Location and selection of Clay-colored Sparrow nests by Brown-headed Cowbirds Paula Grieef Department of Zoology, University of Manitoba Winnipeg, Manitoba, Canada E-mail:

Introduction

The Brown-headed Cowbird (Molothrus ater) is an avian brood parasite that lays its eggs in nests of other birds, the hosts. The hosts then raise the offspring. Parasitism often incurs a cost in lost reproductive effort and is considered an important cause of nest failure (Ricklefs 1969). Cowbirds have been known to parasitize 216 species, although only 139 have successfully raised young cowbirds (Friedmann et al. 1977). These numbers show that although many species are potential hosts, for various reasons, some are better than others. In fact, several studies have shown that cowbirds do not parasitize hosts in proportion to their abundance and actually preferentially parasitize a few species (Fleischer 1986; Wiley 1988).

Before laying their eggs, cowbirds must first choose an appropriate host. Many host characteristics have been discussed by several authors: diet, egg size, breeding synchrony between host and parasite, and whether hosts accept or reject cowbird eggs (Rothstein 1975; Payne 1977; Wiley 1988). Once a cowbird has chosen an appropriate host, it must then locate nests and choose a suitable nest from among those found. In the present study, I am examining three cues that cowbirds might use to locate and select a host nest: (1) host activity, (2) host aggression, and (3) nest concealment.

Host activity may be used by cowbirds to locate nests (Buech 1982). I am testing this by placing out nests and simulating different amounts of host activity (Thompson and Gottfried 1981). Host aggression was used to test if the nest-cue hypothesis could operate. This hypothesis states that cowbirds use host aggression to locate nests. It predicts that the more aggressive individuals should be parasitized more frequently because their nests are easier to locate (Robertson and Norman 1977; Smith 1981). It also requires that hosts be aggressive at distances farther from their nests in order to attract cowbirds. This is the aspect that I am testing. Nest concealment will be used to determine if cowbirds actually choose nests, rather than parasitize them at random. Predation is a major cause of nest failure for most passerine species (Ricklefs 1969). It has been shown that better-concealed nests suffer less predation (McLean et al. 1986). Cowbirds should therefore lay in the better-concealed nests because they will also benefit from a decreased predation rate (Gates and Gysel 1978).

This study involves a grassland species, the Clay-colored Sparrow (Spizella pallida). Knapton (1978) found that better-concealed nests were more successful because they were preyed upon less frequently. He also stated that nest defense appears to be ineffective in preventing predation. This may also apply to parasitism as it's sometimes considered as a form of predation.

Methods

This study was initiated at the University of Manitoba Field Station at Delta Marsh (50°10'N, 98°22'W) during the spring and summer of 1993. A second field season is planned for 1994. The area is an old-field succession dominated by snowberry (Symphoricarpos occidentalis) and wild rose (Rosa sp.).

I located nests by searching the habitat daily. For each nest found, I took a concealment value in eight directions by assigning a value of 0-5 corresponding to decreasing visibility of the nest, 0 = 100% visibility, 1=80% visibility, etc. (see Holway 1991). I averaged these values and compared them for each nest. I monitored each nest for signs of predation and parasitism. I considered a nest successful if, three days after the last egg was laid, the complete clutch was still intact. I chose a three-day cutoff because I wanted a comparable time frame for both parasitism and predation. Parasitism does not usually occur after the first few days of incubation (Sealy 1992).

The host-activity experiment consisted of placing out old Clay-colored Sparrow nests for eight days. Nests received one of five treatments: no eggs, full clutch (four eggs), one egg per day, one egg per day + model Clay-colored Sparrow or one egg per day + model + song (Thompson and Gottfried 1981). The first three treatments were part of the 1993 field season. Treatments four and five will be part of the 1994 field season. The five treatments simulate different amounts of host activity. I monitored nests daily for eight days for signs of predation and parasitism. I conducted comparisons between treatments and between nest failure types. The artificial eggs used were made of plaster-of-Paris (Rothstein 1970) and painted to resemble Clay-colored Sparrow eggs.

To quantify host aggressiveness, I used models placed 0.5 m, 2.5 m or 4.5 m from the nest to determine at what distance Clay-colored Sparrows react to a model. I recorded the responses of the Clay-colored Sparrows and later transcribed them (see Smith et al. 1984; Hobson and Sealy 1989), classifying them as follows: distance from model, vocalizations, attacks, foraging, perching and incubating. In 1993, I used a female cowbird as the model to seek support for the nest-cue hypothesis. In 1994, I will use a visual predator and an olfactory predator as models to determine if aggressive behaviour may function to deter or attract predators. If the sparrows are equally aggressive at all distances then the behaviours may have a deterrent function. If on the other hand, the sparrows are more aggressive closer to the nest then the behaviours may inadvertently attract predators. A third set of model testing will also be conducted using four models at 0.5 m to determine if Clay-colored Sparrows recognize various nest threats to their productivity.

Results

During the 1993 season, I found 62 Clay-colored Sparrow nests. Of these nests, 6 were parasitized (9.7%), 6 were predated (9.7%) and 50 were successful (80.6%). There was no significant difference for all concealment values taken at both eye-level and nest-level for the three possible outcomes. The majority (> 56%) of nests fell into a concealment value range of 4.01-5.00, i.e. highly concealed. Nest outcome was not related to either nest height (Fisher's exact test, p = 0.769) or supporting vegetation (Fisher's exact test, p = 0.710). Snowberry was the dominant plant species with 74% of nests built in this species.

I conducted two runs of nest placements (the first from 2 to 10 June and the second from 10 to 18 June) to increase sample size. I was able to combine the two runs because there was no difference between them (Fisher's exact test, p = 0.917). None of the three treatments (no eggs (N = 33), full clutch (N = 33) and one egg/day (N = 34) affected the nest outcomes (Chi-square test, p = 0.787). It is interesting, however, to note that none of the nests was parasitized. Concealment also had no effect on outcome for any treatment at both nest-level and eye-level.

With the female cowbird model, I tested 17 nests at the three distances. Only four behaviours recorded were significant, distances 2 m and 2-5 m, chips, and perch changes (Table 1). None of the more aggressive behaviours, such as close passes was significant. This is perhaps because they occurred rarely.

Discussion

The non-significant findings suggest that there is no relationship between nest concealment and nest outcome. The major predator in this area in the Franklin's ground squirrel (Spermophilus franklinii), an olfactory predator. Due to this fact predation rates are not expected to be related to concealment. Because host species do not benefit from a decreased predation rate with increasing concealment a cowbird egg also would not benefit. It then follows that cowbirds should lay in any nest regardless of its concealment. Indeed, this is what I found. There was no significant trend of increased parasitism rates with increasing concealment values.

The three levels of host activity simulated did not appear to be enough to elicit parasitism, as no parasitism was observed. Predation, however, did occur and seemed to be independent of the level of host activity. One possibility is that perhaps a critical level of activity or a critical type of activity was not simulated. It is this critical point that must be reached before cowbirds cue in on the hosts and their nests.

Host aggression towards a female cowbird at three distances was used to test if the nest-cue hypothesis could operate. Aggressive behaviours were infrequent and therefore probably cannot be used as a cue for cowbirds. Cowbirds may, however, be able to use the distance of the host from itself, the frequency of chips, and the number of perch changes as cues to the presence of a nest. Clay-colored Sparrows spend more time closer to the model and perch-change and chip more frequently when the model is closer to the nest. There is also a gradation in responses for < 2 m, chips and perch changes; all three increase in frequency as the distance from the nest decreases. These three cues may be used by cowbirds to locate nests. It is these three behaviours that may allow the nest-cue hypothesis to operate because they occur at the furthest distance. They are not, however, highly aggressive behaviours. These behaviours may also give the cowbird an indication of host quality. If sparrows can spend more time near the nest and not be away foraging, this might indicate a better quality host, i.e. the sparrows may be superior at foraging in a shorter time frame and can therefore spend more time near the nest (Arcese and Smith 1988). Cowbirds may be able to detect this and select these nests because their young may have a better chance of surviving.

Summary

This study has examined some of the possible cues used by cowbirds to locate and select host nests. Three of six objectives of the project were met in the 1993 field season. I looked at host activity using nest placements, and host aggression using a female cowbird model to discover their role in nest location. I also looked at nest concealment to see if it had a role in nest selection. I needed to do more research to further isolate the cues that may be used individually or in combination by cowbirds. Two of these cues will be further examined in the 1994 season; host activity will be further investigated by adding two treatments to the nest placements, and host aggression will be looked with respect to predators using a visual and an olfactory predator. I will also test enemy recognition using four models.

Acknowledgements

I wish to thank Dr. Spencer G. Sealy for his guidance throughout this study. I gratefully appreciate the field assistance of D. Beattie, K. Caldwell, D. Froese, G. Goulet and L. Zdrill. Thanks to the staff at Delta Marsh for all their hospitality. Statistical advice was provided by the University of Manitoba Statistical Advisory Service. This project is partially funded by an NSERC Research Grant to Dr. S.G. Sealy.

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