Sharon A. Gill
Department of Zoology, University of Manitoba
Winnipeg, Manitoba, Canada
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The evolution of Yellow Warbler alarm calls
Sharon A. Gill Department of Zoology, University of Manitoba Winnipeg, Manitoba, Canada |
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Studies comparing the responses of populations sympatric with brood parasites to populations allopatric with brood parasites have provided insight into evolutionary processes involving brood parasites and their hosts (e.g., Cruz and Wiley 1989; Davies and Brooke 1989; Soler 1990; Soler and Møller 1990; Briskie et al. 1992). Similar studies have focused on evolutionary processes between predators and their prey (e.g., Giles and Huntingford 1984; Goldthwaite et al. 1990; Towers and Coss 1990; Foster 1994). These studies have inferred the loss of an adaptive behaviour in the absence of a selection pressure (e.g., Cruz and Wiley 1989; Goldthwaite et al. 1990; Foster 1994), the maintenance of a behaviour due to phylogenetic constraints (e.g., Towers and Coss 1990), and the presumptive evolution of a behaviour in the presence of a selection pressure (e.g., Briskie et al. 1992). These studies contribute to our understanding of the evolution of behaviour, but few have permitted the inference of the direction of evolutionary change (but see Bolles 1988; Cruz and Wiley 1989; Goldthwaite et al. 1990; Towers and Coss 1990; Foster 1994), or considered the influence of multiple selection pressures on a behavioural repertoire (but see Giles and Huntingford 1984; Goldthwaite et al. 1990; Towers and Coss 1990).
Yellow warblers (Dendroica petechia) have a complex alarm call system consisting of four calls that they utter during nest defence (Ficken and Ficken 1965; Hobson et al. 1988; Hobson and Sealy 1989; Gill and Sealy, ms). Chip and metallic chip calls grade into each other and are given in response to mammalian and avian predators (Hobson et al. 1988; Gill and Sealy, ms), but also in a wide variety of non-threatening contexts (Ficken and Ficken 1965; Spector 1991; pers. obs.). Seet calls are uttered preferentially to brown-headed cowbirds (Molothrus ater) and usually uttered in conjuction with nest-protection behaviour (Hobson and Sealy 1989; Neudorf et al., ms.; Gill and Sealy, ms). The fourth call, the warble call, is given in tandem with distraction displays in response to nest predators (Gill and Sealy, ms).
Given that the seet call is uttered in such a restrictive context, Briskie et al. (1992) speculated that this call and the associated nest-protection behaviour evolved under the pressure of cowbird parasitism. To test this hypothesis, they compared the responses to a female cowbird model of a population sympatric with cowbirds to a population allopatric with cowbirds. They found that individuals in the population allopatric with cowbirds rarely uttered seet calls and never performed nest-protection behaviour in response to the cowbird, which supports their hypothesis (Briskie et al. 1992). However, they did not consider the responses of this population to an avian nest predator. Thus, these behaviours may have evolved in response to avian nest predation, but now are used in response to cowbirds (i.e. exaptation, Gould and Vrba 1982). In this scenario, avian nest predators and playbacks of seet calls, but not cowbird models, would elicit seet calls and nest-protection behaviour in the population allopatric with cowbirds. By contrast, if these behaviours evolved due to cowbird parasitism, seet calls and nest-protection behaviour should not be elicited by any stimuli in that population.
I tested the hypothesis that seet calls evolved due to cowbird parasitism by (1) presenting cowbird and predator models, and playbacks of alarm calls to a population allopatric with cowbirds (at Churchill, Manitoba), and (2) comparing the responses of that population to a population sympatric with cowbirds (Delta Marsh, Manitoba; Chapter 1). I predicted that (1) yellow warblers in the population allopatric with cowbirds would not respond with seet calls or nest-protection behaviour to any stimuli, and (2) that the populations would differ in their responses to the models and playbacks.
I conducted research from 18 May to 7 July 1993 and 19 May to 16 June 1994 on property of the University of Manitoba Field Station (Delta Marsh) and Portage Country Club (PCC), west of the Assiniboine River diversion, and east of the diversion in the town of Delta. All sites are located at the south end of Lake Manitoba (see map in Sealy 1980, see MacKenzie 1982, Mackenzie et al. 1982 for description of study site). West of the diversion, yellow warblers nested in the forested dune-ridge that separates Lake Manitoba from the surrounding marsh. In the town of Delta, yellow warblers nested in sandbar willow groves situated along roads.
From 17 June to 7 July 1994, I studied a population of yellow warblers allopatric with cowbirds near the town of Churchill, MB (see Figure 1 in Briskie et al. 1992). Yellow warblers nested in willow-birch thickets that lined gravel ridges, streams, and ice ridges around lake and pond shores (Jehl and Smith 1970; Briskie, ms; see Johnson 1987 for description of habitat). The dominant shrubs in these areas are dwarf birch (Betula glandulosa), short-capsuled willow (Salix brachycarpa), hoary willow (S. candida), flat-leaved willow (S. planifolia), and green alder (Alnus crispa; Johnson 1987). Most warblers built their nests in flat-leaved willows, usually towards the centre of the thicket (Briskie, ms; pers. obs.).
At both study sites, I located nests during building, egg-laying and incubation stages, and flagged them from at least 2 m away. Nests were checked daily during nest-building and egg-laying periods until clutch completion, and after clutch completion, nests were monitored irregularly until the eggs hatched.
At Delta Marsh, I presented taxidermic mounts of a fox sparrow, common grackle and female brown-headed cowbird near yellow warbler nests. Fox sparrows are a semi-novel stimulus to yellow warblers because they migrate through the study area (see Hobson and Sealy 1989a). Common grackles (female: 100 g, male: 127 g) are considerably larger than warblers, but at Delta Marsh, grackles are the smallest known avian predator of both eggs and nestlings (Sealy 1994). Furthermore, responses of some species are not influenced by model size (Robertson and Norman 1977; Neudorf and Sealy 1992). Common grackles have been observed preying on adult passerine birds the size of yellow warblers (Davidson 1994). Although Davidson (1994) observed only one grackle taking adult passerines during migration, her observations call into question my assumptions that grackles are solely egg and nestling predators and that nest owners do not perceive the models as threats to themselves.
At Churchill, I presented models of a fox sparrow, gray jay and female cowbird. Cowbirds occur only accidently in the Churchill area, with the nearest breeding record is approximately 600 km SW of Churchill (Allen 1945; Jehl and Smith 1970; Godfrey 1986; Figure 6). Fox sparrows nest uncommonly at Churchill (Godfrey 1986), but they were heard singing in areas in which yellow warblers nested (pers. obs.). I did not present a common grackle to nesting yellow warblers because grackles occur infrequently in the Churchill area (Godfrey 1986), although there are several breeding records (Jehl and Smith 1970). Instead, I presented to yellow warblers a study skin of a gray jay (Perisoreus canadensis) attached to a wire clip. Gray jays are common egg and nestling predators of passerine nests (Strickland and Ouellet 1993), and they occur in the boreal forests adjacent to the nesting habitat of yellow warblers (Godfrey 1986; pers. obs.). Briskie (ms) speculated that gray jays may prey upon yellow warbler nests, but that the major predators of warbler nests were arctic fox (Alopex lagopus) and red fox (Vulpes fulva). Previous studies have found that yellow warblers respond differently to mammalian nest predators than avian nest predators (compare Hobson et al. 1988 and Gill and Sealy, ms; see also Buitron 1983; Knight and Temple 1988). American crows (Corvus brachyrhynchos) are the other common avian nest predator on warbler nests (Briskie, ms). Because of the crow’s large size, warblers may respond to them as a predator on themselves (e.g., Buitron 1983). Thus, neither crows nor foxes would be appropriate stimuli for a population comparison of the alarm calls towards an avian nest predator.
I tested nests at the egg-laying (2 to 5 days) stage. To avoid habituation or positive reinforcement, I tested each nest once with the models (Knight and Temple 1986a,b). I tested nests after 0700 (Central Standard Time) by which time yellow warblers had finished laying (S. G. Sealy and D. L. Neudorf, unpubl. data), and continued testing nests up to 1800. Observations were made from a blind positioned 5 - 10 m from the nest set up 15 min prior to testing to allow nest owners to habituate to the blind. Occasionally, I did not use a blind, but was positioned as far back as possible while maintaining a clear view of the nest.
After 15 min and if the nest owners were out of the area, I quickly positioned the model approximately 0.5 m from the nest, clipped to vegetation and facing the nest. If the nest owners were in the area after the 15-min period, I waited to present the model until both left the area. I presented the models in random order and separated successive tests by at least 15 min to reduce carry-over aggression (Knight and Temple 1986a, b). The 5-min testing interval began when a nest owner arrived within 5 m of the nest. As none of the birds tested was colour-banded, I assumed that when two individuals responded, they were the nest owners. Occasionally other yellow warblers responded, however, these individuals neither approached the model as closely nor responded as intensely to it as did the presumed nest owners. I recorded whether 1 or 2 birds responded, and the sex and behaviours elicited for the entire 5-min trial.
Once one of the nest owners responded, I quantified responses using the method of Smith et al. (1984) and Hobson and Sealy (1989a). I recorded the following responses displayed by both male and female nest owners: (a) alarm calls: seet and chip calls (Hobson and Sealy 1989a), metallic chip (Ficken and Ficken 1965), warble calls that have not been previously recorded (see Results) and zeep calls (Ficken and Ficken 1965); (b) silent watching; (c) distance of nest owner from model (< 2 m, 2-5 m, > 5 m); (d) distraction displays (see description in Hobson and Sealy 1989a); (e) close passes; (f) hovers; (g) strikes; (h) perch changes; (i) nest-protection behaviour, or sitting in the nest; (j) displacement activities such as preening, bill wiping, foraging, feeding female and/or nestlings; (k) singing by male and begging by female; and (l) nest owners out of area. Categories b, c, d, i, and l were recorded as the number of 10-second intervals in which they occurred (maximum value = 30 intervals), while the remaining categories were recorded as the number of times they occurred in the 5-minute trial. If both nest owners called during the model trial I could not assign vocalizations to a particular sex. Therefore, I combined male and female calling for all nests. I recorded observations using a hand-held tape recorder and transcribed the tapes later. Details on the non-vocal responses of yellow warblers (categories c - l) are provided in Gill and Sealy (ms) and Gill (unpubl. data).
During model presentations, I recorded the first 2 min of alarm calls elicited by the models using a Uher 4000 Report-L, Sennheiser ME 88 microphone with fixed windscreen and K3 low frequency filter, and Ampex Precision magnetic tapes. Tape speed was always set at 19 cm / sec to make the best quality recording possible (Spector 1991). The Sennheiser microphone is highly directional, and suitable when the microphone cannot be positioned close to the sound source, as was the case during model trials.
From the recordings, I made 1-min playback tapes of seet or chip calling for each nest. The 1-min length of playback is appropriate to prevent habituation of the receivers (Falls 1982). Moreover, parasitism by cowbirds (Sealy et al. 1995) and predation (Sealy 1994) occur within 1-2 mins. I selected a section of tape 2-30 sec long from which I made a 1-min playback using the Uher Report-L recorder, a Sony TCM-5000EV recorder and Sony Metal SR tapes. The template varied considerably in length because nest owners varied the length of time they spent calling. I recorded the section used to make the playback in all cases. I could not control for call rate in the playbacks because I made the tapes in the field, and the nature of yellow warbler alarm calling behaviour is such that they usually give more than one call per second. Thus, call rate varied between call types (seet = 114.8 ± 11.0 per min playback, chip = 37.9 ± 3.6). Call rate has been shown to influence responsiveness in some species (Leger and Owings 1978; Leger et al. 1979; Weary and Kramer 1995), but not others (Harris et al. 1983).
I performed the playback experiment one or two days after model testing, at the same nests at which model testing was done. Playbacks were delayed by two days only in cases of rain. Fifteen minutes prior to testing I set out a blind and placed an Audio-Technica amplified speaker 1 m below the nest, either on the ground or in a crotch of a tree. The speaker positions were appropriate as yellow warblers vocalized from the ground and from perches (pers. obs.). The speaker was connected by a 10-m cord to the Sony TCM-5000EV recorder. Although I did not measure the loudness of the calls, I played back the calls at what appeared to be their typical level. Loudness does not affect the responses of several species to playbacks of their alarm calls (e.g., Seyfarth et al. 1980a; Weary and Kramer 1995; but see Leger et al. 1979). I randomized the order of the playbacks.
Each playback trial consisted of observations made one min before and during the playback, and five mins of behavioural observation after the playback. The comparison of behaviours before, during and after playback is necessary to show that nest owners changed their behaviours in response to the playbacks. I recorded behaviours directly into a field notebook in which I had delineated 7 min into 10-sec intervals. I recorded the same behaviours as in the model presentation (see above). I also recorded the proportion of nest owners that moved closer to the speaker during the trial, because the distance categories used in model presentation do not necessarily show if this occurred.
At Churchill, I played back seet and chip alarm calls and a control playback of tape noise to yellow warblers at egg-laying and early incubation stages. I did not expect warblers at Churchill to seet call, therefore, I played seet calls recorded from warblers at Delta Marsh to warblers at Churchill. At one nest I played a chip call loop also recorded from Delta Marsh. However, this loop contained considerable background noise. Background noise was not obvious during playbacks at Delta Marsh where noise in the environment, especially wind, masked most noise on the tape loop (pers. obs.). At Churchill, however, this noise was clearly audible to me, and thus, presumably to the warblers. Therefore, I made a playback chip tape from Churchill warblers.
I used nonparametric statistics to analyze the results because the data were not normally distributed. To determine whether model type (sparrow, grackle or cowbird) or call type (seet, chip or noise) influenced the vocal and nest defence responses elicited, I used FriedmanÕs tests blocked by nests. Blocking by nests reduces variance because secondary variables characteristic of the nest owners (e.g., age, experience) are held constant among stimuli (Kamil 1988). The Friedman test is equivalent to a parametric two-way analysis of variance on the ranked data (Conover and Iman 1981). When significant differences resulted (p < 0.05), I used Fisher’s protected least significant difference (FPLSD) test on the ranks to determine which stimuli elicited significantly different responses. The FPLSD test is equivalent to non-parametric multiple comparisons (Conover and Iman 1981). To determine whether the proportion of nest owners that uttered alarm calls differed among models or call types I used chi-square tests. I used Wilcoxon two-sample tests to determine whether responses to the models or calls differed between Churchill warblers and Delta Marsh warblers.
Yellow warblers, especially females, frequently vocalized during model presentation at the egg-laying stage (Table 1). They gave significantly more seet calls to the cowbird model than the sparrow or grackle. Most females (62.9 %) uttered seet calls as they performed nest-protection behaviour (Hobson and Sealy 1989a; Gill and Sealy, ms), but after entering their nests females usually stopped calling. Yellow warblers uttered significantly more chip calls in response to the grackle model than the other models, and more to the sparrow than the cowbird model. Yellow warblers gave metallic chip calls (Ficken and Ficken 1965) to cowbird and grackle models only, although this was not significant. Metallic chips were uttered intermittently during bouts of chip calling, although one male uttered metallic chips throughout the grackle trial. Warble calls were elicited only by the grackle model. One male nest owner uttered warble calls when he performed distraction displays. Females were silent most frequently during cowbird trials, whereas males were silent equally during all trials.
At Churchill, female yellow warblers responded similarly regardless of model type (Table 2). Three nest owners uttered seet calls in response to the models. One female uttered seet calls to all the models, and also performed nest-protection behaviour in response to the cowbird. Once in the nest the female was silent for the remainder of the trial. The male at this nest did not appear during the trials. One male uttered seet calls in response to the cowbird model, and changed perches within 1 m of the nest during the entire trial. The female of this nest did not respond aggressively to the cowbird model, but she was in the nesting area during the trial. A third female apparently seet called in response to the jay model. This female gave ‘zeep’ calls to the cowbird and sparrow, which appeared to grade into the seet calls recorded in response to the jay. Females uttered chip calls more frequently in response to the jay model than the cowbird or sparrow model, but this was not significant. Females chipped intermittently throughout the trials, while changing perches. Metallic chip calls were uttered by only one nest owner each in response to the cowbird and jay models. The jay model elicited warble calls at one nest. The cowbird and jay models elicited zeep calls at five nests, and the sparrow elicited zeep calls at four nests. While uttering zeep calls, females changed perches. The greatest part of the trial was spent silently watching the models or ignoring the models and returning to the nest to resume incubation. Only one female showed true nest-protection behaviour in response to the cowbird, although up to six females returned to their nests and incubated during the trials.
Females uttered significantly more seet calls in response to the seet playback than the chip or control playback (Table 3). Yellow warblers uttered more chip calls to the chip and noise playbacks than the seet playback, although this relationship was not significant. Warblers uttered metallic chip calls only in response to the control playback, but this was not significant. No warbles were recorded in response to any of the playbacks. Females spent a similar amount of time silent during all playbacks. Females performed nest-protection behaviour significantly more in response to the seet playback than chip and control playbacks.
The responses of female yellow warblers differed slightly among models (Table 4). Nest owners gave more chip calls during the chip playback than the seet or control playback but this was not significant. One female nest owner seet called and performed nest-protection behaviour in response to the seet playback. No other birds uttered seet calls or performed nest-protection behaviour during the playbacks. Metallic chip and warble calls were not uttered during any playback. Nest owners uttered zeep calls in response to the chip and control playback, but this was not significant. Females tended to be silent more during the seet and control playback than the chip playback, but this was not significant. Females tended to incubate more frequently during the control playback.
Overall, Delta warblers uttered more alarm calls than Churchill warblers during model trials (Tables 1 and 2). Delta warblers uttered significantly more seet calls to all models than Churchill warblers (Wilcoxon two-sample test, tR = - 5.07, df = 1, P = 0.0001; tR = - 1.99, df = 1, P = 0.0466; and tR = - 3.00, df = 1, P = 0.0027, for cowbird, predator and sparrow models, respectively) . Delta warblers uttered significantly more chip calls to the predator than Churchill warblers (tR = -2.41, df = 1, P = 0.0158). Warblers in both populations rarely uttered metallic chip and warble calls to any model (tR < 0.53, df = 1, P > 0.50). Delta warblers never uttered zeep calls during model testing. Therefore, Churchill warblers uttered more zeep calls to cowbird and predator models (tR = 3.32, df = 1, P = 0.0009; tR = -3.32, df = 1, P = 0.0009, and tR = -2.93, df = 1, P = 0.0033) for cowbird, predator and sparrow models, respectively). Churchill warblers were silent significantly more to the sparrow than Delta warblers (tR = 2.13, df = 1, P = 0.0335). Females at Delta performed nest-protection behaviour significantly more in response to cowbird than Churchill females (tR = -2.31, df = 1, P = 0.0211).
Delta warblers uttered significantly more seet calls (tR = - 2.06, df = 1, P = 0.0396) and more frequently performed nest-protection behaviour (tR = - 2.51, df = 1, P = 0.0121) to the seet playback than Churchill warblers (Tables 3 and 4). By contrast, Churchill warblers uttered significantly more chip calls in response to the chip playback than Delta warblers (tR = 1.94, df = 1, P = 0.0522). Females at Churchill were silent more than Delta females during the control playback (tR = 2.28, df = 1, P = 0.0228). The remaining behaviours did not differ significantly between populations for any playback (tR < 1.45, df = 1, P > 0.10)
Yellow warblers allopatric with cowbirds rarely uttered seet calls or performed nest-protection behaviour in response to cowbird and jay models, and seet and chip playbacks. These results do not support the hypothesis that these behaviours evolved in response to avian nest predation, and are currently used in the context of brood parasitism (i.e. Gould and Vrba 1982). Instead, the results support Briskie et al.’s (1992) hypothesis that seet calls and nest-protection behaviour evolved in response to cowbird parasitism. Interestingly, Robertson and Norman (1977) found that the aggressive responses of several species, including yellow warblers, towards cowbirds did not vary between populations in recent (Ontario) and ancient (Manitoba) sympatry with cowbirds (Mayfield 1965; see also Burgham and Picman 1989). Whether the population in recent sympatry gave seet calls and nest-protection behaviour is unknown because these authors used a subjective scoring index that did not provide details on yellow warbler responses. Burgham and Picman (1989) in another Ontario population found that yellow warblers performed nest-protection (although they misidentified this behaviour as premature incubation), but they did not describe alarm calls that were uttered. Nevertheless, Robertson and Norman’s (1977) and Burgham and Picman’s (1989) findings suggest that yellow warblers in recent sympatry have evolved a level of aggression within 200 years equal to that of yellow warblers in ancient sympatry with cowbirds. Thus, brood parasitism is an important selective pressure that influenced the evolution of nest-defence behaviours in yellow warblers (Robertson and Norman 1977; Briskie et al. 1992; this study).
Brood parasitism has driven the evolution of parasitic egg rejection by cowbird and cuckoo (Cuculus spp.) hosts. Several host species in areas of ancient sympatry with brood parasites eject parasitic eggs naturally or artifically introduced into their nests, whereas populations allopatric or recently sympatric with brood parasites rarely eject these eggs (Davies and Brooke 1989; Brown et al. 1990; Soler 1990; Soler and MØller 1990; Rothstein 1990; Briskie et al. 1992; but see Zuñiga and Redondo 1992). Briskie et al. (1992) further showed American robins (Turdus migratorius) do not eject conspecific eggs indicating that it is cowbird parasitism rather than conspecific brood parasitism that selected egg ejection in this species (see also Sealy et al. 1989; Sealy and Bazin, in press). By contrast, although eastern phoebes (Sayornis phoebe) and clay-colored sparrows (Spizella pallida) sometimes desert parasitized nests, they do so in response to partial clutch reduction associated ocasionally with parasitism, rather than in response to parasitism per se (Rothstein 1986; Hill and Sealy 1994; but see Davies and Brooke 1988; Moksnes and Roskaft 1989). These authors concluded that not all apparently adaptive behaviours evolved under in response to brood parasitism (Rothstein 1986; Hill and Sealy 1994).
Given that seet calls and nest-protection behaviour (hereafter anti-cowbird behaviours; Gill and Sealy, ms) evolved in response to cowbird parasitism (Briskie et al. 1992; this study), the expression of these behaviours was not expected in the Churchill population. However, three yellow warblers uttered seet calls and one female performed nest-protection behaviour in response to the models, and one of these females performed these behaviours in response to the seet call playback. Briskie et al. (1992) never recorded nest-protection behaviour in response to a cowbird model in this population, although one (n = 15) female uttered seet calls. Similarly, low levels of egg rejection have been documented in populations that are not parasitized (Davies and Brooke 1989; Briskie et al. 1992) or that have been parasitized only recently (Soler 1990; Soler and MØller 1990; but see Zuñiga and Redondo 1992).
Briskie et al. (1992) proposed two possible mechanisms for the presence of anti-cowbird behaviours in the Churchill population. First, Briskie et al. (1992) suggested that gene flow may be responsible for the presence of anti-cowbird behaviour in the Chuchill population. Gene flow refers to the incorporation of genes from one population into another (Futuyma 1986), and hence this hypothesis assumes that the Churchill population is distinct from populations that are parasitized. Yellow warblers range continuously from Delta Marsh to Churchill (AOU 1983; Godfrey 1986), but morphological evidence suggests that these two populations are separate subspecies, D.p. aestiva and D. p . amnicola, respectively (Raveling and Warner 1976; Godfrey 1986). Browning (1994) suggested that yellow warblers in Manitoba are comprised of three subspecies, a northern population (parkesi, which includes Churchill), a central population (amnicola), and a southern population (aestiva, which includes Delta Marsh). Subspecies were assigned on plumage differences among populations (Raveling and Warner 1976; AOU 1983; Godfrey 1986; Browning 1994). Recently, Klein (1992) found that yellow warblers are more phenotypically variable than genotypically variable, therefore, subspecies classification should be viewed cautiously until genetic analysis is performed.
Gene flow among populations results from dispersal of individuals from one population to another, either in the hatch year, i.e. natal dispersal, or after breeding, i.e. breeding dispersal (reviewed by Gauthreaux 1982; Greenwood and Harvey 1982). Breeding dispersal of male yellow warblers is low in an Ontario population as over 70% of males returned to the territory in which they nested the previous year (Studd and Robertson 1989). The extent of breeding dispersal in female yellow warblers is unknown, however, females birds generally disperse farther than males (Gauthreaux 1982; Greenwood and Harvey 1982). The return of individuals to the area in which they were hatched (Gauthreaux 1982) is low in all passerines even when post-fledgling mortality is taken into account (reviewed in Weatherhead and Forbes 1994). Yellow warblers from the Delta Marsh population have a relatively higher natal philopatry (11.3 % of 1152 fledglings banded; J. V. Briskie in Weatherhead and Forbes 1994) compared with a second population (none of 81 nestlings banded returned to unstated location; R. J. Robertson in Weatherhead and Forbes 1994). Although the extent of dispersal is unknown for the Churchill population, Browning (1994) noted that plumage characteristics intergraded between the amnicola (aestiva ?) and parkesi (amnicola ?) populations. Therefore, it is likely that anti-cowbird behaviours are maintained in the Churchill population through gene flow from the central population (see also Soler and MØller 1990; but see Zuñiga and Redondo 1992). By contrast, Davies and Brooke (1989) indicated that the maintenance of egg rejection in Iceland birds could not result from gene flow due to the large barrier (Atlantic Ocean) separating populations allopatric and sympatric with cuckoos.
Second, Briskie et al. (1992) suggested that a founder effect may be responsible for the presence of the behaviours, which are presumably maintained because of low costs (e.g., Cruz et al. 1985; Cruz and Wiley 1989). Founder effect is the principle that individuals founding a new and isolated colony carry only a fraction of the total genetic variation present in the source population (Futuyma 1986). However, there is no evidence to suggest that the Churchill warblers formed an isolated colony at any time during their evolutionary history (c.f. Pielou 1991; Dawson 1992). Rather, the radiation of yellow warblers may have followed receding ice as has been suggested for other wood warblers (Mengel 1964). Thus, founder effect does not adequately explain the presence of anti-cowbird behaviours in the Churchill population.
Invoking founder effect to explain the rare occurence of anti-cowbird behaviours in the Churchill population contradicts the argument that the absence of these behaviours is plesiomorphic (i.e. the primitive condition). As applied by Briskie et al. (1992), this hypothesis implies that all individuals in the founding Churchill population displayed these behaviours at one time in evolutionary history and, over time most descendents of these founders secondarily lost the behaviours. Accordingly, the absence of anti-cowbird behaviours in the Churchill population is apomorphic (i.e. the derived condition; e.g., Foster 1994), and not plesiomorphic as suggested by Briskie et al. (1992). If the Churchill population was descended from a parasitized population during the radiation of yellow warblers, Churchill warblers may have inherited these behaviours (e.g., Davies and Brooke 1989), in which case the absence of the anti-cowbird behaviours is apomorphic.
Authors have incorrectly assumed that by showing differences in behaviour between populations that experience different selection pressures, they can infer the direction of evolutionary change (e.g., Davies and Brooke 1989; Soler 1990; Soler and Moller 1990; Briskie et al. 1992; but see Bolles 1988; Cruz and Wiley 1989; Goldthwaite et al. 1990; Towers and Coss 1990; Foster 1994). This clearly is not true, as this type of study cannot resolve whether a population has secondarily lost the behaviour (i.e. absence is apomorphic) or whether a population originally lacked the behaviour (i.e. absence is plesiomorphic; see above). This question can be resolved only by mapping the behaviour of interest onto independently derived cladograms that show the phylogenetic relationships among populations (Brooks and McLennan 1991; but see Frumhoff and Reeve 1994). Foster (1994) recently used this method to study the diversionary display of three-spined sticklebacks in six freshwater populations and two marine forms. Foster (1994) found that the absence of diversionary displays was the apomorphic condition, a finding that is contradictory to the assumption of many authors that the expression of complex behaviours is apomorphic. Model presentation and playback experiments on other yellow warbler populations and an outgroup to control for phylogeny (Harvey and Pagel 1991), in conjuction with cladistic methods (Brooks and McLennan 1991), must be performed to assess whether seet calls and nest-protection behaviour represent a plesiomorphic or apomorphic condition.
I thank Dr. Spencer G. Sealy for guidance throughout this study. I thank D. Beattie, J. Briskie, K. Caldwell, D. Froese, J. Loranzana, A. McEwen, G. McMaster, G. Stinson, B. Whittam and L. Zdrill for assistance in the field. L. Armstrong of the University of Manitoba Statistical Advisory Service provided statisitcal advice. Thanks to everyone at Delta Marsh and the Churchill Northern Studies Centre for their hospitality, and to the Portage Country Club for permitting me to work on their property. Funding was provided by and NSERC research grant to Dr. S. G. Sealy and an NSERC Postgraduate Scholarship, Faculty of Science Postgraduate Scholarship, and grants from the Frank M. Chapman Memorial Fund and Northern Studies Training Program to myself.
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