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Keywords:

  • birds;
  • body size;
  • dominance;
  • islands;
  • survival

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

Differences between island- and mainland-dwelling forms provide several classic ecological puzzles. Why, for instance, are island-dwelling passerine birds consistently larger than their mainland counterparts? We examine the ‘Dominance hypothesis’, based on intraspecific competition, which states that large size in island passerines evolves through selection for success in agonistic encounters. We use the Heron Island population of Capricorn silvereyes (Zosterops lateralis chlorocephalus), a large-bodied island-dwelling race of white-eye (Zosteropidae), to test three assumptions of this hypothesis; that (i) large size is positively associated with high fitness, (ii) large size is associated with dominance, and (iii) the relationship between size and dominance is particularly pronounced under extreme intraspecific competition. Our results supported the first two of these assumptions, but provided mixed evidence on the third. On balance, we suggest that the Dominance Hypothesis is a plausible mechanism for the evolution of large size of island passerines, but urge further empirical tests on the role of intraspecific competition on oceanic islands versus that on mainlands.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

Morphological differences between island- and mainland-dwelling forms have played an important role in stimulating evolutionary and ecological hypotheses (for recent reviews see Blondel, 2000; Grant, 2001). One of the best known examples of a consistent difference between island and mainland forms is what Van Valen (1965) called the ‘island rule’, which states that small-bodied species tend to get bigger and more robust when they invade an island, whereas large-bodied species tend to get smaller and less robust. This island rule has been demonstrated in both mammals (Lomolino, 1985) and birds (Grant, 1968; Clegg & Owens, 2002). The overall aim of this study is to investigate the ecological basis of one half of this island rule: the increase in size among small-bodied forms, using passerine birds as our model system.

The traditional explanation for morphological shifts in island-dwelling passerines is that the low species-richness of oceanic islands leads to low interspecific competition, which in turn leads to an ‘expanded niche’ and selection for more generalist foraging behaviour and a more generalist morphology (reviewed by Grant, 1998, 2001; Whittaker, 1998; Blondel, 2000). This traditional hypothesis has not, however, been supported in the two explicit tests that have been performed to date (Werner & Sherry, 1987; Scott et al., in press), in which detailed observations on individually-marked birds revealed that apparently generalist populations were in fact made up of a diversity of individual specialists. Moreover, in neither population was there evidence that large body size or long bill length was associated with generalist foraging behaviour. Thus, there is reason to doubt whether changes in foraging niche are the full explanation for morphological shifts in isolated island-dwelling passerines.

An alternative explanation for morphological changes in insular passerines, and especially the large body size typical of those forms, is that island-dwelling populations experience unusually strong intraspecific competition and selection therefore favours any traits that confer an advantage in agonistic interactions (MacArthur, 1961; Mees, 1969; Kikkawa, 1976, 1980b; Schwaner & Sarre, 1988; Case, 1993; Clegg & Owens, 2002). In agreement with this ‘Dominance Hypothesis’, island populations of passerines have often been reported to occur at unusually high densities (e.g. MacArthur, 1961; Crowell, 1962; Grant, 1966; Mees, 1969; MacArthur et al., 1972; Wright, 1980; Werner & Sherry, 1987; Blondel et al., 1988), social dominance is an important determinant of individual fitness in some island populations of passerines (e.g. Smith et al., 1991), and in nonisland populations dominant individuals are commonly larger than subordinates (e.g. King, 1973; Patterson, 1977; Persson, 1985; Craig & Douglas, 1986; Saito, 1996). Remarkably, however, there is no direct evidence that social dominance and fitness is associated with large size in the unusually large races that typify island-dwelling passerines. The basic assumptions of the Dominance Hypothesis remain, therefore, untested in this context.

We used the Heron Island population of Capricorn silvereyes (Zosterops lateralis chlorocephalus), a large-bodied island race of white-eye (Zosteropidae), to test three assumptions of the Dominance Hypothesis in the context of a contemporary island population. The Capricorn silvereye is ideally suited to this type of test because, as well as being 40% larger than its mainland counterpart with proportionately longer and thicker bill, it is also well-established that this population occurs at an unusually high nesting density, that agonistic interactions are common at feeding sites, that dominance affects fitness components, and that the large insular body size is unlikely to be due to neutral genetic mechanisms such as drift and founder events (Kikkawa, 1980b; Catterall et al., 1982; Kikkawa & Wilson, 1983; Jansen, 1990; Kikkawa & Catterall, 1990; Clegg et al., 2002a, b). Finally, there is also direct evidence that, contrary to the predictions of the traditional explanation for large size in insular passerines, there is no simple association between large size and generalist foraging behaviour in this large-bodied island race (Scott et al., in press).

The specific aims of this study were, therefore, to (i) test for a relationship between fitness and size, (ii) determine whether there is an association between dominance and size under field conditions, and (iii) test whether an experimental manipulation of the level of intraspecific competition would lead to a change in the relationship between dominance and size. As it has already been demonstrated in this population that there is no straightforward relationship between size and reproductive success (Catterall et al., 1982; Kikkawa & Wilson, 1983; Kikkawa & Catterall, 1990), or between size and adult survival (J. Kikkawa & S.I. Robinson-Wolrath, unpublished analyses), the index of fitness that we chose to measure was over-winter survival by first-winter individuals (henceforth referred to as juveniles). Given that more than 50% of juveniles typically die before reaching their first breeding season (Kikkawa, 1980b), the potential for selection during this stage is great. We experimentally manipulated the level of intraspecific competition to mimic the post-cyclone conditions that Kikkawa (1980b) predicted may be particularly important for size selection in this population.

Study population

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

The study population of Capricorn silvereyes was on Heron Island (23°26′S, 151°57′E), a coral cay comprising approximately 16 ha, located 72 km east of the central Queensland coast (Mees, 1969; Kikkawa, 1973). The Capricorn silvereye is a small (approximately 14 g), sexually monomorphic, greenish-yellow passerine that is largely restricted to the coral cay islands of the Capricorn and Bunker-group islands at the Southern region of the Great Barrier Reef, Queensland, Australia. Its diet includes insects, fruit (figs and Pipturus) and nectar (Jansen, 1987). The Capricorn silvereye race is distinguished from the Australian mainland race (Z. l. familiaris) by its more yellow plumage, larger body size and thicker bill (Mees, 1969; Kikkawa, 1970, 1976). The island race is up to 40% heavier, and up to 20% larger for all other morphological measurements relative to the mainland race (Kikkawa, 1973; Degnan, 1993; Clegg et al., 2002b; Scott et al., in press).

This study was conducted during the nonbreeding season from mid January to late September 1999. The population of Capricorn silvereyes on Heron Island is sedentary and this population had not been subjected to mass cyclone-induced mortality during the 5 years prior to 1999. During the period of this study, therefore, the population comprised approximately 100 breeding pairs, plus between 221 (January) and 63 (September) independent first-year birds. The age structure was not known in detail, however. In the breeding season immediately prior to this study, the peak of breeding attempts took place from late October to mid November 1998, with a few scattered attempts in late November and early December 1998. This is typical for Capricorn silvereyes (Wilson, 1970; Kikkawa & Wilson, 1983; Robertson, 1996). In the nonbreeding season flocking occurs (Kikkawa, 1977) and the population experiences a high degree of mortality, with between 50 and 75% of juveniles dying before the beginning of the next breeding season (Kikkawa, 1980b).

The population of Capricorn silvereyes on Heron Island has been the subject of a long-term study of ecology and evolution, and over 90% of the population is individually colour-banded (Kikkawa, 1997). This long-term study has, therefore, provided longitudinal records of the size of known individuals from which we can estimate the age at which Capricorn silvereyes are fully grown in terms of the morphological traits that we study here. The age of adult size in this population is 23 days after hatching, which is remarkably short compared with some studies of temperate-dwelling passerines. Hence, to minimize the chances of measuring partly-grown individuals, we did not begin catching or measuring juvenile silvereyes until at least 60 days after the end of the peak of the breeding season.

Silvereyes were caught using fruit-baited wire traps or mist-nests, and then colour-banded. Unless an individual was used in further experiments, juveniles were caught and measured only once. After banding a series of morphological measurements were recorded, including: weight (g); wing length (maximum flattened chord from carpal joint to tip of the longest feather); tail length (base to the tip of the central feather); tarsus length (length of the tarso-metatarsus); culmen length (culmen tip to posterior lip of nostril); and culmen depth (measured at the anterior end of the nostril) (Kikkawa, 1997). Weight was recorded to the nearest 0.5 g. Wing and tail length was measured to the nearest 0.5 mm, and all other body measurements were measured to the nearest 0.05 mm. Repeatability of all measurements was over 85% (Lessells & Boag, 1987). Juvenile birds were identified on the basis of skull ossification and the colour of an individual's legs, which are dark grey in juveniles and pale grey in adults (J. Kikkawa, personal communication).

Sex and age as potential confounding factors

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

Although sex and age are well known potential confounding factors in observational studies of the relationship between size and behaviour, we do not control for their effects in the analyses presented here. This omission is for five nonindependent reasons. First, for most of the individuals used in these analyses we do not have data on either sex or age (precise date of fledging) because juvenile Capricorn silvereyes are sexually monomorphic and we did not monitor the population in detail during the 1998 breeding season. If we were to restrict our analyses to those for which we know both sex and exact fledging data we would not be able to carry out statistically meaningful analyses. Secondly, we have strong empirical data suggesting that neither age nor sex is associated with the morphological measurements of silvereyes that we study here. Using the data we do have on sex and age in juvenile silvereyes we have been unable to detect any significant, or near-significant effect of either sex or age at fledging on any measure, or composite measure, of morphology (J. Kikkawa, unpublished data). This is true whether we restrict our analyses to the individuals of known age and sex, or if we analyse age and sex separately (J. Kikkawa & F. Frentiu, unpublished data). It is also true irrespective of whether we control for other potentially confounding variables, such as year and season. Thirdly, we are confident that there is a substantial heritable genetic component to variation in many aspects of morphology based on comparisons between known relatives (F. Frentiu & I.P.F. Owens, unpublished data). Fourthly, as mentioned above, we only measured individuals that were known to be over the age at which Capricorn silvereyes are fully-grown (23 days). Finally, all of the behavioural data used in this study were collected more than 16 weeks after the end of the peak of the 1999 breeding season, thus minimizing the chances of any partly-grown juveniles being present on the island. In the case of the aviary experiments, the silvereyes were more than 6 months old when they were measured and observed.

When taken together, we believe that these five factors considerably diminish the risk that any association that we find between size and behaviour are because of the confounding effects of either age or sex. Our approach is also in line with many previous studies of the Capricorn silvereyes, which have all failed to find any significant effects of this type.

Measuring juvenile survival

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

In order to determine whether there was a relationship between size and survival we observed the population over winter, which is the period of highest mortality among both adults and juveniles (Kikkawa, 1980b). Population censuses were conducted from mid to late January 1999, from the same time as the banding of new individuals, and again in September 1999, which allowed us to estimate survival over the winter mortality period. During each census three experienced silvereye researchers made a progressive tally of all individuals seen each day. Observations were made for three hours both in the morning and afternoon at numerous sites across the island. At each of these sites a feeding station was set up which involved attracting the silvereyes by placing apples in a tree. In addition to these observation sessions, we carried out intensive and systematic searches throughout the island using a grid system. This is the standard method used for censuses of this population (e.g. Kikkawa, 1980b; Catterall et al., 1982, 1989) and is known to provide reliable estimates of population size and individual survival (see Brook & Kikkawa, 1998; McCallum et al., 2000). The population estimate asymptote occurred within 2 weeks which was used to determine the length of the census.

Census data were used to test for associations between morphology and survival. Variation in survivorship was measured as a binary variable, with those birds surviving from January to September being assigned a fitness value of 1, and those which died being assigned a fitness value of 0 (Kikkawa, 1980b). Because we had measured several morphological traits, we carried out principal component analyses to reduce the number of highly correlated morphological measurements down to two summary indices (Table 1a).

Table 1.  Factor loadings for principal component analyses of morphological variation.
Morphological variables(a)(b)(c)
Survival analysisIntraspecific competition analyses
Natural observationsExperimental observations
PC1PC2PC1PC2PC1PC2
  1. PC1 and PC2 refer to the first and second components for each principal component analysis. Loadings are shown for principal component analyses based on individuals used in test of (a) size-biased over-winter survival among juvenile silvereyes, (b) intraspecific competition under natural conditions, and (c) intraspecific competition under experimental conditions. Separate principal component analyses were undertaken for each part of the study to maximize variation among individuals.

Weight (g)+0.27+0.54+0.22+0.48+0.36+0.38
Wing length (mm)+0.47−0.35+0.45+0.34+0.55−0.28
Tail length (mm)+0.44−0.34+0.41+0.49+0.43−0.61
Tarsus length (mm)+0.37+0.45+0.38−0.44+0.39+0.41
Culmen length (mm)+0.33−0.41+0.43−0.25+0.30+0.47
Culmen depth (mm)+0.49+0.28+0.49−0.39+0.37−0.06

Multivariate logistic regression models were used to test for correlations between variation in survival and variation in the two morphological principal components (see Janzen & Stern, 1998). After testing for associations between morphology and survival, we calculated the directional selection gradient (β) on each principal component (Lande & Arnold, 1983) using the approach described by Janzen & Stern (1998). The standard errors of β were calculated by multiplying the standard errors of the variable coefficient by the logistic coefficient (Janzen & Stern, 1998).

Natural observations of intraspecific competition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

The proportion of fights won by individual juvenile Heron Island silvereyes was determined using repeated behavioural observations at feeding stations between May and August (winter-flocking phase) (see Kikkawa, 1980b, 1987; Kikkawa & Wilson, 1983). Behavioural observations were carried out at artificial feeding stations (as described above). The juveniles observed were chosen haphazardly from those that were feeding at the station. For each individual we noted the outcome of all observed agonistic encounters (won/lost) with conspecifics. We followed Kikkawa's (1961, 1980b methodology for the behavioural indicators of winning and losing agonistic encounters. The outcome of such contests is unambiguous, with the ‘loser’ withdrawing at some stage of the escalation (Kikkawa, 1961). To be included in the analyses, a juvenile had to have been watched on at least four separate occasions, each of which had to contribute more than 60 s of behavioural observation. For such birds, we summed across the four observation periods to obtain a measure of the overall proportion of fights won (Kikkawa, 1980b; Kikkawa & Kakizawa, 1981; Kikkawa & Wilson, 1983).

Multivariate General Linear Models were used to test for associations between variation in morphology and variation in the proportion of fights won. Again, a principal component analysis was used to reduce the multiple morphological measurements down to just two indices (Table 1b). We performed a new principal component analysis on just those juveniles for which we had behavioural data in order to maximize variance among the individuals in the model. However, for all the analyses presented in this study the results remain qualitatively unchanged whether the principal component analysis is performed on all juveniles together, or separately for each type test.

Experimental manipulation of intraspecific competition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

Aviary experiments were carried out to determine if experimental manipulation of the level of intraspecific competition for food would lead to changes in the relationships between body size and either dominance rank or feeding behaviour. Twenty-three replicate trials were performed in an aviary (1.5 m wide × 1.5 m deep × 2 m high) on Heron Island between May and August 1999. For each trial, five juvenile silvereyes were caught at a single location on Heron Island and measured immediately. All five individuals were caught within 1 h. The individuals used were the first five caught, unless any had been used in previous trials in which case they were released. No juvenile was included in a trial more than once. Also, we did not select individuals in order to maximize variation in morphology. Once in the aviary the silvereyes were allowed to habituate for 24 h and then exposed to two experimental treatments on consecutive days in random order. The two experimental treatments were designed to mimic ‘benign’ and ‘harsh’ feeding conditions, respectively (Williams et al., 1972; Wilson, 1994). The benign treatment was designed to invoke weak intraspecific competition, whereas the harsh treatment was intended to invoke strong intraspecific competition, such as has been predicted to occur on Heron Island after a cyclone (Kikkawa, 1980b). Under the benign treatment, the birds were fasted for one and a half hours after dawn and then provided with two portions of apple. Under the harsh treatment the birds were fasted for 2 h after dawn and provided with only one portion of apple. Food deprivation in both treatments was required to ensure that birds, within each treatment, had similar motivations to feed and were prepared to compete for access to food sources, allowing agonistic and feeding behaviour to be recorded (Williams et al., 1972; Ewald, 1985; Wilson, 1994). After being subjected to the treatments in the morning they were fed normally for the rest of the day. The level of aggression shown by individuals was strikingly higher under the harsh treatment than for the same individuals under the benign treatment (Robinson, 1999).

During experimental periods the agonistic and foraging behaviours of individuals were recorded over a period of 20 min. The outcomes of agonistic interactions between individuals were used to construct dominance hierarchies (criteria for assigning outcomes of agonistic encounters summarized in previous section; for details see Kikkawa, 1968, 1980a). The result was that each bird from an observation period was assigned a rank from 1 (subordinate) to 5 (dominant). A separate dominance hierarchy was constructed for each replicate under each of the two treatment regimes (Boyd & Silk, 1983). Similarly, by observing the order in which individuals obtained access to the portions of apple we calculated a foraging rank with the top rank being given to the individual that was first to begin feeding. In cases where two or more birds gained access at the same time we gave tied ranks.

We used General Linear Models to test for the effects of the experimental treatment (‘harsh’ vs. ‘benign’) on the associations between morphology and dominance rank and between morphology and foraging rank. Principal components were used to summarize morphological variation and, in order to maximize variance among individuals, we once again performed a separate principal component analysis on just those individuals used in the aviary trials. Two models were constructed, with dominance rank and foraging rank as the dependent variables, respectively. In both models we fitted trial as a random factor, treatment as a fixed factor nested within trial, and the principal components as fixed continuous variables. The Dominance Hypothesis predicts that, not only should large body size be associated high dominance and feeding rank, but also that this association between morphology and behaviour should become more intense under conditions of harsh intraspecific competition. For both models we therefore then tested for a interaction between treatment and the principal components.

Survival vs. morphology

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

The survival analyses were carried out on 221 juveniles that were known to be alive on the island at the end of January 1999. Of this original group of birds, 158 juveniles (71%) could not be relocated by the beginning of the next breeding season in September 1999. Given the extremely low rate of dispersal from Heron Island, and the completeness of the census, we assumed that these missing birds had died over winter.

The first step in our survival analysis was to carry out a principal component analysis on the morphological variables. The factor loadings for this are shown in Table 1a. PC1 is an index of overall body size, whereas PC2 is positively associated with high body weight and long, narrow bills.

The second step was to use the calculated PC1 and PC2 measures in logistic regressions. The results of the multivariate logistic regressions are shown in Table 2. There was a significant positive association between the likelihood of survival and PC1 (Fig. 1a), but no significant association with PC2 (Fig. 1b). The same pattern was also found if the principal component analyses were performed across all silvereyes used in the study, rather than just those used for this survival analysis.

Table 2.  Associations between morphological principal components and the likelihood of survival, based on multivariate logistic regression models.
Principal componentEstimate of coefficientχ2P-valueSelection coefficient (±SE)P-value
  1. Full model: χ2 = 5.81, d.f. = 2, P = 0.05*.

  2. Degrees of freedom = 1 in univariate chi-square tests. SE refers to standard error. PC1 and PC2 refer to first and second principal components, respectively. Asterisks denote statistical significance *P < 0.05; **P < 0.001.

Intercept0.6816.12<0.001**  
PC10.234.41<0.05*+0.04 (±0.02)<0.05*
PC2−0.161.29>0.25−0.01 (±0.04)>0.50
image

Figure 1. Relationships between over-winter survival of juveniles and (a) PC1 (body size) and (b) PC2 (body shape). Fitted lines are based on logistic regression models, where 1 indicates survival.

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Natural observations of intraspecific competition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

We obtained sufficient data to calculate the proportion of fights won for 57 individual juvenile silvereyes. The principal component factor loadings on morphological characters for these juvenile silvereyes are shown in Table 1b. PC1 is an index of overall body size, whereas PC2 is positively associated with large body-to-bill size ratio.

Our regression models revealed a significant positive association between the proportion of fights won and PC1, but no significant association between the proportion of fights won and PC2 (Table 3). Also, the same pattern was revealed if the principal component analyses were performed across all silvereyes used in the study, rather than just those used for this analysis of natural intraspecific competition.

Table 3.  Associations between morphological principal components and the proportion of fights won in natural agonistic encounters, based multivariate least-squares regression models.
FactorF ratiod.f.P-valueEstimate of coefficient (±SE)
  1. Full model: r = 0.35, F = 3.44, d.f. = 2,45, P = 0.05*.

  2. SE refers to standard error. PC1 and PC2 refer to first and second principal components, respectively. Asterisks denote statistical significance *P < 0.05, **P < 0.001, ***P < 0.0001.

Intercept10.67<0.001*** 
PC16.07<0.01**+0.06 (±0.02)
PC20.22>0.50

Experimental manipulations of intraspecific competition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

The principal component analysis factor loadings for the juvenile silvereyes used in the aviary experiments are shown in Table 1c. PC1 is an index of overall body size, whereas PC2 is positively associated with high body weight and robust bill measurements.

Our multivariate General Linear Model of dominance rank, revealed that there was a positive association between dominance rank and PC1, but that there was no significant interaction between PC1 and our experimental treatment (Table 4). Our model of foraging rank, on the contrary, revealed that there was a positive association between foraging rank and PC2, but again that there was no significant interaction between PC2 and our experimental treatment (Table 5). No other factors or interactions were significant in either model. Together, these results indicate that, in our aviary experiments, large body size was generally associated with high dominance rank and that relatively high body weight and robust bill proportions were generally associated with high foraging rank, but that our experimental treatment of ‘harsh’ and ‘benign’ competition had no significant effect on the form of these relationships between morphology and behaviour.

Table 4.  Effect of experimental manipulation of the level of intraspecific competition on the association between morphological principal components and dominance rank.
Factorχ2P-valueEstimate of coefficient (±SE)
  1. Full model: d.f. = 49, χ2 = 9.76, P = 1.00.

  2. Associations are based on general linear models. PC1 and PC2 refer to first and second principal components, respectively (see Table 1c). Asterisks denote statistical significance. **P < 0.001.

Trial2.981.00
Treatment [trial]0.481.00
PC19.25<0.01**0.19(±0.06)
PC20.250.640.05(±0.11)
Treatment × PC10.240.630.03(±0.06)
Treatment × PC20.100.75−0.03(±0.11)
Table 5.  Effect of experimental manipulation of the level of intraspecific competition on the association between morphological principal components and foraging rank.
Factorχ2P-valueEstimate of coefficient (±SE)
  1. Full model: d.f. = 49, χ2 = 8.05, P = 1.00.

  2. Associations are based on general linear models. PC1 and PC2 refer to first and second principal components, respectively (see Table 1c). Asterisks denote statistical significance *P < 0.05.

Trial1.691.00
Treatment [trial]1.081.00
PC10.700.400.05(±0.06)
PC25.350.02*0.25(±0.11)
Treatment × PC10.180.670.03(±0.06)
Treatment × PC20.680.41−0.09(±0.11)

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

Our results broadly support the assumptions of the Dominance Hypothesis for large size in insular passerines. For each of the three stages of our study we obtained two principal components that summarized variation in six morphological measurements (PC1 and PC2). In each case the first principal component proved to be a index of overall body size, while the second principal component was a more complex index of body shape. In our subsequent analyses we found that, as predicted by the Dominance Hypothesis, the indices of overall body size were significantly positively associated with over-winter survival, and the proportion of fights won in natural agonistic encounters. These findings support the first two assumptions that we tested. With respect to the third assumption, that the association between dominance and body size should be most pronounced under conditions harsh intraspecific competition, we obtained mixed evidence. Consistent with this assumption we found that large body size was correlated with high dominance rank. However, contrary to expectation our experimental manipulation of the level of intraspecific competition (‘harsh’ versus ‘benign’ conditions) had no significant effect on this relationship between size and dominance. Similarly, although heavy-bodied individuals with robust bills tended to feed for more time than did lighter-bodied, fine billed individuals, our experimental treatment did not significantly change this pattern. On balance, therefore, the third assumption requires further testing.

We cautiously suggest that our results provide a new perspective on morphological evolution in insular passerines. Whereas the traditional explanation assumes that the most important effect of low species diversity is low interspecific competition and selection for niche expansion and ecological generalism, our results suggest that an equally important effect of low species diversity on islands is to facilitate high population densities, intense intraspecific competition and selection for success in agonistic interactions. Thus, while both explanations are based on the importance of the low species-diversity of oceanic islands, they differ in the relative importance of inter- vs. intraspecific competition. As previous work on the Heron Island population of silvereyes has already shown that there is no straightforward relationship between body size and foraging generalism in this population (Scott et al., in press), it is intriguing that we have found some support for a relationship between size and dominance.

Having made the case for the Dominance Hypothesis, it is important to emphasize that our results do not prove unambiguously that intraspecific competition is the key to ‘giant’ insular passerines. Several issues remain to be addressed. First, it must be kept in mind that, in this study, we have been examining the current maintenance of the traits in question, rather than on the evolutionary origin of those traits. Hence, although our results suggest that there is ongoing positive directional selection on body size because of the success that large size confers in agonistic encounters, we cannot prove that selection for dominance is what caused the Capricorn silvereyes to become large in the first place. A second general issue is that our results, while supportive of the Dominance Hypothesis, do not prove its generality. We have presented data from a single winter, of a single cohort, of a single population, of a single island race. More tests are required on other island-dwelling passerines (see also Scott et al., in press).

Thirdly, it should be remembered that we have simply tested three of the assumptions of the Dominance Hypothesis. Other assumptions remain to be tested, such as that of additive genetic variance in the behavioural traits of interest. More important, however, is a replicated test of the key prediction of the Dominance Hypothesis: that intraspecific competition is a more important agent of selection in oceanic island populations than it is in corresponding mainland populations. Until this is demonstrated it remains possible that the associations between fitness and body size that we have shown here are also found in mainland silvereye populations.

A final general puzzle is how our results on large body size in insular passerines relate to the overall island-rule, which states that, while small-bodied forms tend to get bigger on islands, large-bodied forms tend to become smaller. Here we have investigated only half of that rule – the increase in size among small-bodied forms – and have suggested that this is linked to the need to be socially dominant in high density island populations. But what about the other half of the island rule – the decrease in size among large-bodies forms? More specifically, if the Dominance Hypothesis applies to small animals, why does it not apply to large ones? Two explanations seem most likely: either the assumptions of the Dominance Hypothesis are violated in large-bodied forms, or large animals face a different form of selection. With respect to the first of these explanations, the Dominance Hypothesis is based on the assumptions that: (1) island populations live at high densities; (2) that high densities lead to high intraspecific competition, and (3) that large body size confers an advantage in agonistic encounters. We know of no a priori reason why these assumptions are less valid in large-bodied animals than they are in small-bodied ones. One possibility is that island-dwelling large animals do experience selection for large body size through the dominance mechanism, but that this selection is counterbalanced by another form of selection. A plausible counterbalancing factor would be the energetic needs incurred through maintaining large body size (Kendeigh, 1972). Acting simultaneously, selection for dominance counterbalanced by selection for energetic requirements could explain the island rule. The need to be dominant over conspecifics selects for large size, while the need to be energy-efficient selects for moderate size. Hence, when released from interspecific interactions, insular populations tend to move towards an intermediate size, with small-bodied forms getting bigger (because of selection for fighting ability) and large-bodied forms getting smaller (because of selection to minimize energy requirements).

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References

We thank Mark Blows, Terry Burke, Sonya Clegg, Francesca Frentiu, Anne Goldizen, Peter Grant, Jiro Kikkawa, Kate Lessells, Jonathan Losos, Hamish McCallum, Katrina McGuigan, Trevor Price, Susan Scott, Jamie Smith and an anonymous reviewer for discussion and/or help in the field; and The Australian Research Council for financial support. This work was carried out under permits from Environment Australia and Queensland Parks and Wildlife Service with approval from the University of Queensland Animal Ethics Committee.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study population
  6. Sex and age as potential confounding factors
  7. Measuring juvenile survival
  8. Natural observations of intraspecific competition
  9. Experimental manipulation of intraspecific competition
  10. Results
  11. Survival vs. morphology
  12. Natural observations of intraspecific competition
  13. Experimental manipulations of intraspecific competition
  14. Discussion
  15. Acknowledgments
  16. References
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