5.6	7.6 ± 3.0	0.1885
Responses are given as mean ± s.e. Conventions as in Table 2.

Clay-colored Sparrows did not react aggressively to the model Common Grackle, as only two behaviours recorded were significant, distances <2 m and >5 m (Table 4). The sparrows spent more time closer to the model when it was closer to the nest and more time farther from the model when it was farthest from the nest. This indicates that the birds centred their behaviour around the nest, not around the model. Perch changes showed a strong but nonsignificant trend, decreasing in frequency as the model was placed farther from the nest. Only the quiet chip was significant for the Fox Sparrow model (Table 5). Chipping decreased as the model was placed farther away, though not significantly.

Table 4. Summary of responses of Clay-colored Sparrows to Common Grackle models presented at three distances from the nest, and results of Friedman test and associated multiple comparisons.
	Distance (m)
Response	0.5 (7)	2.5 (8)	4.5 (10)	p-value
< 2 m	9.6 ± 4.21	2.9 ± 2.02	0.02	0.0111
> 5 m	3.1 ± 2.51	14.6 ± 5.22	20.2 ± 4.12	0.0361
Perch changes	29.3 ± 11.5	17.0 ± 4.4	15.2 ± 3.2	0.1027
Responses are given as mean ± s.e. Conventions as in Table 2.

Table 5. Summary of responses of Clay-colored Sparrows to Fox Sparrow models presented at three distances from the nest, and results of Friedman test and associated multiple comparisons.
	Distance (m)
Response	0.5 (4)	2.5 (4)	4.5 (4)	p-value
Chip	5.0 ± 4.7	0.3 ± 0.3	0.0	0.1537
Quiet chip	0.5 ± 0.51	2.8 ± 0.62	0.5 ± 0.51	0.0029
Responses are given as mean ± s.e. Conventions as in Table 2.

Discussion

The different responses to the female cowbird model compared to the control suggests that Clay-colored Sparrows recognize cowbirds as a specific threat. Indeed, they responded more aggressively to the cowbird model as more time was spent near the nest and they chipped more frequently (Table 2).

Clay-colored Sparrows apparently did not distinguish between different predators (Table 1). Nor did they react aggressively to the predators. The sparrows only gave quiet chips to the predator models. Bent (1968) described the quiet chip or "tsip" call as a communication call. This suggests that the birds are not disturbed by the presence of the predator models. Another fact that suggests that they were not disturbed is that the birds centred their behaviours around the nest and not the model as would be expected if the model posed a real threat. Neudorf and Sealy (1992) were the first to test an avian predator, the Common Grackle, along with a female cowbird and a control. They showed that some species reacted differently toward the predator and brood parasite whereas others did not. My study is one of the few to test two different types of predators, avian and mammalian (see also Knight and Temple 1988). These tests allowed me to determine if responses varied for different predators and if predators are recognized as unique threats. Clay-colored Sparrows were not highly aggressive and apparently did not recognize unique predators (Tables 3 and 4).

The Fox Sparrow elicited some aggression, perhaps due to its similarity in shape and size to a cowbird. The only significant behaviour was quiet chipping, which suggests that the birds were not disturbed. However, chipping was greatest at the closest distance indicating that they were slightly disturbed as chip calls were given as alarm calls (Walkinshaw 1944, Bent 1968). Other studies have found only low levels of aggression elicited from a variety of hosts when presented with a Fox Sparrow model (Hobson and Sealy 1989, Neudorf and Sealy 1992). Clay-colored Sparrows, therefore, may recognize a shape or size and not individual species but due to the small sample size for Fox Sparrow, it is impossible to say with certainty if they recognize cowbirds per se or simply shape and size. Neudorf et al. (unpubl. data) found that bill shape was more important than plumage colour or pattern in recognizing cowbirds. Fox Sparrows and cowbirds have similar bills, although it is slightly shorter in the former. This similarity may account for the slight aggressive responses recorded in these studies.

Sealy et al. (1995) placed a female cowbird on the nest and found that the Clay-colored Sparrows responded aggressively and even knocked the model off the nest. This study demonstrates that Clay-colored Sparrows can be aggressive but may respond aggressively only to threats right at the nest (see also Neudorf and Sealy 1994, Sealy et al. 1995). They may not react until the threat is at or on the nest so that their behaviour does not reveal the position of their well-concealed nests.

Distance testing

Host aggression generally has been assumed to deter predators (Blancher and Robertson 1982, Buitron 1983). However, several workers have suggested that host aggression may be used by both predators and parasites to locate nests (Smith 1981, Smith et al. 1984, Hobson et al. 1988). McLean et al. (1986) found that alarm vocalizations attracted avian predators. Wiley (1988) found that cowbirds were attracted to areas where residents aggressively defended against intruders. These behaviours seem to be "maladaptive" unless they indeed deter parasites and predators, and the nest owners benefit by this behaviour (Smith et al. 1984).

Clay-colored Sparrows responded but not aggressively at all three distances, which reveals that aggression could be used as a cue to locate nests. Aggressive behaviours were, however, observed infrequently and were not elicited by all individuals, therefore, aggressive behaviours probably cannot be used as reliable cues for cowbirds. However, for those individuals that exhibit aggressive behaviour, cowbirds may be able to cue in on them, opportunistically. Cowbirds may be able to use the distance between them and the host, the number of perch changes, and the frequency of chips as cues to the presence of a nest. Clay-colored Sparrows spent more time closer to the model, changed perches and chipped more frequently when the model was closer to the nest. There was also a gradation in responses for <2 m, perch changes and chips, which increased in frequency as the distance from the nest decreased. Chip calls indicate that adults were disturbed and may indicate aggression. These three behaviours may be used by cowbirds to locate nests. For most of the responses, the two closest distances seem to be similar and the responses more frequent than the third and farthest distance. This may indicate a threshold distance, where the adults ignore the model cowbird (intruders) until a certain distance (somewhere between 2.5 and 4.5 m) and then respond as the threat increases (intruder closer to nest).

Clay-colored Sparrows did not react aggressively toward the two predator models. The responses that were significant were not aggressive behaviours and therefore host aggression cannot be used as a cue by predators to locate nests. The quiet chip was given frequently and seems to be given most often when the birds are foraging or communicating with one another (pers. obs.). This suggests that the birds were not disturbed. The prediction of increased aggression as the predator model was placed nearer to the nest was not supported for either type of predator. Clay-colored Sparrows may not want to draw attention to the nest and, therefore, are not aggressive towards predators, thus eliminating this as a nest-finding cue or perhaps they know they cannot deter predators (Knight et al. 1985, Sealy 1994).

Model testing at different distances from the nest has only been done in one other study (Gill et al., unpubl. data). These results show that responses vary with distance and that this is important when looking at nest success with respect to cowbird and predator models because conclusions about nest finding cues and behaviours that attract predators and parasites (McLean et al. 1986) may vary depending on testing distance.

None of the five cues (host activity, host aggression, nest concealment, nest height, or supporting vegetation) examined in this study was used by parasites or predators to locate Clay-colored Sparrow nests. These cues may work to varying degrees, either alone or in combination, for other species. Nest finding is probably species-specific, with nests of some species being found more readily. The question, therefore, still remains: what specific cues do parasites and predators use to find Clay-colored Sparrow nests?

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Bell, W.J. 1991. Searching behaviour: the behavioural ecology of finding resources. Chapman and Hall, London, England. pp. 358.

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