Present address: School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK. E-mail: firstname.lastname@example.org
The life-history basis of latitudinal diversity gradients: how do species traits vary from the poles to the equator?
Article first published online: 5 MAR 2002
Journal of Animal Ecology
Volume 71, Issue 1, pages 79–87, January 2002
How to Cite
Cardillo, M. (2002), The life-history basis of latitudinal diversity gradients: how do species traits vary from the poles to the equator?. Journal of Animal Ecology, 71: 79–87. doi: 10.1046/j.0021-8790.2001.00577.x
- Issue published online: 5 MAR 2002
- Article first published online: 5 MAR 2002
- Received 12 March 2001; revision received 20 September 2001
- geographical range;
- phylogenetically independent contrasts;
- species richness
- Top of page
- Materials and methods
1.Latitudinal variation among species in life-history traits is often suggested to contribute to high tropical species richness. However, traditional methods of analysing such variation rarely control for phylogeny and latitudinal range overlap between species, potentially giving misleading results.
2.Using a method of pairwise independent contrasts which overcomes these problems, I tested for latitudinal variation among bird species in a number of traits which have been linked, theoretically or empirically, with both latitude and species richness.
3.This method indicates strong support for Rapoport’s Rule and decreasing clutch size towards the equator in both hemispheres, but only partial support for decreasing body size and ecological generalism towards the equator.
4.Indirect measures of sexual selection (sexual dichromatism and size dimorphism) show no variation with latitude; an apparent increase in dichromatism towards the equator is shown to be an artefact of phylogeny.
5.Many of the associations between life history and latitude were not detected by traditional cross-species analyses, highlighting the importance of incorporating phylogeny and overlap in studies of geographical life-history variation. Establishing associations between life-history traits and latitude does not prove, but is a necessary prerequisite for, a link between these traits and latitudinal diversity gradients.
- Top of page
- Materials and methods
Why are there so many species in the tropics? One general explanation is that latitudinal gradients in environmental conditions influence speciation or extinction rates, either directly or by modifying species’ life histories. Thus, many explanations for latitudinal diversity gradients are based on the assumption of consistent latitudinal variation in life-history attributes, for example geographical range size or ecological specialization (Fischer 1960; Pianka 1966; Rohde 1992; Rosenzweig 1995; Rohde 1999; Gaston 2000). However, the first step in linking a life-history trait to latitudinal diversity gradients is to demonstrate convincingly that the trait itself is associated with latitude. Unfortunately, the vast majority of cited evidence for latitudinal variation in life-history traits among species comes from studies which suffer from a number of analytical problems. Often, geographical elements such as gridcells, lines of latitude or latitudinal bands are used as datapoints (e.g. Stevens 1989), but these are non-independent because many species are counted repeatedly. This form of non-independence is overcome by the midpoint method, where each latitudinal band includes only those species whose latitudinal midpoint falls within that band (Rohde, Heap & Heap 1993), or by the cross-species method, which treats each species as an independent datapoint (e.g. Letcher & Harvey 1994; Blackburn & Gaston 1996a, 1996b). However, these approaches still suffer from two additional potential problems.
First, they fail to control for the possible confounding effects of phylogeny (Pagel, May & Collie 1991). This is important because closely related species are likely to be more similar in their biology than more distantly related species due to more recent common ancestry (Harvey & Pagel 1991), and are more likely to inhabit the same geographical region. This means that an apparent latitudinal gradient in a life-history trait may result from changing representation of different phylogenetic groups across latitudes, rather than any mechanistic association between latitude and life history (Cushman, Lawton & Manly 1993; Blackburn & Gaston 1996a). Hawkins & Lawton (1995), for example, found that geographical body size variation among butterfly species could be attributed mostly to the changing representation of different families at different latitudes. Under traditional methods of analysis, therefore, it is difficult to rule out phylogeny as an explanation for latitudinal variation in life history.
The second problem is that traditional methods fail to consider latitudinal overlap of geographical ranges. The latitudinal position a species occupies is a function not only of the latitudinal midpoint, but also the north–south extent of its geographical range. Two species may differ in their latitudinal midpoints, but the latitudinal range of one species may largely or entirely overlap that of the other (Fig. 1). In such cases, populations of both species will be subject largely to similar latitude-related environmental conditions. Within any given taxon, such cases are probably very common because closely related species will tend to inhabit the same geographical regions. Therefore, analyses using the midpoint or cross-species methods may be prone to unusually high Type 2 statistical error. Ideally, comparisons between species should take into account not only the difference in latitudinal midpoints, but the degree to which the latitudinal ranges of species overlap.
Several previous studies have applied some form of phylogenetic control in measuring latitudinal gradients in life-history traits (Pagel et al. 1991; Letcher & Harvey 1994; Taylor & Gotelli 1994; Blackburn & Gaston 1996b; Edwards & Westoby 1996; Gaston & Blackburn 1996; Cowlishaw & Hacker 1997; Blackburn, Gaston & Lawton 1998), but none have addressed explicitly the problem of latitudinal range overlap. In this paper I present an analysis which simultaneously overcomes these two problems. I use pairwise phylogenetically independent contrasts (Felsenstein 1985; Harvey & Pagel 1991) to test for latitudinal variation among bird species in the following traits which have been linked, theoretically or empirically, with both species richness and latitude.
- 1Latitudinal extent of geographical ranges. Stevens (1989) suggested that decreasing latitudinal range extent towards the equator (Rapoport’s Rule) inflates species richness in the tropics due to spillover of species beyond their usual range limits. Latitudinal range extent may also be a proxy for ecological specificity, which may influence rates of speciation or extinction (see below).
- 2Body size. Decrease in body sizes towards the equator associated with Bergmann’s Rule (e.g. James 1970; Blackburn & Gaston 1996a; Gaston & Blackburn, 2000) may increase rates of cladogenesis if small body size is associated with higher speciation or lower extinction rates (e.g. Hutchinson & MacArthur 1959; Dial & Marzluff 1988).
- 4Sexual selection. Moller (1998) predicted that increased parasite impact on host fitness in the tropics should lead to stronger parasite-mediated sexual selection and more extravagant sexual ornamentation at lower latitudes. The theoretical link between sexual selection and speciation is long-standing (Darwin 1871; Lande 1981), and recent empirical tests support this link (Barraclough, Vogler & Harvey 1998; Owens, Bennett & Harvey 1999).
- 5Ecological specificity. Tropical species have long been assumed to utilize a narrower range of habitat and food types than temperate species (e.g. Dobzhansky 1950; MacArthur 1969; but see Price 1991). Specialization or niche restriction has been suggested to increase species richness by enabling greater coexistence (Hutchinson & MacArthur 1959; Hutchinson 1959; MacArthur 1965) or increasing the likelihood of speciation (see Cracraft 1985).
This is not intended to be a complete list of life-history traits which potentially contribute to high tropical species richness. The traits examined here are those which can be measured relatively accurately and for which data are readily available for a wide range of bird species. Because few southern hemisphere examples exist for some well-known patterns, such as Rapoport’s Rule (Rohde 1996; Gaston, Blackburn & Spicer 1998) or the decrease in clutch size towards the equator (Yom-Tov 1994), I examine patterns both worldwide and within each hemisphere. I assume that if life history is not independent of latitude, then pairs of closely related species which differ in the latitudinal positions of their geographical ranges should also differ with respect to life-history traits. The simple question addressed here is, do life-history traits differ more often than expected by chance, and in predicted directions, among latitudinally separated pairs of bird species?
Materials and methods
- Top of page
- Materials and methods
SELECTION OF SPECIES PAIRS
From published species-level phylogenies of bird taxa, I identified 69 phylogenetically independent pairs of species (contrasts) in which the two species differed in the latitudinal position of their breeding ranges (see Appendix I for a list of the species pairs and references for the phylogenies used). Species were chosen so as to maximize the number of contrasts from each phylogeny which were suitable for inclusion in this analysis, and for which data on the relevant life-history traits were obtainable for both species. Contrasts were chosen before life-history data were collected, minimizing the possibility of bias in choosing contrasts which varied with latitude in the traits of interest.
To ensure a sufficient degree of latitudinal separation between the two species in each contrast, two criteria were applied: (1) the latitudinal midpoints of the geographical ranges of the two species must be separated by at least 10° of latitude, regardless of hemisphere, and (2) the amount of latitudinal overlap between the two ranges must be no more than 25% of the latitudinal extent of either species. In most cases I was able to find contrasts in which there was very little or no latitudinal overlap between the two species’ ranges.
For each species I recorded latitudinal range extent, mean female mass, mean clutch size, two indirect measures of sexual selection (sexual size dimorphism and sexual plumage dichromatism), habitat specificity and food specificity, using a wide range of published sources (data set and references are available on request). Where differing values were given by different sources, or a range of values given by one source, I used mean values. Sexual size dimorphism was measured as 1 –[weight of smaller sex/weight of larger sex]. Latitudinal range extent was measured in two ways. The first way was to measure the range of latitudes covered by the species, regardless of hemisphere; thus if a species’ range extends from 35° north to 10° south, its latitudinal extent would be 35°. The second way was to measure the entire latitudinal distance between the northern and southern extremes of the species’ range; the same species would therefore have a latitudinal extent of 45°. These two measures differ only in species whose ranges cross the equator. Sexual dichromatism was also measured in two ways. First, a species was classed as dichromatic simply if it was represented by separate male and female illustrations in guidebooks, and monochromatic if only one illustration was provided. Secondly, species were subjectively classed as 0 (monochromatic), 0·5 (slightly dichromatic) or 1 (strongly dichromatic). This was carried out with the list of species randomized to lessen the possibility of bias in assigning the level of dichromatism. The number of breeding habitats and food types used by each species were determined using categories modified from Owens et al. (1999). The habitat categories used were (1) saltwater, (2) freshwater lakes and wetlands, (3) running water, (4) forest, (5) woodland and scrub and (6) open habitats. The food type categories were (1) vertebrate carrion, (2) vertebrate prey, (3) invertebrate prey, (4) nectar and pollen, (5) fruit and seeds and (6) leaves, stems and roots.
CALCULATION AND ANALYSIS OF CONTRASTS
Contrasts in latitudinal range, dichromatism and habitat and food specificity were calculated additively, as VH– VL, where VH and VL are the values for the higher-latitude species and lower-latitude species, respectively. For female mass, size dimorphism and clutch size, it was inappropriate to calculate contrasts additively because the magnitude of trait values influences the size of the contrast (e.g. the contrast between 80 and 100 is 10 times the size of the contrast between 8 and 10). For these traits, contrasts were calculated as 1–[VS/VG], where VS and VG are the smaller and the greater of the two values. Contrasts in which the lower-latitude species had the higher trait value were arbitrarily given a negative sign. Under the null hypothesis that life-history traits are independent of latitude, the number of positive and negative contrasts, and their magnitudes, should be approximately equal. I used non-parametric Wilcoxon signed-ranks tests to test for significant departures from this expectation.
To examine whether latitudinal variation in life-history traits occurs both north and south of the equator, I carried out additional tests on northern and southern hemisphere contrasts separately. Northern hemisphere contrasts were defined as those in which both species’ geographical ranges lie entirely north of 10° south, and southern hemisphere contrasts were those where both species’ ranges lie entirely south of 10° north.
I also compared the results obtained using phylogenetically independent contrasts with those obtained by traditional cross-species analyses. I analysed relationships between trait values and latitudinal midpoint using least-squares regression (logistic regression for sexual dichromatism), treating all species in the dataset as independent observations without controlling for phylogeny. Log-transformation of the continuous variables before analysis gave acceptably normal error distributions and stable variances.
RANDOMIZATION TESTS FOR SAMPLING BIAS
Applying the minimal-overlap criterion for selecting species pairs imposes restrictions on the types of pairs that can be selected, principally that it will be difficult for two species with large latitudinal ranges to be selected as a pair. Despite this, there ought to be no possibility of sampling bias towards or against a latitudinal effect on latitudinal range generated by this restriction. This is because if there is no real association between latitude and latitudinal range, it should be no more likely for a high-latitude species with large range to be paired with a low-latitude species of small range than for a high-latitude species of small range to be paired with a low-latitude species of large range, or for two species of small range to be paired. To test this formally I carried out a simple randomization test, in which I reshuffled the values for latitudinal midpoint and latitudinal range among the species included in the analyses. I selected randomly, without replacement, one species from the reshuffled list. I then selected further species at random until one was able to be paired with the first species under both of the imposed criteria (described above). Pairs were thus selected until the list was exhausted. For each pair I calculated the contrasts in latitudinal range and tested for an effect of latitudinal midpoint on latitudinal range, using the same statistical methods as in the main analysis. A significant effect would indicate an artefactual latitudinal trend in latitudinal range caused by the method of choosing species pairs.
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- Materials and methods
Results of Wilcoxon signed-ranks tests for all contrasts (Table 1a) show that the two measures of latitudinal range extent, clutch size, habitat specificity and food specificity show significant positive associations with latitude. In other words, bird species inhabiting higher latitudes are likely to occupy a greater range of latitudes, have a greater north–south geographical distribution, larger clutches and broader ecological niches than their lower-latitude close relatives. Female mass, sexual size dimorphism and the two measures of sexual dichromatism show no significant variation with latitude. Results of cross-species regressions in which phylogeny is not controlled for are not wholly consistent with the independent contrast results (Table 1a).
|Trait||Method of analysis|
|Latitudinal range 1||69||+||0·0004||149||+||0·0004|
|Latitudinal range 2||69||+||0·03||148||+||0·07|
|Clutch size||40||+||< 0·0001||121||+||0·0001|
|Dichromatism 1||17||0·42||143||–||< 0·0001|
|Dichromatism 2||26||0·16||143||–||< 0·0001|
|(b) Northern hemisphere|
|Latitudinal range 1||28||+||< 0·0001||74||+||0·0001|
|Latitudinal range 2||29||+||0·0009||74||+||0·0003|
|(c) Southern hemisphere|
|Latitudinal range 1||23||+||0·002||53||0·83|
|Latitudinal range 2||22||+||0·01||53||0·9|
In the northern hemisphere (Table 1b), the two measures of latitudinal range extent, clutch size and habitat specificity show highly significant positive associations with latitude. Other traits show no significant associations. In the southern hemisphere (Table 1c), latitudinal range extent and clutch size are also positively associated with latitude. Additionally, female mass becomes significantly positively associated with latitude in the southern hemisphere. Cross-species regressions fail to detect many of the significant associations indicated by the independent contrasts analysis.
Results of the randomization tests for sampling bias in the analyses of latitudinal range are non-significant under Wilcoxon signed-ranks tests, for both measures of latitudinal range (n = 65 pairs, P = 0·11 and n = 61 pairs, P = 0·45, respectively). This indicates that the association between latitudinal midpoint and latitudinal range extent is not subject to sampling bias resulting from the method of choosing independent contrasts.
- Top of page
- Materials and methods
This study confirms a number of long-held assumptions of latitudinal variation in life-history traits among bird species (see Introduction), although it does not support the more recent prediction of latitudinal variation in the degree of sexual selection (Moller 1998). Latitudinal variation in life history among species may arise in three ways: (1) natural selection imposed by environmental conditions; (2) selective extinction of species from high-latitude assemblages; and (3) selective recolonization of high latitudes following periods of climate change. This study cannot distinguish between these mechanisms, but the explicit inclusion of phylogeny and latitudinal range overlap in the analyses, and the testing of patterns in both hemispheres, means that it can reject the possibility that patterns have arisen simply through phylogenetic or geographical bias. It appears therefore that latitude does influence many aspects of avian life history which potentially determine rates of speciation or extinction.
There has been little previous evidence for Rapoport’s Rule (decreasing latitudinal range extent towards the equator) in the southern hemisphere, most tests having shown equivocal results (Gaston et al. 1998; Gaston 1999). This has led to doubts about the generality of Rapoport’s Rule and suggestions it may be a relatively local pattern, existing primarily in the northern hemisphere at latitudes above approximately 30–40° (Rohde et al. 1993; Rohde 1996; Gaston et al. 1998; Gaston 1999). My results, however, suggest that in birds there is indeed a strong Rapoport effect in the southern hemisphere, and that the pattern may in fact be more general than is currently recognized. The pattern is nevertheless not as strong in the southern compared to the northern hemisphere, most probably because smaller continental land masses result in a more moderate increase in climatic seasonality with latitude (Gaston 1999). This probably explains why the Rapoport effect in the southern hemisphere has been difficult to detect in the past: more sensitive analytical methods which are less prone to Type 2 statistical error are needed to detect the pattern. My results for the southern hemisphere (Table 1c) appear to bear this out: the effect of latitude on latitudinal range is significant when independent contrasts are used, but nonsignificant when cross-species analyses are used.
The results are also strongly supportive of the traditional view that clutch size among bird species increases away from the equator (Lack 1954). As with Rapoport’s Rule, however, there have been very few tests for this pattern within the southern hemisphere, and these have indicated either a weak or non-existent effect of latitude on clutch size (Moreau 1944; Yom-Tov 1987, 1994). Again, one possible explanation for this is that none of the previous tests used phylogenetically explicit methods for analysing clutch size variation with respect to latitude, so may have lacked the power to detect patterns. However, my analysis indicates strong latitudinal effects on clutch size in both northern and southern hemispheres, regardless of whether or not phylogenetically indpendent contrasts are used (although patterns are much stronger when independent contrasts are used, despite greatly reduced sample sizes). Clutch size variation with latitude may therefore be another pattern which is more general than it currently appears. Further studies using appropriate analytical methods are needed to confirm this.
Other results only partly support predicted patterns of latitudinal variation. Female mass increased with latitude in the southern hemisphere, supporting the predictions of Bergmann’s Rule (James 1970), but showed no latitudinal effect in the northern hemisphere. Moller (1998) also used independent contrasts to examine bird body size differences across latitudes, and found no body size variation with latitude. Both mine and Moller’s results contrast with a large-scale analysis by Blackburn & Gaston (1996a) which showed a strong interspecific decrease towards the equator in mean body size of birds in the New World, in both hemispheres. It is difficult to explain why these results differ, but one possibility is that Blackburn & Gaston’s analyses used a different method of controlling for the effect of phylogeny. They did not apply phylogenetically independent contrasts using the method of Felsenstein (1985), but rather they found that the pattern of smaller body size towards the equator was repeated within taxa more often than expected by chance. This minimizes the likelihood that the pattern results entirely from phylogenetic artefact, although Blackburn & Gaston conclude that the lack of a strong association within taxa suggests that there may be some truth in explanations for latitudinal variation in body size among species based on phylogenetic history.
The predicted patterns of increasing ecological specificity towards the equator were supported in the northern hemisphere, but not the southern hemisphere. If it is assumed that more seasonal or variable climates promote ecological generalism (see Letcher & Harvey 1994), then there are several reasons to expect stronger latitudinal variation in ecological specificity in the northern hemisphere. Land extends to higher latitudes in the northern hemisphere, so bird species are distributed across a wider range of climatic zones. Seasonality is enhanced further by the vast continental land masses of the northern hemisphere: much of the land area of Eurasia and North America is far from the moderating influence of the ocean. Glaciation during the Pleistocene was more extensive and severe in high northern compared to high southern latitudes (Markgraf & Hope 1995), which may have led to selective extinction of species which were more specialized, and hence less tolerant of climatic fluctuation. Finally, high northern latitudes may have been selectively recolonized by ecological generalists following glacial retreat at the end of the Pleistocene (Price, Helbig & Richman 1997). An alternative explanation for the observed increase in ecological specificity towards the equator is that it results not from a real pattern but from sampling bias. Measures of niche breadth often increase with a species’ abundance because less abundant species are more likely to be recorded from fewer sites than they actually occupy (Gaston 1994). If tropical species are less abundant than those at higher latitudes, this could create a bias in the number of recorded habitats or food types for any given species. However, this would not explain the lack of an observed latitudinal gradient in niche breadth in the southern hemisphere, unless patterns of latitudinal variation in abundance differed between hemispheres.
The analyses of the two indirect measures of sexual selection (size dimorphism and dichromatism) do not support predicted patterns of latitudinal variation. When cross-species analyses are used, sexual dichromatism increases significantly towards the equator. At first, this does appear to support the prediction of Moller (1998) that increased parasite impact on host fitness in the tropics should lead to stronger parasite-mediated sexual selection and more extravagant sexual ornamentation. However, when phylogeny is controlled for, these relationships disappear, suggesting that the apparent latitudinal variation in dichromatism is due to phylogenetic bias and not to selection for greater dichromatism at lower latitudes. The phylogenetic bias probably stems from the high representation of taxa in my dataset which are largely dichromatic and tend to inhabit low latitudes, particularly the families Bucerotidae (hornbills), Pittidae (pittas) and Maluridae (fairy-wrens).
Overall, the discrepancies between results from independent contrasts and traditional cross-species analyses indicate the susceptibility of traditional analyses to both Types 1 and 2 statistical error, emphasizing the importance of controlling for phylogeny and range overlap in the analysis of geographical patterns of life history. The results do not prove that the life-history traits associated with latitude contribute to high species richness in the tropics. However, establishing a convincing link between life history and latitude is a necessary prerequisite to any claim that life history underlies latitudinal diversity gradients. It seems clear that the latitude at which a species lives does influence numerous aspects of its life history, which in turn may determine its likelihood of speciating or of going extinct.
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- Materials and methods
Many thanks to Ian Owens and Klaus Rohde for comments. I am supported by a University of Queensland Graduate School Award.
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- Materials and methods
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- Materials and methods
List of phylogenetically independent pairs of bird species used in the analysis
|Family||Lower-latitude species||Higher-latitude species||Phylogeny reference|
|Alcidae||Synthliboramphus antiquus||Synthliboramphus wumizusumi||Friesen et al. 1996|
|Anatidae||Anas aucklandica||Anas chlorotis||Johnson & Sorenson 1999|
|Anatidae||Anas versicolor||Anas puna||Johnson & Sorenson 1999|
|Anatidae||Anas acuta||Anas bahamensis||Johnson & Sorenson 1999|
|Anatidae||Anas rubripes||Anas fulvigula||Johnson & Sorenson 1999|
|Anatidae||Anas castanea||Anas bernieri||Johnson & Sorenson 1999|
|Anatidae||Anas penelope||Anas sibilatrix||Johnson & Sorenson 1999|
|Anatidae||Anas querquedula||Anas hottentottus||Johnson & Sorenson 1999|
|Anatidae||Anas clypeata||Anas smithii||Johnson & Sorenson 1999|
|Anatidae||Speculanas specularis||Amazonetta brasiliensis||Johnson & Sorenson 1999|
|Bucerotidae||Tockus leucomelas||Tockus deckeni||Kemp 1995|
|Bucerotidae||Tockus bradfieldii||Tockus fasciatus||Kemp 1995|
|Bucerotidae||Tockus monteiri||Tockus albocristatus||Kemp 1995|
|Bucerotidae||Ocyceros birostris||Ocyceros gingalensis||Kemp 1995|
|Bucerotidae||Buceros hydrocorax||Buceros rhinoceros||Kemp 1995|
|Bucerotidae||Anorrhinus austeni||Anorrhinus galeritus||Kemp 1995|
|Bucerotidae||Penelopides manillae||Penelopides exarhatus||Kemp 1995|
|Cacatuidae||Callocephalon fimbriatum||Probosciger aterrimus||Brown & Toft 1999|
|Cacatuidae||Cacatua sanguinea||Cacatua alba||Brown & Toft 1999|
|Certhiidae||Polioptila californica||Polioptila albiloris||Zink & Blackwell 1998|
|Ciconiidae||Mycteria leucocephala||Mycteria cinerea||Slikas 1997|
|Corvidae||Chlamydera maculata||Chlamydera nuchalis||Christidis & Schodde 1992|
|Corvidae||Ptiloris paradiseus||Ptiloris magnificus||Christidis & Schodde 1992|
|Falconidae||Falco naumanni||Falco punctatus||Seibold, Helbig & Wink 1993|
|Falconidae||Falco rusticolus||Falco cherrug||Seibold et al. 1993|
|Fringillidae||Serinus scotops||Serinus capistratus||Badyaev 1997|
|Fringillidae||Serinus alaria||Serinus nigriceps||Badyaev 1997|
|Fringillidae||Serinus flaviventris||Serinus donaldsoni||Badyaev 1997|
|Fringillidae||Serinus citrinella||Serinus canicollis||Badyaev 1997|
|Fringillidae||Serinus leucopterus||Serinus burtoni||Badyaev 1997|
|Fringillidae||Serinus totta||Linurgus olivaceus||Badyaev 1997|
|Fringillidae||Serinus citrinipectus||Serinus dorsostriatus||Badyaev 1997|
|Fringillidae||Carduelis barbata||Carduelis spinescens||Badyaev 1997|
|Fringillidae||Agelaius thilius||Agelaius xanthopthalmus||Johnson & Lanyon 1999|
|Fringillidae||Pseudoleistes virescens||Agelaius icterocephalus||Johnson & Lanyon 1999|
|Fringillidae||Agelaius tricolor||Agelaius humeralis||Johnson & Lanyon 1999|
|Fringillidae||Quiscalus major||Quiscalus lugubris||Johnson & Lanyon 1999|
|Fringillidae||Molothrus ater||Scaphidura oryziphora||Johnson & Lanyon 1999|
|Fringillidae||Spizella arborea||Spizella atrogularis||Zink & Dittmann 1993|
|Fringillidae||Pipilo chlororus||Pipilo ocai||Zink, Weller & Blackwell 1998|
|Fringillidae||Pipilo crissalis||Pipilo albicollis||Zink et al. 1998|
|Fringillidae||Zonotrichia querula||Zonotrichia capensis||Zink, Dittmann & Rootes 1991|
|Fringillidae||Ammodramus bairdii||Ammodramus aurifrons||Zink & Avise 1990|
|Fringillidae||Diglossa carbonaria||Diglossa humeralis||Hackett 1995|
|Furnariidae||Cranioleuca obsoleta||Cranioleuca vulpina||Garcia-Moreno, Arctander & Fjeldsa 1999a|
|Gruidae||Grus monachus||Grus nigricollis||Krajewski & Fetzner 1994|
|Gruidae||Grus vipio||Grus rubicunda||Krajewski & Fetzner 1994|
|Gruidae||Anthropoides virgo||Anthropoides paradisea||Krajewski & Fetzner 1994|
|Hirundinidae||Riparia riparia||Riparia cincta||Sheldon, Whittingham & Winkler 1999|
|Maluridae||Malurus pulcherrimus||Malurus amabilis||Christidis & Schodde 1997|
|Maluridae||Malurus cyaneus||Malurus coronatus||Christidis & Schodde 1997|
|Maluridae||Malurus melanocephalus||Malurus alboscapulatus||Rowley & Russell 1997|
|Maluridae||Stipiturus mallee||Stipiturus ruficeps||Christidis & Schodde 1997|
|Maluridae||Stipiturus malachurus||Clytomyias insignis||Rowley & Russell 1997|
|Pardalotidae||Sericornis magnirostris||Sericornis beccari||Joseph & Moritz 1993|
|Pittidae||Pitta sordida||Pitta steerii||Erritzoe & Erritzoe 1998|
|Pittidae||Pitta nympha||Pitta megarhyncha||Erritzoe & Erritzoe 1998|
|Pittidae||Pitta versicolor||Pitta elegans||Erritzoe & Erritzoe 1998|
|Scolopacidae||Limnodromus scolopaceus||Limnodromus griseus||Avise & Zink 1988|
|Sturnidae||Toxostoma redivium||Toxostoma occelatum||Zink et al. 1999|
|Sulidae||Morus bassanus||Papasula abbotti||Friesen & Anderson 1997|
|Sylviidae||Sylvia curruca||Sylvia leucomelaena||Blondel, Catzeflis & Perret 1996|
|Sylviidae||Phylloscopus schwarzii||Phylloscopus affinis||Price et al. 1997|
|Sylviidae||Phylloscopus trochilus||Phylloscopus lorenzii||Price et al. 1997|
|Sylviidae||Phylloscopus proregulus||Phylloscopus chloronotus||Price et al. 1997|
|Sylviidae||Phylloscopus borealis||Phylloscopus magnirostris||Price et al. 1997|
|Sylviidae||Phylloscopus plumbeitarsus||Phylloscopus trochiloides||Price et al. 1997|
|Trochilidae||Heliodoxa branickii||Heliodoxa gularis||Gerwin & Zink 1989|
|Trochilidae||Metallura aeneocauda||Metallura williami||Garcia-Moreno, Arctander & Fjeldsa 1999b|