Flight costs of long,sexually selected tails in hummingbirds

The elongated tails adorning many male birds have traditionally been thought to degrade flight performance by increasing body drag. However, aerodynamic interactions between the body and tail can be substantial in some contexts, and a short tail may actually reduce rather than increase overall drag. To test how tail length affects flight performance, we manipulated the tails of Anna''s hummingbirds (Calypte anna) by increasing their length with the greatly elongated tail streamers of the red-billed streamertail (Trochilus polytmus) and reducing their length by removing first the rectrices and then the entire tail (i.e. all rectrices and tail covert feathers). Flight performance was measured in a wind tunnel by measuring (i) the maximum forward speed at which the birds could fly and (ii) the metabolic cost of flight while flying at airspeeds from 0 to 14 m s⁻¹. We found a significant interaction effect between tail treatment and airspeed: an elongated tail increased the metabolic cost of flight by up to 11 per cent, and this effect was strongest at higher flight speeds. Maximum flight speed was concomitantly reduced by 3.4 per cent. Also, removing the entire tail decreased maximum flight speed by 2 per cent, suggesting beneficial aerodynamic effects for tails of normal length. The effects of elongation are thus subtle and airspeed-specific, suggesting that diversity in avian tail morphology is associated with only modest flight costs.     

Hummingbirds pay a high cost for a warm drink

Endotherms must warm ingested food to body temperature. Food warming costs may be especially high for nectar-feeding birds, which can ingest prodigious volumes. We formulated a mathematical model to predict the cost of warming nectar as a function of nectar temperature and sugar concentration. This model predicts that the cost of warming nectar should: (1) decrease as a power function of nectar concentration, and (2) increase linearly with the difference between body temperature and nectar temperature. We tested our model on rufous hummingbirds (Selasphorus rufus). A typical experiment consisted of feeding birds nectar of a given concentration at 39°C (equivalent to body temperature) and then at 4°C, and vice versa. We used the percentage change in metabolic rate between the two food temperatures to estimate the cost of warming nectar. The model's predictions were accurately met. When birds had to hover rather than perch during feeding bouts, estimated food-warming costs were only slightly lower. The cost of warming nectar to body temperature appears to be an important yet overlooked aspect of the energy budgets of nectar-feeding birds. Hummingbirds feeding on 5% sucrose solutions at 4^oC have to increase their metabolic rate by an amount equivalent to that elicited by a 15°C drop in ambient temperature.Abbreviations AE assimilation efficiency - C nectar concentration - H' cost of warming food to body temperature - SDA specific dynamic action - T_a ambient temperature - T_b body temperature - T_n nectar temperatureCommunicated by: G. Heldmaier     

Pollen transfer by hummingbirds and bumblebees, and the divergence of pollination modes in Penstemon

We compared pollen removal and deposition by hummingbirds and bumblebees visiting bird-syndrome Penstemon barbatus and bee-syndrome P. strictus flowers. One model for evolutionary shifts from bee pollination to bird pollination has assumed that, mostly due to grooming, pollen on bee bodies quickly becomes unavailable for transfer to stigmas, whereas pollen on hummingbirds has greater carryover. Comparing bumblebees and hummingbirds seeking nectar in P. strictus, we confirmed that bees had a steeper pollen carryover curve than birds but, surprisingly, bees and birds removed similar amounts of pollen and had similar per-visit pollen transfer efficiencies. Comparing P. barbatus and P. strictus visited by hummingbirds, the bird-syndrome flowers had more pollen removed, more pollen deposited, and a higher transfer efficiency than the bee-syndrome flowers. In addition, P. barbatus flowers have evolved such that their anthers and stigmas would not easily come into contact with bumblebees if they were to forage on them. We discuss the role that differences in pollination efficiency between bees and hummingbirds may have played in the repeated evolution of hummingbird pollination in Penstemon.     

Mimicry on the QT(L): genetics of speciation in Mimulus

Ecological studies suggest that hummingbird-pollinated plants in North America mimic each other to increase visitation by birds. Published quantitative trait locus (QTL) data for two Mimulus species indicate that floral traits associated with hummingbird versus bee pollination results from a few loci with major effects on morphology, as predicted by classical models for the evolution of mimicry. Thus, the architecture of genetic divergence associated with speciation may depend on the ecological context.     

Hummingbird blood traits track oxygen availability across space and time

Predictable trait variation across environments suggests shared adaptive responses via repeated genetic evolution, phenotypic plasticity or both. Matching of trait–environment associations at phylogenetic and individual scales implies consistency between these processes. Alternatively, mismatch implies that evolutionary divergence has changed the rules of trait–environment covariation. Here we tested whether species adaptation alters elevational variation in blood traits. We measured blood for 1217 Andean hummingbirds of 77 species across a 4600-m elevational gradient. Unexpectedly, elevational variation in haemoglobin concentration ([Hb]) was scale independent, suggesting that physics of gas exchange, rather than species differences, determines responses to changing oxygen pressure. However, mechanisms of [Hb] adjustment did show signals of species adaptation: Species at either low or high elevations adjusted cell size, whereas species at mid-elevations adjusted cell number. This elevational variation in red blood cell number versus size suggests that genetic adaptation to high altitude has changed how these traits respond to shifts in oxygen availability.     

Genetic structuring and secondary contact in the white‐chested Amazilia hummingbird species complex

Pleistocene climate cycles have been recognized to be a major driver of postglacial northward range expansion of North American bird populations. During glacial maxima, allopatric lineages that were reproductively isolated might have come into secondary contact with one another during expansion periods and the genetic signatures of past hybridization as a result of secondary contact events should produce detectable hybrid zones. The white‐chested hummingbirds Amazilia violiceps and A. viridifrons, constitute a species complex showing phenotypic similarity across its range. One exception is the subspecies found in the Central Depression of Chiapas (A. viridifrons villadai), which shares some plumage traits with the endemic but allopatric green‐fronted populations in Oaxaca. Phylogenetic relationships, taxonomy and species limits among violiceps, viridifrons and villadai have been controversial for decades. We assessed genetic structure of populations and introgression in this species complex by analysing 95 individuals at ten nuclear microsatellites and morphology. Bayesian analysis yielded four clusters. However, only two clusters generally match previously described mtDNA haplogroups, one parental taxon in the south (villadai) and a cluster with two admixed taxa (viridifrons and violiceps) that cannot be attributed to any pure parental population. High genetic admixture was recorded in the violiceps/viridifrons range, probably as a consequence of a postglacial northern expansion of violiceps. Signs of admixture and gene flow between violiceps/viridifrons and villadai were low. Historical and contemporary migration rates and Approximate Bayesian computations support a scenario of divergence with gene flow: a Pleistocene basal split separating A. violiceps and the other two clades are derived from a second split (villadai and viridifrons) or from a merger of violiceps and villadai into viridifrons due to gene flow.     

Pedilanthus diazlunanus (Euphorbiaceae): pollination by hymenopterans in a bird-pollinated genus

Pollination in the genus Pedilanthus is commonly effected by hummingbirds. Pollination by vespid wasps in Pedilanthus is documented for the first time based on observations of Pachodynerus nasidens and Eumenes americanus pollinating Pedilanthus diazlunanus. An hypothesis concerning the mechanism by which hummingbird pollination was replaced by insect pollination in Pedilanthus is advanced based on observations of insects on P. bracteatus.     

Reproduction in an Orchid Can Be Resource‐Limited over its Lifetime1

Most orchids studied thus far show long‐term resource adjustments to increases in fruit production within a flowering season, but none of these offers rewards to their potential pollinators. If nectar production is energetically expensive, then resources utilized to produce fruits and seeds may be even more limited in pollinator‐rewarding orchids than in non‐rewarding ones. Thus, resource adjustments may be more dramatic or entirely different in nectar producing plants. In this study, we performed artificial hand‐pollinations for two consecutive flowering seasons in die nectar producing orchid Comparettia falcata, and tested whether or not fruit set, seed set, and seed viability were limited by the quantity of pollinations or by resources. In addition, we compared mechanisms of short‐term (fruit abortion within seasons) and long‐term consequences (percent change in leaf length and change in flower number per plant between seasons, probability of shoot and inflorescence production, and mortality) between hand‐ pollinated and unmanipulated plants. The relationships among plant traits related to vegetative size and reproduction also were examined. Hand‐pollinations showed some negative effects. Fruit set was higher in hand‐pollinated plants in the first season but was similar to the controls in the second. Seed set was significantly lower and abortions were higher than in unmanipulated plants. On the other hand, some of our measurements were unaffected by die hand‐pollination treatment. Unexpectedly, there were no significant differences between groups in percent change in leaf length, change in flower number per plant between seasons, or die probability of shoot and inflorescence production. Although there was a strong correlation between leaf size and die number of flowers produced within a season, associations between leaf size and traits related to current or future reproduction were not consistent. Like other epiphytic orchids, pollination limitation occurred within a single season in C. falcata., but increases in fruit production also resulted in reduced lifetime fitness as estimated by a compounded fitness index. Contrary to all other epiphytic orchids studied, long‐term adjustments to increased fruit production in C. falcata through reduction in future growth or flower and inflorescence production were either minor or lacking. Our results suggest that the nature of plant strategies associated with resource constraints during sexual reproduction may be dependent on whether or not plants have evolved traits that are costly.     

Phylogenetic structure of specialization: A new approach that integrates partner availability and phylogenetic diversity to quantify biotic specialization in ecological networks

Biotic specialization holds information about the assembly, evolution, and stability of biological communities. Partner availabilities can play an important role in enabling species interactions, where uneven partner availabilities can bias estimates of biotic specialization when using phylogenetic diversity indices. It is therefore important to account for partner availability when characterizing biotic specialization using phylogenies. We developed an index, phylogenetic structure of specialization (PSS), that avoids bias from uneven partner availabilities by uncoupling the null models for interaction frequency and phylogenetic distance. We incorporate the deviation between observed and random interaction frequencies as weights into the calculation of partner phylogenetic α‐diversity. To calculate the PSS index, we then compare observed partner phylogenetic α‐diversity to a null distribution generated by randomizing phylogenetic distances among the same number of partners. PSS quantifies the phylogenetic structure (i.e., clustered, overdispersed, or random) of the partners of a focal species. We show with simulations that the PSS index is not correlated with network properties, which allows comparisons across multiple systems. We also implemented PSS on empirical networks of host–parasite, avian seed‐dispersal, lichenized fungi–cyanobacteria, and hummingbird pollination interactions. Across these systems, a large proportion of taxa interact with phylogenetically random partners according to PSS, sometimes to a larger extent than detected with an existing method that does not account for partner availability. We also found that many taxa interact with phylogenetically clustered partners, while taxa with overdispersed partners were rare. We argue that species with phylogenetically overdispersed partners have often been misinterpreted as generalists when they should be considered specialists. Our results highlight the important role of randomness in shaping interaction networks, even in highly intimate symbioses, and provide a much‐needed quantitative framework to assess the role that evolutionary history and symbiotic specialization play in shaping patterns of biodiversity. PSS is available as an R package at https://github.com/cjpardodelahoz/pss.