Ecological niche divergence or ecological niche partitioning in a widespread Neotropical bird lineage

Ecological niche divergence is generally considered to be a facet of evolution that may accompany geographic isolation and diversification in allopatry, contributing to species’ evolutionary distinctiveness through time. The null expectation for any two diverging species in geographic isolation is that of niche conservatism, wherein populations do not rapidly shift to or adapt to novel environments. Here, I test ecological niche divergence for a widespread, pan-American lineage, the avian genus of martins (Progne). Despite containing species with distributions that go from continent-spanning to locally endemic, I found limited evidence for niche divergence across the breeding distributions of Progne, and much stronger support for niche conservatism with patterns of niche partitioning. The ancestral Progne had a relatively broad ecological niche, similar to extant basal Progne lineages, and several geographically localized descendant species occupy only portions of the larger ancestral Progne niche. I recovered strong evidence of breeding niche divergence for four of 36 taxon pairs but only one of these divergent pairs involved two widespread, continental species (Southern Martin P. elegans vs. Gray-breasted Martin P. chalybea). Potential niche expansion from the ancestral species was observed in the most wide-ranging present-day species, namely the North American Purple Martin P. subis and P. chalybea. I analyzed populations of P. subis separately, as a microcosm of Progne evolution, and again found only limited evidence of niche divergence. This study adds to the mounting evidence for niche conservatism as a dominant feature of diversifying lineages. Even taxa that appear unique in terms of habitat or behavior may still not be diversifying with respect to their ecological niches, but merely partitioning ancestral niches among descendant taxa.


Introduction
Species' ecological niches, like morphological traits, evolve via natural selection, thus altering their ecological niches and geographic potential through time (Engler et al., 2021).These processes are particularly important for diversification among closely related species, as niche divergence and diversification can lead to or reinforce speciation (Hu et al., 2015;Cuervo et al., 2021;Şahin et al., 2021).Such ecological shifts can lead to dramatic shifts in geographic distributional potential, and may follow predictable patterns through evolutionary time (Cobos et al., 2021).For example, insular species often shift from lowland to montane situations through time (Ricklefs & Cox, 1972;Kennedy et al., 2022), whereas the opposite tendency (highlands to lowlands) may dominate in continental settings (van Els et al., 2021).
Despite frequent opportunities for species to adapt and evolve with respect to their ecological niches, ecological niche conservatism appears to be the norm within most species (Peterson, Soberón & Sanchez-Cordero, 1999;Peterson, 2011;Khaliq et al., 2015;García-Navas & Westerman, 2018), at least with respect to coarse-resolution environmental conditions (Comte, Cucherousset & Olden, 2017).Such conservatism has been argued to be a contributing factor in diversification dynamics, especially in systems in which conserved niches through time force populations into allopatry and allow independent evolution to occur (Prigogine, 1987;Vrba, 1993;Kozak & Wiens, 2006).Indeed, in birds, secondary contact is often identified as a driving force for character divergence, including in ecological niches (Endler, 1977;Seddon & Tobias, 2007;McCormack, Zellmer & Knowles, 2009).
Three major scenarios for niche evolution are thus available for allopatric and parapatric populations sharing a recent common ancestor: (1) descendant populations become wholly allopatric and undergo no appreciable niche differentiation; (2) descendant populations occupy 4 different parts of their ancestor's ecological niche in allopatry or parapatry, adapting to these specific conditions and partitioning ecological space upon subsequent secondary contact; and (3) one or more descendant populations are able to adapt to new environments and occupy novel ecological niches (Figure 1).Scenario 1 appears to be the norm, but these patterns may be overridden in deeper time by the open exchange of genes between populations after secondary contact, as has occurred with raven lineages (Corvus spp.) in North America (Omland, Baker & Peters, 2006).Scenario 2 is perhaps best exemplified by Poecile chickadees within North America: two lineages (Carolina Chickadee P. carolinensis and Black-capped Chickadee P. atricapillus) have a narrow but distinct hybrid zone within which each species can survive, with the socially dominant P. carolinensis slowly pushing northwards as the climate warms (Mostrom, Curry & Lohr, 2020).These situations can also lead to complicated hybrid zones, as in taxa that now exist in secondary contact after retreating to different Pleistocene refugia (e.g., members of the Yellowrumped Warbler Setophaga coronata complex) (Hubbard, 1969;Milá, Smith & Wayne, 2007).Scenario 3 is perhaps less frequent, but is likely manifested within genera such as Baeolophus titmice, in which the interior western Juniper Titmouse Baeolophus ridgwayi exists in drier, more xeric conditions than all of its congeners (Cicero, Pyle & Patten, 2020).
One genus that likely has undergone multiple modes of diversification is the martin genus Progne (Aves: Hirundinidae) (Table 1).These aerial insectivores are distributed from central Canada to southern Argentina, nesting in cavities in trees, rocks, man-made structures, and even occasionally in the ground (Allen & Nice, 1952;Pistorius, 1975).Northern and southern representatives of the genus are migratory, with several resident taxa overlapping with these species in the Tropics during non-breeding periods.Movements of Progne species appear to be complex, with some species undergoing seasonal inter-regional movements between primary and secondary wintering areas thousands of kilometers apart (Siddiqui, 2017).Even daily movements of Progne can cover large distances, with coastal, sea-level-nesting Peruvian Martins P. murphyi reaching elevations as high as 1500 m (Parker, Stotz & Fitzpatrick, 1996;Luo, 2020) and Purple Martins P. subis foraging as high as 1889 m above the ground (Helms et al., 2016), sometimes quite far from nest sites (Corman, 2005).The combination of aerial feeding habits and migratory behaviors enables Progne to cover long distances as migrants and as vagrants, as demonstrated by records P. subis, a species that breeds broadly across central North America and winters in central South America, from Alaska, Ireland, Scotland, the Azores, and the Falklands [Malvinas] (eBird, 2012;Quigley, 2018;Brown, Airola & Tarof, 2021).
Martins show high levels of phenotypic conservatism, such that winter distributions for many species are unknown or are only being confirmed in recent years; many migrant or awayfrom-breeding-range individuals (especially among the 'white-breasted' martin complex of the Caribbean and Mexico) are not identifiable in the field (Fang & Schulenberg, 2020;García-Lau & Turner, 2021;Perlut & Williams, 2021).Distributions vary greatly in extent: the Galápagos Martin P. modesta (Roper, 2020) and P. murphyi (Luo, 2020) inhabit geographically limited xeric coastal areas whereas most widespread species are found across continents either seasonally or year round.
An extreme example is the Gray-breasted Martin P. chalybea, which breeds across the entire latitudinal breadth of the Tropics from Mexico to Argentina (Lagasse, 2020).
Given that this genus is monophyletic and its phylogenetic relationships have been well documented (Moyle et al., 2008;Brown, 2019), it is possible to explore the evolutionary history of ecological niche traits.Studying ecological niche evolution in Progne provides an opportunity to examine niche differentiation in a wide-ranging genus that has evolved migratory and locally endemic resident forms that are largely conserved morphologically, as well as to test whether a geographically widespread species complex has evolved to take advantage of new ecological niches or has merely partitioned the ancestral species' ecological niche.Martin species occur in allopatry, parapatry, and sympatry in their breeding distributions, allowing for tests of niche conservatism between pairs of species that are of varying relatedness and that possess varying levels of geographic overlap.Reconstructing niche evolution in this genus will also shed light on what kinds of evolutionary shifts lead to local endemism, and will provide a continental comparison to island speciation cycles.

General Data Cleaning
Occurrence data for all Progne martin species were downloaded from the Global Biodiversity Information Facility (GBIF) on 21 Feb 2022 (Global Biodiversity Information Facility, 2022).Data were processed using R 4.1.2,4.2.0, and 4.2.1 (R Core Team, 2022) , relying on the general data packages of tidyverse (Wickham et al., 2019) and data.table(Dowle & Srinivasan, 2019), and the general spatial packages maptools (Bivand & Lewin-Koh, 2022), raster (Hijmans, 2022), rgdal (Bivand, Keitt & Rowlingson, 2022), and sf (Pebesma, 2018).Data were reduced to presence-absence data based at localities, with duplicate sightings of particular species from single sites removed.Distributions of individual species were superimposed on country borders using rnaturalearth (South, 2017) and rnaturalearthhires (South, 2022), and occurrences of each species were compared to known distributions for each taxon (Billerman et al., 2020).Outlier occurrences for each taxon were removed or re-identified depending on their location, the taxonomic history for the species in question, and on the species' residency status in a given region (see Supplemental Materials).These steps are necessary given the number of misidentifications and low-confidence identifications in the database at the time of download, and given the long-distance dispersal ability of Progne.Based on published distributions and breeding records, I created dispersal areas by hand based on major biogeographic barriers (e.g., straights, crests of mountains) referred to as Ms for identifying potentially erroneous points and, for allopatric complexes like P. subis, assigning breeding records to the correct geographic populations (Soberón & Peterson, 2005;Cooper et al., 2021).I also ran a custom 'rarefy' code provided by Dr. J. D. Manthey (Texas Tech University) to thin occurrences to be ≥20 km to one another using the R package fossil (Vavrek, 2011).This last step was particularly important near large population centers or known colonies for rarer taxa, where records can be much denser than rural areas, potentially introducing biases in the models.
Occurrence data were largely restricted to April-July (Northern Hemisphere) and October-January (Southern Hemisphere) for migratory taxa to compare breeding niches, with minor adjustments for individual species based on their migratory patterns (see Supplemental Material).
I focused on summer distributions as winter distributions are less well-known and are datadepauperate, especially for long-distance migrants.These restrictions ensured that comparisons of taxa encompassed similar phenological periods, and that records could be identified with greater confidence, as species' ranges are known to be largely discrete in breeding season.Non-breeding distributions of many migratory taxa remain incompletely known, such that whether non-breeding distributions of several species pairs are wholly syntopic is unknown (Perlut, Klak & Rakhimberdiev, 2017;Fang & Schulenberg, 2020;Turner, 2020;Brown, Airola & Tarof, 2021;García-Lau et al., 2021;García-Lau & Turner, 2021;Perlut & Williams, 2021).
These steps were also applied to subspecies of Progne subis for analyses within a polytypic migratory taxon.I did not repeat these analyses with Brown-chested Martin P. tapera, the other polytypic Progne with migratory populations, as the distributions of migratory and non-migratory populations do not appear to be discrete, and many records within the GBIF database are not identified to subspecies.For Progne subis, three subspecies are described, but the limits of their distributions are not well defined (Brown, Airola & Tarof, 2021).Specifically, among western populations, lines of evidence for subspecies assignment, behaviors, and nesting preferences vary greatly across the species' range.As such, I subdivided P. subis into the following populations for analysis: nominate P. s. subis of eastern North America; core P. s. arboricola in the Rocky Mountains as far south as the Mogollon Rim; P. s. hesperia of the Sonoran desert; Pacific coast P. s. arboricola of California north to British Columbia; and interior Mexican populations of unknown taxonomic status from the mountains south of the Mogollon Rim to southwestern Mexico (Brown, Airola & Tarof, 2021).

Environmental Data
Environmental data were drawn from the ENVIREM dataset (Title & Bemmels, 2018).I removed count-format data restricting the data to continuous, raster-format variables representing terrestrial conditions.I retained elevation in the analyses but I removed the terrain roughness index, as elevation is known to affect the physiology of birds (and thus may affect nest site selection) but general terrain roughness is likely to affect Progne only indirectly, given that single species can be found under very diverse topographic conditions (Dubay & Witt, 2014).The remaining environmental variables were transformed via principal components analyses using the function 'rda' in the R package vegan (Oksanen et al., 2022).The first principal component was mostly explained by variation in potential evapotranspiration (96.3%), annual potential evapotranspiration (1.2%) and thermicity (a metric comparing minimum and average temperatures; 1.2%).The second principal component was likewise dependent on evapotranspiration, with the primary explaining variables of annual evapotranspiration (70.1%), thermicity (18.7%), and maximum temperature of the coldest month (5.7%).
Data were further restricted to variables less affected by temperate latitude seasonality to reflect potential differences within populations' breeding niches better.That is, I retained the ENVIREM variables of annual potential evapotranspiration, Thornthwaite aridity index, climatic moisture index, Emberger's pluviometric quotient (a measure for differentiating Mediterranean climates), minimum temperature of the warmest month (generally corresponding with the breeding season for migratory taxa), potential evapotranspiration of the driest quarter, potential evapotranspiration of the warmest quarter, potential evapotranspiration of the wettest quarter, and topographic wetness index (Title & Bemmels, 2018).The top remaining explanatory variables for the first principal component were annual evapotranspiration (70.3%) and Emberger's pluviometric quotient (27.8%).For the second principal component, they were Emberger's pluviometric quotient (71.5%) and annual evapotranspiration (27.3%).I only used the first two principal components to analyze variable importance and to look at niche differentiation, and I used variables directly for creating ecological niche models.I proceeded with all subsequent analyses using this subset of environmental data.

Environmental Comparisons
To assess different 'ecopopulations' (i.e., populations as defined by unique environments) and to identify how well-partitioned Progne taxa are ecologically, I performed linear discriminant analyses in R, using the 'lda' function in the package MASS (Venables & Ripley, 2002).These tests partition individuals based on the environmental data, and suggest hypotheses for group assignments for individuals among known group assignments and groups presumed based on environmental characteristics (Cooper et al., 2021).I performed these tests both for all Progne species and within P. subis.I also verified whether the number of taxa recognized is supported by the environmental data by performing gap-statistic analyses of k-means clusters using the function 'fviz_nbclust' in the R package factoextra (Kassambara & Mundt, 2020).

Niche Modeling and Comparisons
For each species, I created ecological niche models using a presence-only method, minimum volume ellipsoids, following Cooper et al. (2021).Specifically, I fit minimum volume ellipsoids with an inclusion level of 90% using the 'cov.mve'function in the R package MASS (Venables & Ripley, 2002).I departed from the previous pipeline with respect to thresholding to create a presence-absence map.Whereas this threshold was previously determined by applying a normal distribution cutoff to the Mahalanobis distances within the data (Cooper et al., 2021), these data are often non-normal, and such thresholding can augment bias in the dataset.These distances are better fit by a Gaussian distribution, given the overwhelming number of points close to the centroid and the right skew of the data.Gaussian distributions fitting each ellipsoid were determined using the R function 'fitdist' in the package fitdistrplus (Delignette-Muller & Dutang, 2015); models were then thresholded to the following inclusion levels: 75%, 85%, 90%, 95%, and 99%.These models were run for every Progne species, as well as for subpopulations of P. subis.I performed pairwise niche comparisons of species following Warren et al. (2008) and expanded by Cooper et al. (2021) utilizing the comparative statistic of Schoener's D calculated using the R package dismo (Hijmans et al., 2021).Schoener's D returns a value between 0 and 1, with 0 being wholly different ecological niche models and 1 being identical ecological niche models.For each pair of species A and B, I computed the observed value of Schoener's D for A and B, and for two test distributions derived from 100 ecological niche models created from random points within each population's accessible area (M) (Warren, Glor & Turelli, 2008;Glor & Warren, 2011;Cooper et al., 2021).Specifically, I defined a test statistic as the 'true' comparison between A and B using the species' ecological niche models, and then derived the two random distributions by comparing the model of A to the models derived from random points from across the accessible area (M) of B and vice versa (Glor & Warren, 2011;Cooper et al., 2021).I considered tests above the 97.5% confidence interval for both random distributions to be a very clear failure to reject the null hypothesis, and clear confirmation that A and B should be considered to have highly similar, near-equivalent ecological niches (Glor & Warren, 2011;Cooper et al., 2021).If the statistic fell below the 2.5% confidence interval, A and B were assessed to have divergent ecological niches, and thus be a rejection of the null hypothesis.Many other outcomes are possible, most indicating that the contrast between is indistinguishable from random, or that the two tests are not both significant in the same direction.These tests could be indicative of a 'spectrum' of ongoing niche diversification, but I treat them separately; in the end, however, they are not rejections of the null hypothesis of ecological niche conservatism (Cooper et al., 2021).

Historical Niche Reconstruction
Historical niche reconstructions were performed using the R packages ellipsenm (Cobos et al., 2022), geiger (Pennell et al., 2014), nodiv (Borregaard et al., 2014), and nichevol (Cobos, Owens & Peterson, 2020), taking into account accessible areas (Ms) for each species and the lack of access of many species to all climatic combinations experienced by the genus (Saupe et al., 2018).I used the same ecological characters as described above for the pairwise comparisons for consistency (Cooper & Soberón, 2018).I used a phylogenetic tree of species' relationships based on a UCE study of the family Hirundinidae provided by Clare E. Brown, Subir Shakya, and Fred Sheldon (Brown, 2019).This tree is missing two taxa due to poor DNA reads, namely Galápagos Progne modesta and Cuban P. cryptoleuca Martins.I added P. cryptoleuca to the tree as sister to Caribbean Martin P. dominicensis halfway between the node and the base of the dendrogram based on phylogenetic information indicating that these taxa form a sister-pair (Moyle et al., 2008).I performed nichevol analyses of each ENVIREM character to assess how ecological niches shifted through time, and to identify which species experience the largest evolutionary shifts.shifted through time, and to identify which species experience the largest evolutionary shifts.

General Note on Results
Many of the results presented herein involve models that are not static and depend on dynamic parameters; as a result, exact values may vary between iterations, but general patterns were consistent between all iterations I performed within this study.

Environmental Differences and Clustering
I observed broad ecological niche overlap within the genus generally, with respect to individual environmental variables and in terms of environmental variables transformed by principal components analysis.Species that occupy different extreme habitats (e.g., desert vs. rainforest) may show differentiation along individual environmental axes that reflect these differences, but few taxa showed overall differences in realized niches (see Supplementary Material).Using gap-statistic analysis, I determined the number of 'ecospecies' within the genus Progne to be 5 (see Supplemental Material).However, when classifying the full set of occurrence data into five groups using k-means, none of these ecospecies corresponded clearly to any described taxon, and no individual taxon is fully within any k-means group (Supplemental Material).Using discriminant function analyses, the most accurately reconstructed species from environmental data were P. murphyi (90% clustered together) and P. subis (94%).The greatest confusion was related to Gray-breasted Martin P. chalybea, a species that is wide-ranging both geographically and environmentally.Most individuals of Cuban Martin P. cryptoleuca, Caribbean Martin P. dominicensis, Sinaloa Martin P. sinaloae, and Brown-chested Martin P. tapera, were ascribed to the same group as P. chalybea (Table 2).Within P. subis, I found that the most-supported number of ecopopulations in this clade is 1.This result holds true even when all taxa except nominate P. s. subis are compared.Despite this, discriminant function analyses of the three currently recognized subspecies of P. subis had a high level of success in discriminating taxa, with accuracies of 96% for P. s. arboricola, 96% for P. s. hesperia, and 99% for P. s. subis (see Supplemental Materials).Subdividing these taxa further into the five groups based on region, habitat, and behavior still showed high support for each described subspecies, with accuracies of 93% for P. s. arboricola, 90% for P. s. hesperia, and 100% for P.
s. subis (Table 3).Additionally, Pacific P. s. arboricola populations were largely differentiable from interior arboricola populations, with 82% of individuals being correctly identified and only 12% of individuals erroneously assigned as interior arboricola.Mexican populations were split between being considered their own entity (40%) and being considered an extension of interior arboricola (32%).

Ecological Niche Divergence
Niches appeared to be largely conserved within the genus Progne, with a failure to reject the null hypothesis in 69% of pairwise tests; significant failures to reject with respect to both comparisons were only found in two pairwise comparisons: P. subis vs.Southern Martin P. elegans and P. subis vs. P. tapera) (Table 4A; Figure 2).Only four pairwise tests showed definitive ecological niche divergence between test taxa: P. chalybea vs. P. elegans, P. chalybea vs, P. murphyi, P. cryptoleuca vs. P. elegans, and P. cryptoleuca vs. P. murphyi.Rejection of the null hypothesis for only one of the two pairwise comparisons was observed in six more comparisons, most involving combinations with P. dominicensis, P. tapera, P. murphyi, and P. modesta (Table 4A).Instances of divergence were not limited to widespread species or limited-range species, with one instance of divergence found between two wide-ranging species (Progne chalybea and P. elegans), two instances between widespread and restricted-range species (P.chalybea and P. murphyi, P. elegans and P. cryptoleuca), and one between two restricted-range species (P.murphyi and P. cryptoleuca).Conversely, ecological niche comparisons that were most similar (i.e., the null of conservatism was the least likely to be rejected) involved the ecologically diverse P. subis.
Among subpopulations of Progne subis, no comparisons were able to conclusively reject the null hypothesis of niche conservatism (Table 4B).Only two pairwise comparisons rejected the null hypothesis for one of the two comparisons: P. s. arboricola (Rocky Mountains) vs. P. s. unknown (Mexico), and P. s. hesperia vs. P. s. unknown (Mexico).The most similar taxa appeared to be P. s. arboricola (Pacific Coast) and both P. s. arboricola (Rocky Mountain) and P. s. hesperia; however, it is unclear whether this similarity is informative or merely within the variation of the spectrum of what can be considered niche conservatism.

Ecological Niche Reconstructions
Reconstructions were created for each variable independently.The ancestral Progne was found to have a broad ecological niche in most aspects, though (in some cases) not quite as broad as the most basal extant species, P. tapera (Figure 3; see Appendix 1).Reconstructions consistently showed instances of niche contraction in geographically localized taxa, especially among the restricted-range species such as P. murphyi, P. sinaloae, and P. cryptoleuca.Niche expansion was observed for several traits, such as Emberger's Pluviometric Quotient (designed to separate Mediterranean climates), but such expansions were mostly observed in widespread taxa or taxa that breed at high latitudes.Some taxa also experienced niche expansion with respect to their sister species, further indicating flux in the occupied environmental areas within the clade.Wide ranging taxa, especially P. subis and P. chalybea, demonstrate this niche expansion with respect to their most recent common ancestor with their less widespread sister taxa.Most niches, however, were similar to the broad ancestral niche, or were contained within the space of the broad ancestral niche.Unsurprisingly, some taxa, especially insular taxa, inhabit ecological niches that cannot be characterized completely owing to limited environmental conditions being present within the species' accessible area.

Discussion
I found widespread support for niche conservatism within Progne.Historical reconstructions demonstrate that members of Progne are largely overlapping in ecological space both with their congeners and the presumed niche of their ancestral taxon.Thus, current niche similarities are not the result of ecological niche convergence, and observed divergences appear to be the result of niche partitioning wherein species niches contract with respect to their ancestral state.The most similar ecological niches were found among basal martins with broad distributions, namely between P. subis and both P. tapera and P. elegans.
I found only a few instances of ecological niche divergence overall, with these divergences being between non-sister taxa.Two of the incidences of niche divergence were between P. elegans, the outgroup of the 'white-breasted' martin clade, and species found in the same clade (P.chalybea and P. cryptoleuca).This group includes multiple morphologically conservative species that are not always identifiable in the field, but that possess largely discrete breeding ranges in diverse habitats, ranging from mid-elevation montane forests to coastal scrub, palms, and urban areas (Fang & Schulenberg, 2020;García-Lau & Turner, 2021;Perlut & Williams, 2021).

Ecological niche conservatism in Progne
I was unable to reject the null hypothesis of niche conservatism for most of Progne, providing further evidence that organisms are unlikely to have rapid niche shifts while diversifying (Peterson, Soberón & Sanchez-Cordero, 1999).Evolutionary reconstructions indicate that the ancestral Progne was a South American species with a broad ecological niche, and that it was at least partially migratory (Brown, 2019), similar to the ecology of the extant Progne tapera.The next basal groups included P. subis, a long-distance North American migrant, and the rangerestricted species pair P. murphyi and P. modesta.It appears that P. subis is descended from a South American species (Brown, 2019), perhaps similar to the recent North-to-South Hemisphere colonization of Barn Swallows Hirundo rustica in Argentina (Martínez, 1983).Similar north-tosouth shifts and geographic isolation appear to have led to diversification within the 'whitebreasted' martins as well, resulting in three allopatric species breeding across the North American and Caribbean tropics (Brown, 2019).Such events demonstrate the importance in geographic isolation for driving diversification within Progne, and support the idea of Progne undergoing a 'geographic radiation' of speciating while maintaining similar ecological niches among the descendant species (Peterson, Soberón & Sanchez-Cordero, 1999;Simões et al., 2016).Ecological niche differences exist among a few extant Progne, but these appear to be the result of ecological niche partitioning and not true ecological niche shifts.As species have colonized isolated regions and have continued to adapt regionally, they have experienced niche contractions, possibly as a result of specializing to conditions in these regions, or moderate niche shifts, wherein they have expanded their ecological tolerance within their specific geographic distributions.The former is demonstrated well by Progne murphyi, a species restricted to the coast of Peru.The restricted ecological niche of P. murphyi is reminiscent of taxon cycles in the Caribbean: species diversify, and descendant species become more restricted (and sometimes more ecologically specialized) through time (Ricklefs & Cox, 1972;Engler et al., 2021).Results within the genus Progne indicate that tests of ecological niche divergence should try to account for ancestral niche states to understand whether observed niche evolution is novel or reflective of a different evolutionary process, such as partitioning of the ancestral state.
Progne subis appears to be a microcosm of these phenomena, with the species as a whole occupying a broad ecological niche, with individual populations evolving for specific environmental conditions within the overall niche space.Within P. subis, evidence for niche divergence is lacking, with results reflecting a continuum of variation between descendant populations partitioning environments than any true ecological shift.In southwestern North America, cactus-nesting P. s. hesperia are found in close geographic proximity with montane P. s. arboricola, yet I found ecological niche overlap between these taxa that differ drastically with respect to habitat preference.These populations have niches within the broader Progne subis niche reminiscent of other extant species occupying subsets of the broader Progne niche elsewhere (e.g., P. murphyi and P. sinaloae), further supporting the notion of niche partitioning and niche specialization across a geographic mosaic, as opposed to significant ecological niche evolution leading to large ecological shifts.Diversification via niche partitioning of a broader ancestral state may be more common than is realized in continental taxa, given the propensity of groups like Zosterops to undergo taxon cycles in montane regions (Ricklefs & Cox, 1972;Melo, Warren & Jones, 2011;Pearson & Turner, 2017;Engler et al., 2021), and the propensity for related populations to partition and specialize on different food sources across space and time (Benkman et al., 2009;Cenzer, 2016;Alonso et al., 2020).

Assessments of niche in geographically widespread groups
Assessments of ecological niches are still frequently performed across political boundaries or study area boundaries, and thus do not account for the bias introduced by including sites and associated environments not accessible to the study taxa over relevant time periods (Soberón & Peterson, 2005;Barve et al., 2011;Owens et al., 2013;Peterson & Anamza, 2015;Cooper & Soberón, 2018;Song et al., 2020).These oversights can bias niche estimates and therefore bias distribution models in current environments.These issues can also compound when comparing ecological niches through time to understand their evolution (Saupe et al., 2018).
Another less discussed oversight, however, is that of population-level variation within species.What constitutes a species can be contentious (Watson, 2005), so what is recognized as a species in the literature can vary greatly between taxonomic authorities (Barrowclough et al., 2016;Garnett & Christidis, 2017;Raposo et al., 2017).The effects of these oversights are easily missed in studies focusing on 'species-level' diversification.I focused on sets of populations of P. subis, and results illustrated the processes that are occurring on a more unitary, fine-scale basis within this one species.

Conclusions and Future Directions
Future research on Progne should focus on understanding relationships between populations across ranges of species (particularly P. subis) and in clarifying breeding and nonbreeding distributions within the genus.Several species and populations are poorly known, most notably populations of P. subis and P. sinaloae in western Mexico.Ecological analyses such as those developed here are useful for helping identify ecological specialization between closely related taxa, even when those taxa occupy portions of the broad ancestral niche, and can guide future efforts to plan sampling for phylogeographic analyses.Ecological niche analyses can also focus more on the temporality of ecological niches, specifically considering nest sites, wintering sites, and the seasonal occupancy of each species or population, to understand environmental conditions necessary for species survival and to understand how niches are or are not conserved through annual cycles (Nakazawa et al., 2004;Peterson et al., 2005).Ecological niche divergence is not a driver of or an inevitable consequence of 12 Tables 460   Table 1A.

English Name
Scientific Name Distribution

Cuban Martin
Table 1.Part A: a list of all Progne species with subspecific breakdown for all taxa except subis.
Part B: distributions of study populations within P. subis used in this paper and their respective subspecific groups.
Table 2.  Table 3. Percent of individuals correctly clustered to original subspecies within P. subis using environmental data in a discriminant function analysis, with original (correct) taxon assignments being shown as rows and predicted taxon cluster being shown as columns, with accurate groupings falling along the bolded diagonal.Values shown are rounded percents and thus may not exactly add to 100, and taxa are listed alphabetically.

Progne taxa chalybea cryptoleuca dominicensis elegans modesta murphyi sinaloae subis tapera chalybea
Percent of individuals correctly clustered to original species using environmental data in a discriminant function analysis, with 471 original (correct) taxon assignments being shown as rows and predicted taxon cluster being shown as columns, with accurate groupings 472 falling along the bolded diagonal.Values shown are rounded percents and thus may not exactly add to 100, and taxa are listed Figure 1.A non-exhaustive demonstration of different ways geographic range and ecological

Figure 2 .
Figure 2.An example test of niche equivalency.Density histograms show the distribution of

Figure 3 .
Figure 3. Ecological niche reconstructions for aridity tolerance for the genus Progne, with