# Strength of the ‘island rule’ in birds is positively associated with absence of avian predators

Raquel Ponti, Claire Doutrelant, Rita Covas

► To cite this version:

Raquel Ponti, Claire Doutrelant, Rita Covas. Strength of the ‘island rule’ in birds is positively associated with absence of avian predators. Biology Letters, 2023, 19 (3), 10.1098/rsbl.2022.0536. hal-04210806

HAL Id: hal-04210806

https://hal.science/hal-04210806v1

Submitted on 19 Sep 2023

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Strength of the ‘island rule’ in birds is positively associated with absence of avian predators

Raquel Ponti<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Claire Doutrelant<fnref n="4" /> and Rita Covas<fnref n="1" />,<fnref n="2" />,<fnref n="5" />

<figure><img src="image_2.png" /></figure>

ID RP, 0000-0003-3485-5528; CD, 0000-0003-1893-3960; RC, 0000-0001-7130-144X

Received: 15 November 2022

Accepted: 3 March 2023

**Subject Areas:**

ecology, evolution

**Keywords:**

ecological release, body mass, insularity, raptors, competitors

**Author for correspondence:**

Raquel Ponti

e-mail: raquelponti@gmail.com

The similar characteristics shared by island environments have been shown to lead to common patterns of adaptations in island species, commonly referred to as the ‘insularity syndrome’. A well-known example is the ‘island rule’, where large species become smaller on islands and small species become larger, leading to well-known cases of dwarfism and gigantism. This pattern was recently verified on a global scale, but the mechanisms underlying it have been poorly investigated. Here, we focused on the role of released pressure from predation and competition experienced by island birds. Using 120 pairs of endemic island species and their mainland sister relatives, we first verified that the island rule was detected in our dataset, and then evaluated the effects of the numbers of raptors and interspecific competitors on the evolution of the insular species’ body mass. We found a strong effect of predation on body mass evolution, with a stronger island rule for species occurring on islands with no raptors, while the pattern disappears in their presence. However, we did not find an effect of competition on this pattern. Our study shows the importance of considering ecological interactions for understanding patterns of body size evolution, and the exceptions to those patterns.

## 1. Introduction

Island ecosystems share several unique characteristics, such as reduced area, species diversity and isolation [1,2], which make them simplified natural laboratories for studying species’ evolutionary patterns and processes. These insular geographical features provide conditions that modulate the lifestyle traits of the new settlers, from life history to physiology and morphology [3–6]. Since these features are repeated on islands worldwide, they give rise to parallel evolution resulting in common patterns of adaptation to the island environment—the so-called ‘insularity syndrome’ [7,8].

Shared patterns of morphological variation in insular species are well-known textbook examples of adaptation [9–11]. One of the most striking patterns is the ‘island rule’ [12,13], where large species evolve toward smaller body sizes on islands (e.g. the dwarf elephants of Mediterranean islands) and small-sized species evolve toward larger forms (e.g. the Mauritian dodo, which was a giant pigeon; [14]). Although the generality of the island rule has been somewhat controversial [15–19], large-scale studies on different groups support this general

<footnote n="1">¹CIBIO-InBio, Centro de Investigação em Biodiversidade e Recursos Genéticos, Laboratório Associado, University of Porto, Campus Agrário de Vairão, Vairão, 4485-661 Portugal</footnote>

<footnote n="2">²BIOPOLIS Program in Genomics, Biodiversity and Land Planning, CIBIO, Campus de Vairão, 4485-661 Vairão, Portugal</footnote>

<footnote n="3">³Departamento de Biología Vegetal y Ecología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Leioa, Bizkaia, España</footnote>

<footnote n="4">⁴CEFE, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France</footnote>

<footnote n="5">⁵FitzPatrick Institute of African Ornithology, DST-NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa</footnote>
trend [20], including a recent large-scale study on birds, mammals, reptiles and amphibians [21]. However, while the island rule is now well-established, we still have a poor understanding of the mechanisms affecting the specific patterns of body size evolution that led to the island rule.

The main mechanisms put forward to explain body size shifts on islands have been resource limitation, climate, release from predation pressure and a decreased number of competitors [8,22,23], and it has been theorized that under these conditions island species are free to evolve to optimal body sizes where fitness is maximum [24]. Also, morphological changes are expected to be associated with the evolution of life-history traits, such as growth rate, longevity and fecundity [5,25–27].

Predation is a particularly strong selective pressure known to affect body size evolution [28]. In mainland areas, small-bodied species are thought to have an adaptive advantage of being small in order to be less detectable or attractive to predators [29]. On islands where predator species are less numerous, an increase in body size could be advantageous for small-sized species if that increases their capacity for habitat exploitation and keeping a territory [30,31]. Large-sized species could evolve to smaller forms in the absence of, or reduced, predator pressure as they would not need to outrun or confront predators [32,33]. Being smaller for large organisms is also expected to be beneficial when living on islands, and especially small islands, given the limited area and resources available [23].

Changes in interspecific competition on islands could also influence body size. The species impoverishment on islands is usually associated with increased population densities [23,34,35], and hence stronger intra-specific competition, which can favour an increase in the body size of small-bodied species if that increases their competitive ability [36,37]. Increased intraspecific competition was associated with gigantism in Mediterranean lizards [38], and competition in two Japanese mouse species was linked to their opposing body size changes (one increased and the other decreased), in agreement with the ecological release hypothesis [39]. In large-bodied species, however, the reduction of competition is associated with dwarfism, as it has been shown in ungulates [40].

Previous empirical studies on the factors associated with changes in body size have used island geography (island area, isolation) and climate as a proxy of resource availability and number of predators and competitors [21,23,41]. The results have been mixed, with some studies finding that body size evolution is associated with island area or isolation [23], but not others [21].

Here, we set to investigate the effects of the numbers of predators and interspecific competitors on body size evolution. We use the body mass of 240 species of endemic island birds and their mainland counterparts (belonging to 11 bird orders and 34 families). We first test the island rule with our dataset and then investigate the association between this pattern and the number of avian predators (raptors) and interspecific competitors (number of sympatric species in the same genus or family). We expected that fewer predators and competing species would lead to a stronger increase in the body size of small-sized birds and a decrease in body size for large birds. In other words, we expect to find more pronounced island rule patterns when there are few or no predators or competitors on the islands.

## 2. Methods

### (a) Body mass and island rule index

We used a database of 240 species: 120 insular endemic birds from 46 different archipelagos and their mainland closest relatives at the same latitude [4], see electronic supplementary material, appendix S1). Most of the study islands were oceanic, with some exceptions such as Corsica, and were located at least 50 km from the nearest mainland. The pairs of bird species belong to 11 different orders and 34 families.

Mass information was obtained from the museum specimen information, when possible, from three female and three male specimens per species. When the mass information was lacking (museum: N = 104 specimens and 40 species), we used information from the literature: Birds of the World [42], the Handbook of Avian Body Masses [43] and AVONET [44]. We used averaged mass data for both female and male specimens, when possible, per species for analyses (electronic supplementary material, appendix S2). For mainland species, we used the mass information corresponding to the subspecies geographically closest to the island. If no subspecies were defined, we used the average mass value considering all populations.

We calculated the body mass ratio proposed by Benítez-López et al. [21] (lnRR) calculated as the natural logarithm of the division between the mean body mass of the island species (Mi) and the mean body mass of their closely related mainland species (Mm):

$$ \ln RR = \ln \frac{M_i}{M_m} $$

This body mass index was calculated for each pair of sister species, where values above 0 indicate a shift of insular species body mass towards bigger forms compared to the mainland sister species (gigantism); and values below 0 correspond to a decrease in insular body size compared to their mainland relative (dwarfism).

### (b) Species distributions and latitude

We used Birdlife species distribution maps [45] to calculate the average latitude of their distribution and the overlapping competitors and predator species per each focal species. We first randomly selected 10 000 points from each species point shapefiles, using the function 'spsample' of the 'sp' package for R [46]. We then obtained the latitude as the mean of the y-axis for each species' distribution points.

### (c) Presence and number of co-occurring predators and competitors

To account for the presence or number of predators, competitors, and the total bird richness on islands, we overlapped the focal species distribution with all bird species distribution maps (Birdlife) and generated a list of co-occurring species. We used the function 'Intersect' in QGIS software. From the overlapped species list, we eliminated seabirds and introduced species. The ecological features of marine species completely differ from terrestrial ones; hence, we did not consider them as potential predators or competitors. Introduced species could currently compete with endemic island species, but we are interested in the evolution of bird morphology before the human introduction of non-native species. Furthermore, we did not expect that species drastically changed since these introductions.

#### (i) Predators

To have a list of all co-occurring predator species, we selected the raptors of the Accipitriformes and Falconiformes orders as
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Presence/absence of raptors changes the relationship between the body mass ratio (lnRR) and the body mass of the mainland species while (b) presence/absence of competitors belonging to the same genus does not, and (c) presence/absence of competitors belonging to the same family on islands has a marginal effect on this relationship. Points and lines in blue indicate absence and in violet presence of predators or competitors. The intercepts correspond to the binary models performed each with one variable (predation, competitors of the same genus or family). See models in electronic supplementary material, table S1 in appendix S4.</figcaption></figure>

predators of our focal species. We used the approach followed by Bliard *et al.* [47] and Gravel *et al.* [48] to exclude some predators based on the mismatch of predator–prey body sizes (i.e. each raptor would predate bird species within a particular body size range). We considered the prey size ranges for each predator species defined by Bliard *et al.* [47]. We did not account for mammals and snakes, given that they represent a minimal predator pressure for insular adult birds compared to avian predators (electronic supplementary material, appendix S3).

## (ii) Competitors

We counted all overlapping species to estimate bird species richness and also, the number of species belonging to the same genus and family as two proxies of the strength of competing interactions. Species within the same genus and family tend to

have similar ecological requirements [49,50]; hence, we consider that higher numbers of phylogenetically related bird species co-occurring in one area were associated with increased competition.

## (d) Statistical analyses

To test our main hypotheses, we constructed phylogenetic generalized least squares models (PGLS) in the 'caper' R package [51]. To account for phylogeny, we used 100 alternative trees from the most complete known phylogenetic source [52]. We created a consensus phylogeny using the function *consensus_edges* from the 'phytools' R package [53].

First, we tested the island rule by relating the body mass ratio to the log₁₀-transformed body mass of the mainland species of each pair. A negative regression slope supports the island rule [21].
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Fixed estimates of PGLS models using non-binary (left) and binary (right) variables for accounting for predation and competition. See electronic supplementary material, table S2 in appendix S4.</figcaption></figure>

Second, we evaluated the effect of predation or competition on body size evolution. Predators and competitors were either defined as binary variables (presence/absence) or as the number of predators or competitors, competitors being the species of the same genus or the same family, separately.

We built two models with body mass ratio of island/mainland species as a dependent variable and including the three variables of interest as explanatory variables: number of predators, number of species from the same genus and number of species from the same family. One of these two complete models included the three ecological factors (predators, competitors) as a presence/absence, and the other included them as a continuous variable. In addition, we included latitude as a proxy for climatic conditions. For these two models, we excluded the bird richness variable from our final models to avoid collinearity, given the high relation to the number of predators ($R^2$ = 0.5, *p*-value < 0.001).

### 3. Results

We found support for the island rule in our data set: as expected, the relationship between body mass ratio and log₁₀-normalized body mass of the island rule is negative and significant ($R^2$=0.12, $F_{1,118}$ = -16.68, *p*-value < 0.0001, electronic supplementary material, figure S1 in appendix S4). The phylogenetic signal for the model was shallow ($\lambda$ = CI: 0-0.24).

In addition, we found that the presence and absence of predators and not the competitors have a significant effect in the island rule (figure 1). Our models including all interacting variables and latitude as a fixed variable (figure 2, electronic supplementary material, table S2 in appendix S4) showed that the island/mainland body mass ratio was associated with the number of predators (estimate: -0.128 [95%CI: -0.182; -0.074], and the interaction of the number of predators and mainland body mass (estimate: -0.028 [95%CI: 0.016; 0.040] ($R^2$ = 0.17, $F_{8,111}$ = 4.124, *p*-value = 0.0002). Specifically, the shifts in body size were more pronounced, and in the direction predicted by the island rule, when there are no predators present (see electronic supplementary material, table S1 in appendix S4).

The number of competitors of the same genus and family, their interactions with mainland body mass, and latitude were not associated with the body mass ratio (lnRR). The model using the predators and competitors as binary factors showed similar results (figure 2) ($R^2$ = 0.19, $F_{8,111}$ = 4.513, *p*-value < 0.0001).

### 4. Discussion

Our results confirm that birds included in this study followed the island rule pattern: large birds tended to evolve toward small body mass, and small birds to evolve toward larger forms, thus providing further indication that the island rule is common in birds (see also [21,41]). However, few studies have focused on examining the factors underlying these specific body size changes. Our results showed that the island rule appears stronger in the absence of avian predators, while the pattern is absent when the number of raptors is high. Finally, we found no clear indication that the number of competitors was associated with body mass evolution.

As predicted, we found evidence that predation constitutes a strong driver of body size evolution in insular birds. Our results provide empirical support to suggestions that a release from predation pressure is one of the key elements underlying morphological evolution on islands [28,54]. When predation pressure is removed, being very small or very large appears to become less advantageous and other ecological and social factors appear to drive body mass evolution. For small species, increasing body size is thought to be advantageous if it increases intraspecific competitive advantage, such as increased access to territories and resource (e.g. [30,55]), while for large species, reducing size is expected to be beneficial to decrease energetic requirements, [56], especially in smaller islands where food resources might be a limiting factor [21,23]. However, even if these are main factors underlying the island rule, additional factors such as feeding and thermal ecology, sexual selection or social system are likely to play an additional role (see also [21,28]),
and large-scale studies combining these factors with predation should provide a better understanding of what favours body size changes in the absence of predation pressure.

We also expected that a release from interspecific competition would have a similar effect on body size evolution, leading to a stronger island-rule effect with fewer competitors. Although our results were in the predicted direction when we tested the number of co-occurring species that belong to the same family, the results were not significant, and we did not find the same trend when considering only the species belonging to the same genus. We predicted that closely related species would act as stronger competitors because of their potentially similar ecological characteristics to the focal species, making their absence more impactful on the evolution of the focal species [49,50]. However, competitive interactions among species are complex, and evolution might respond with rapid evolutionary radiation processes where each species evolves to become adapted to exploit different new resources, thereby avoiding competition [57–59]. The character displacement of related competitors could result in the evolution of opposite body sizes or other morphological characteristics (e.g. one species increasing and the other reducing their body mass [59]), which makes it challenging to understand the global pattern of body change with a subset of species [39]. To conclude about the effect of competition, future analyses should account for other bird species that share similar ecological requirements, such as the same diet or nesting habitats, but are not necessarily close phylogenetically. However, currently, the scarcity of ecological information for most endemic island birds makes this approach challenging.

We did not find a latitude effect in our models, suggesting that, in our data set, the climatic factors are not the main drivers of morphological changes on islands. This contrasts with the results of Benítez López et al. [21], which suggested that climate and thermal ecology were likely to play a role in body mass shifts. However, climate, latitude and species richness are highly correlated and more detailed studies are

needed to better understand how climate might or not interact with the island rule.

Here, by quantifying the number of raptors, we provide empirical results suggesting that predation is a major driver of body size evolution and, when predation is absent, other factors play an important role. Hence, our results also shed light on how the island rule can be broken on many islands. Island colonization, including colonization by predators such as avian raptors, has a stochastic component and islands with similar characteristics may vary in their predator fauna, which can be a determinant for the body size evolution of the other species occurring on the island.

**Data accessibility.** Data are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.k0p2ngfcj [60].

The data are provided in the electronic supplementary material [61].

**Authors' contributions.** R.P.: data curation, formal analysis, investigation, methodology, visualization, writing—original draft; C.D.: conceptualization, data curation, investigation, supervision, validation, writing—review and editing; R.C.: conceptualization, data curation, investigation, project administration, supervision, validation, writing—review and editing.

All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

**Conflict of interest declaration.** We declare we have no competing interests.

**Funding.** R.P. was supported by the Basque Government Department of Education (POS_2021_2_0011). This work was funded by the European Program Synthesys to R.C. and C.D. and a Collection Study Grant from the AMNH to R.C.

**Acknowledgements.** We thank Pierre-André Crochet, Olivier Gimenez, Doris Gomes Julien Claude, Gonçalo Cardoso, Marco Sannolo and Martim Melo for helpful discussions and advice. Paul Sweet and Robert Prys-Jones from the American and British Natural History Museum kindly supported our visits to the museums. Emeline Mourocq helped building the database and conducted preliminary statistical analyses on a subset of the data. Nathalie Grnac, Romain Guerreiro, Andrea Baquero and Thibaut Powolny helped with data collection at the museums. Adrian Skerrett provided useful information on birds of the Seychelles. M. Melo, S. Anderson and R. Bowie gave us access to unpublished phylogenetic data.

## References

1. Grant PR. 1965 The adaptive significance of some size trends in island birds. *Evolution* **19**, 355. (doi:10.2307/2406446)

2. Whittaker RJ, Fernández-Palacios JM. 2007 *Island biogeography: ecology, evolution, and conservation*. Oxford, UK: Oxford University Press.

3. Sagonas K, Pafilis P, Valakos ED. 2015 Effects of insularity on digestion: living on islands induces shifts in physiological and morphological traits in island reptiles. *Sci. Nat.* **102**, 1–7. (doi:10.1007/s00114-015-1301-8)

4. Doutrelant C, Paquet M, Renoult JP, Grégoire A, Crochet PA, Covas R. 2016 Worldwide patterns of bird colouration on islands. *Ecol. Lett.* **19**, 537–545. (doi:10.1111/ele.12588)

5. Sandvig EM, Coulson T, Clegg SM. 2019 The effect of insularity on avian growth rates and implications for insular body size evolution. *Proc. R. Soc. B* **286**, 20181967. (doi:10.1098/rspb. 2018.1967)

6. Covas R. 2012 Evolution of reproductive life histories in island birds worldwide. *Proc. R. Soc. B* **279**, 1531–1537. (doi:10.1098/rspb. 2011.1785)

7. Grant PR. 1998 *Evolution on islands*. Oxford, UK: Oxford University Press.

8. Covas R. 2016 Life-history evolution in island populations of birds. In *Encyclopedia of evolutionary biology* (ed. RM Kliman), pp. 352–358. New York, NY: Academic Press.

9. Grant PR. 1999 *Ecology and evolution of darwin's finches*. Princeton, NJ: Princeton University Press.

10. Pratt HD. 2005 *The Hawaiian honeycreepers Drepanidinae*. Oxford, UK: Oxford University Press.

11. Tokita M, Yano W, James HF, Abzhanov A. 2017 Cranial shape evolution in adaptive radiations of birds: comparative morphometrics of Darwin's finches and Hawaiian honeycreepers. *Phil. Trans. R. Soc. B* **372**, 20150481. (doi:10.1098/rstb. 2015.0481)

12. Van Valen L. 1973 A new evolutionary law. *Evol. Theory* **1**, 1–33.

13. Lomolino MV. 2005 Body size evolution in insular vertebrates: generality of the island rule. *J. Biogeogr.* **32**, 1683–1699. (doi:10.1111/j.1365- 2699.2005.01314.x)

14. Heupink TH, van Grouw H, Lambert DM. 2014 The mysterious spotted green pigeon and its relation to the dodo and its kindred. *BMC Evol. Biol.* **14**, 1–7. (doi:10.1186/1471-2148-14-136)

15. Meiri S, Dayan T, Simberloff D. 2006 The generality of the island rule reexamined. *J. Biogeogr.* **33**, 1571–1577. (doi:10.1111/j.1365-2699.2006. 01523.x)

16. Meiri S. 2008 Evolution and ecology of lizard body sizes. *Glob. Ecol. Biogeogr.* **17**, 724–734. (doi:10. 1111/j.1466-8238.2008.00414.x)

17. Itescu Y, Karraker NE, Raia P, Pritchard PC, Meiri S. 2014 Is the island rule general? Turtles disagree. *Glob. Ecol. Biogeogr.* **23**, 689–700. (doi:10.1111/geb.12149)
18. Leisler B, Winkler H. 2015 Evolution of island warblers: beyond bills and masses. *J. Avian Biol.* **46**, 236–244. (doi:10.1111/jav.00509)

19. Lokatis S, Jeschke JM. 2018 The island rule: an assessment of biases and research trends. *J. Biogeogr.* **45**, 289–303. (doi:10.1111/jbi.13160)

20. Price TD, Phillimore AB. 2007 Reduced major axis regression and the island rule. *J. Biogeogr.* **34**, 1998–1999. (doi:10.1111/j.1365-2699.2007.01796.x)

21. Benítez-López A, Santini L, Gallego-Zamorano J, Milá B, Walkden P, Huijbregts MAJ, Tobias JA. 2021 The island rule explains consistent patterns of body size evolution in terrestrial vertebrates. *Nat. Ecol. Evol.* **5**, 768–786. (doi:10.1038/s41559-021-01426-y)

22. Blondel J. 2000 Evolution and ecology of birds on islands: trends and prospects. *Vie Milieu/Life Environ.* **50**, 205–220.

23. Boyer AG, Jetz W. 2010 Biogeography of body size in Pacific Island birds. *Ecography* **33**, 369–379. (doi:10.1111/j.1600-0587.2010.06315.x)

24. Damuth J. 1993 Cope's rule, the island rule and the scaling of mammalian population density. *Nature* **365**, 748–750. (doi:10.1038/365748a0)

25. Raia P, Barbera C, Conte M. 2003 The fast life of a dwarfed giant. *Evol. Ecol.* **17**, 293–312. (doi:10.1023/A:1025577414005)

26. Palkovacs EP. 2003 Explaining adaptive shifts in body size on islands: a life history approach. *Oikos* **103**, 37–44. (doi:10.1034/j.1600-0706.2003.12502.x)

27. McNab BK, Ellis HI. 2006 Flightless rails endemic to islands have lower energy expenditures and clutch sizes than flighted rails on islands and continents. *Comp. Biochem. Physiol. - A Mol. Integr. Physiol.* **145**, 295–311. (doi:10.1016/j.cbpa.2006.02.025)

28. Runemark A, Brydegaard M, Svensson EI. 2014 Does relaxed predation drive phenotypic divergence among insular populations? *J. Evol. Biol.* **27**, 1676–1690. (doi:10.1111/jeb.12421)

29. Lawlor TE. 1982 The evolution of body size in mammals: evidence from insular populations in Mexico. *Am. Nat.* **119**, 54–72. (doi:10.1086/283890)

30. Michaux JR, De Belloco JG, Sarà M, Morand S. 2002 Body size increase in insular rodent populations: a role for predators? *Glob. Ecol. Biogeogr.* **11**, 427–436. (doi:10.1046/j.1466-822x.2002.00301.x)

31. Runemark A, Sagonas K, Svensson EI. 2015 Ecological explanations to island gigantism: dietary niche divergence, predation, and size in an endemic lizard. *Ecology* **96**, 2077–2092. (doi:10.1890/14-1996.1)

32. Lomolino MV, Sax DF, Palombo MR, van der Geer AA. 2012 Of mice and mammoths: evaluations of causal explanations for body size evolution in insular mammals. *J. Biogeogr.* **39**, 842–854. (doi:10.1111/j.1365-2699.2011.02656.x)

33. Rozzi R. 2018 Space-time patterns of body size variation in island bovids: the key role of predatory release. *J. Biogeogr.* **45**, 1196–1207. (doi:10.1111/jbi.13197)

34. Grant PR. 1968 Bill size, body size, and the ecological adaptations of bird species to competitive situations on islands. *Syst. Biol.* **17**, 319–333. (doi:10.1093/sysbio/17.3.319)

35. Blondel J, Chessel D, Frochot B. 1988 Bird species impoverishment, niche expansion, and density inflation in Mediterranean island habitats. *Ecology* **69**, 1899–1917. (doi:10.2307/1941167)

36. Dayan T, Simberloff D. 1998 Size patterns among competitors: ecological character displacement and character release in mammals, with special reference to island populations. *Mamm. Rev.* **28**, 99–124. (doi:10.1046/j.1365-2907.1998.00029.x)

37. Herrmann NC, Stroud JT, Losos JB. 2021 The evolution of 'ecological release' into the 21st century. *Trends Ecol. Evol.* **36**, 206–215. (doi:10.1016/j.tree.2020.10.019)

38. Pafilis P, Meiri S, Foufopoulos J, Valakos E. 2009 Intraspecific competition and high food availability are associated with insular gigantism in a lizard. *Naturwissenschaften* **96**, 1107–1113. (doi:10.1007/s00114-009-0564-3)

39. Millien V. 2004 Relative effects of climate change, isolation and competition on body-size evolution in the Japanese field mouse, *Apodemus argenteus*. *J. Biogeogr.* **31**, 1267–1276. (doi:10.1111/j.1365-2699.2004.01119.x)

40. Raia P, Meiri S. 2006 The island rule in large mammals: paleontology meets ecology. *Evolution (N Y)* **60**, 1731. (doi:10.1554/05-664.1)

41. Clegg SM, Owens IPF. 2002 The 'island rule' in birds: medium body size and its ecological explanation. *Proc. R. Soc. B* **269**, 1359–1365. (doi:10.1098/rspb.2002.2024)

42. Billerman SM, Keeney BK, Rodewald PG, Schulenberg TS (eds). 2022 *Birds of the World*. Ithaca, NY: Cornell Laboratory of Ornithology. See https://birdsoftheworld.org/bow/home.

43. Dunning J. 2008 *CRC handbook of avian body masses*. Boca Raton, FL: CRC Press.

44. Tobias JA et al. 2022 AVONET: morphological, ecological and geographical data for all birds. *Ecol. Lett.* **25**, 581–597. (doi:10.1111/ele.13898)

45. BirdLife International and Handbook of the Birds of the World. 2018 Bird species distribution maps of the world. Version 2018.1. See http://datazone.birdlife.org/species/requestdis.

46. Pebesma EJ, Bivand RS. 2005 Classes and methods for spatial data in R. *R News* **5**, 9–13.

47. Bliard L, Paquet M, Robert A, Dufour P, Renoult JP, Grégoire A, Crochet PA, Covas R, Doutrelant C. 2020 Examining the link between relaxed predation and

bird coloration on islands. *Biol. Lett.* **16**, 4–8. (doi:10.1098/rsbl.2020.0002)

48. Gravel D, Poisot T, Albouy C, Velez L, Mouillot D. 2013 Inferred food web structure from predator-prey body size relationships. *Methods Ecol. Evol.* **4**, 1083–1090. (doi:10.1111/2041-210X.12103)

49. Grant PR. 1966 Ecological compatibility of bird species on islands. *Am. Nat.* **100**, 451–462. (doi:10.1086/282438)

50. Costa-Pereira R, Araújo MS, Souza FL, Ingram T. 2019 Competition and resource breadth shape niche variation and overlap in multiple trophic dimensions. *Proc. R. Soc. B* **286**, 20190369. (doi:10.1098/rspb.2019.0369)

51. Orme D. 2013 The caper package: comparative analysis of phylogenetics and evolution in R. *R Packag. version* **5**, 1–36.

52. Jetz W, Thomas GHH, Joy JBB, Hartmann K, Mooers AO. 2012 The global diversity of birds in space and time. *Nature* **491**, 1–5. (doi:10.1038/nature11631)

53. Revell LJ. 2012 phytools: An R package for phylogenetic comparative biology (and other things). *Methods Ecol. Evol.* **2**, 217–223. (doi:10.1111/j.2041-210X.2011.00169.x)

54. Wright NA, Steadman DW, Witt CC. 2016 Predictable evolution toward flightlessness in volant island birds. *Proc. Natl. Acad. Sci. U. S. A.* **113**, 4765–4770. (doi:10.1073/pnas.1522931113)

55. Adler GH, Levins R. 1994 The island syndrome in rodent populations. *Q. Rev. Biol.* **69**, 18–19. (doi:10.1086/418744)

56. Lomolino MV. 1985 Body size of mammals on islands: the island rule reexamined. *Am. Nat.* **125**, 310–316. (doi:10.1086/284343)

57. Losos JB, Ricklefs RE. 2009 Adaptation and diversification on islands. *Nature* **457**, 830–836. (doi:10.1038/nature07893)

58. Losos JB, Glor RE, Kolbe JJ, Nicholson K. 2006 Adaptation, speciation, and convergence: a hierarchical analysis of adaptive radiation in Caribbean Anolis lizards. *Ann. Missouri Bot. Gard.* **93**, 24–33. (doi:10.3417/0026-6493(2006)93[24:ASACAH]2.0.C0;2)

59. Melo M, Warren BH, Jones PJ. 2011 Rapid parallel evolution of aberrant traits in the diversification of the Gulf of Guinea white-eyes (Aves, Zosteropidae). *Mol. Ecol.* **20**, 4953–4967. (doi:10.1111/j.1365-294X.2011.05099.x)

60. Ponti R, Doutrelant C, Covas R. 2023 Data from: Strength of the 'island rule' in birds is positively associated with absence of avian predators. *Dryad Digital Repository*. (doi:10.5061/dryad.k0p2ngfcj)

61. Ponti R, Doutrelant C, Covas R. 2023 Strength of the 'island rule' in birds is positively associated with absence of avian predators. Figshare. (doi:10.6084/m9.figshare.c.6470007)