Benjamin G. Freeman

Clutch size declines with elevation in tropical birds

ABSTRACT Clutch size commonly decreases with increasing elevation among temperate-zone and subtropical songbird species. Tropical songbirds typically lay small clutches, thus the ability to evolve even smaller clutch sizes at higher elevations is unclear and untested. We conducted a comparative phylogenetic analysis using data gathered from the literature to test whether clutch size varied with elevation among forest passerines from three tropical biogeographic regions—the Venezuelan Andes and adjacent lowlands, Malaysian Borneo, and New Guinea. We found a significant negative effect of elevation on variation in clutch size among species. We found the same pattern using field data sampled across elevational gradients in Venezuela and Malaysian Borneo. Field data were not available for New Guinea. Both sets of results demonstrate that tropical montane species across disparate biogeographic realms lay smaller clutches than closely related low-elevation species. The environmental sources of selection underlying this pattern remain uncertain and merit further investigation.

Using song playback experiments to measure species recognition between geographically isolated populations: A comparison with acoustic trait analyses

ABSTRACT Geographically isolated populations of birds often differ in song. Because birds often choose mates on the basis of their song, song differentiation between isolated populations constitutes a behavioral barrier to reproduction. If this barrier is judged to be sufficiently strong, then isolated populations with divergent songs may merit classification as distinct species under the biological species concept. We used a dataset of 72 pairs of related but allopatric Neotropical passerines (“taxon pairs”) to compare 2 methods for measuring song divergence between isolated populations: statistical analysis of 7 acoustic traits measured from spectrograms, and field playback experiments that “ask the birds themselves” if they perceive foreign song as conspecific or not. We report 4 main findings: (1) Behavioral discrimination (defined as failure to approach the speaker in response to allopatric song) is nonlinearly related to divergence in acoustic traits; discrimination is variable at low to moderate levels of acoustic divergence, but nearly uniformly high at high levels. (2) The same nonlinear relationship held for both song learners (oscines) and nonlearners (suboscines). (3) Song discrimination is not greater in taxon pairs ranked as species compared to taxon pairs ranked as subspecies. (4) Behavioral responses to allopatric song are symmetric within a taxon pair. We conclude (1) that playback experiments provide a stronger measure of species recognition relevant to premating reproductive isolation than do acoustic trait analyses, at least when divergence in acoustic traits is low to moderate; and (2) that playback experiments are useful for defining species limits and can help address the latitudinal gradient in taxonomy, which arises because species are defined more broadly in the tropics than in the temperate zone. To this end, we suggest that 21 Neotropical taxon pairs that are currently ranked as subspecies, but that show strong behavioral discrimination in response to allopatric song, merit classification as distinct biological species.

Evolution and plasticity: Divergence of song discrimination is faster in birds with innate song than in song learners in Neotropical passerine birds

Plasticity is often thought to accelerate trait evolution and speciation. For example, plasticity in birdsong may partially explain why clades of song learners are more diverse than related clades with innate song. This “song learning” hypothesis predicts that (1) differences in song traits evolve faster in song learners, and (2) behavioral discrimination against allopatric song (a proxy for premating reproductive isolation) evolves faster in song learners. We tested these predictions by analyzing acoustic traits and conducting playback experiments in allopatric Central American sister pairs of song learning oscines (N = 42) and nonlearning suboscines (N = 27). We found that nonlearners evolved mean acoustic differences slightly faster than did leaners, and that the mean evolutionary rate of song discrimination was 4.3 times faster in nonlearners than in learners. These unexpected results may be a consequence of significantly greater variability in song traits in song learners (by 54–79%) that requires song‐learning oscines to evolve greater absolute differences in song before achieving the same level of behavioral song discrimination as nonlearning suboscines. This points to “a downside of learning” for the evolution of species discrimination, and represents an important example of plasticity reducing the rate of evolution and diversification by increasing variability.

Climate, habitat, and geographic range overlap drive plumage evolution

Organismal appearances are shaped by selection from both abiotic and biotic drivers 1–5. For example, Gloger’s rule describes the pervasive pattern that more pigmented populations are found in more humid areas 1,6,7, and substrate matching as a form of camouflage to reduce predation is widespread across the tree of life 8–10. Sexual selection is a potent driver of plumage elaboration 5,11, and species may also converge on nearly identical colours and patterns in sympatry, often to avoid predation by mimicking noxious species 3,4 To date, no study has taken an integrative approach to understand how these factors determine the evolution of colour and pattern across a large clade of organisms. Here we show that both habitat and climate profoundly shape avian plumage. However, we also find a strong signal that many species exhibit remarkable convergence not explained by these factors nor by shared ancestry. Instead, this convergence is associated with geographic overlap between species, suggesting strong, albeit occasional, selection for interspecific mimicry. Consequently, both abiotic and biotic factors, including interspecific interactions, are potent drivers of phenotypic evolution.

Apparent cooperative breeding at a nest of the Silvery-throated Jay (Cyanolyca argentigula) and first nest description

ABSTRACT The Silvery-throated Jay (Cyanolyca argentigula) is a poorly known Neotropical corvid endemic to the Cordillera de Talamanca of Costa Rica and western Panama. In April 2016, we found an active Silvery-throated Jay nest in Chirripó National Park, Costa Rica. Here we provide the first description of the breeding biology of this species, including nest architecture, estimates of incubation and nestling periods, and observations of adults attending the nest. The large cup nest, placement in the crown of a small tree, and breeding phenology are similar to other Cyanolyca species. We observed up to 5 adults associated with this nest, including at least 3 different adults that fed chicks. These observations suggest cooperative breeding at this nest but are also consistent with the possibility of communal breeding. Future studies with color-banded individuals and genetic sampling will be necessary to better understand what seems to be cooperative breeding in the Silvery-throated Jay.

No evidence for a positive correlation between abundance and range size in birds along a New Guinean elevational gradient

ABSTRACT A general pattern in biogeography is that species with high local abundances tend to have large geographic ranges, while species with low local abundances have small ranges. However, many tropical biotas do not show positive abundance–range–size correlations, potentially because eco-climatic stability in the tropics promotes specialisation or because density compensation permits some higher-elevation species to be both abundant and small-ranged. I explored these ideas by studying the abundance–range-size correlation for 37 species of small-bodied understorey New Guinean birds that live along a reef-to-ridgetop elevational gradient. Abundance (capture rates) is not related to range size (elevational breadth) in this dataset. In fact, when conducting phylogenetic regressions, abundance is significantly negatively related to range size. Because species’ abundances do not systematically vary as a function of elevational zone, this pattern is not due to density compensation. Instead, elevational specialisation appears to explain the abundance–range-size correlation, interspecific competition being an important driver of elevational specialisation. If specialised taxa are sometimes able to achieve high local abundances compared to broader-ranged taxa, specialisation may break any consistent association between abundance and range size. Further studies are necessary to test the generality of this explanation.