Global diversification rates of passerine birds

The distribution of species richness in families of passerine birds suggests that the net rate of diversification was significantly higher than average in as many as 7 out of 47 families. However, the absence of excess species richness among the 106 tribes within these families indicates that these high rates were transient, perhaps associated in some cases with tectonic movements or dispersal events that extended geographical ranges. Thus, large clade size among passerine birds need not represent intrinsic key innovations that influence the rate of diversification. Approximately 17 families and 30 tribes have too few species relative to other passerine taxa. Many of these are ecologically or geographically marginal, being especially overrepresented in the Australasian region. Observed intervals between lineage splitting suggest that extinction has occurred ca. 90% as frequently as speciation (waiting times of 1.03 and 0.93 Myr) and that the 47 modern families comprising 5712 species descended from approximately 430 passerine lineages extant 24 Myr ago. Speciation and extinction rates among small, marginal families might be 1-2 orders of magnitude lower.     

Distinct evolutionary patterns of morphometric sperm traits in passerine birds

The striking diversity of sperm shape across the animal kingdom is still poorly understood. Postcopulatory sexual selection is an important factor driving the evolution of sperm size and shape. Interestingly, morphometric sperm traits, such as the length of the head, midpiece and flagellum, exhibit a strong positive phenotypic correlation across species. Here we used recently developed comparative methods to investigate how such phenotypic correlations between morphometric sperm traits may evolve. We compare allometric relationships and evolutionary trajectories of three morphometric sperm traits (length of head, midpiece and flagellum) in passerine birds. We show that these traits exhibit strong phenotypic correlations but that allometry varies across families. In addition, the evolutionary trajectories of the midpiece and flagellum are similar while the trajectory for head length differs. We discuss our findings in the light of three scenarios accounting for correlated trait evolution: (i) genetic correlation; (ii) concerted response to selection acting simultaneously on different traits; and (iii) phenotypic correlation between traits driven by mechanistic constraints owing to selection on sperm performance. Our results suggest that concerted response to selection is the most likely explanation for the phenotypic correlation between morphometric sperm traits.     

Polyterritorialitytion in passerine birds

In some birds, males defend two spatially separated territories and attract females to each one in turn. it has been proposed that this behaviour, referred to as polyterritoriality, allows males to conceal their marital status, thereby deceiving females into accepting polygyny against their best interests. The deception hypothesis has gained general acceptance as an explanation for polyterritoriality in birds, but until recently the empirical basis for the hypothesis rested almost entirely on studies of Swedish populations of the pied flycatcher (Ficedula hypoleuca). New findings, emerging from further studies of the pied flycatcher and of other species, suggest several alternative selection pressures that may have favoured polyterritoriality.     

The effect of corticosterone on standard metabolic rates of small passerine birds

Summary Resting metabolic rates of Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) and pine siskins (Carduelis pinus) were evaluated at thermoneutral temperatures before and after administration of corticosterone (B) at physiological doses. There was no effect of B on basal metabolic rate of either species, but nocturnal metabolic rate varied significantly less over the 3-h period of measurement in B-treated sparrows and siskins than in control birds. These results, coupled with observations of caged birds, suggest that corticosterone has no direct effect on avian resting metabolism but does reduce the responsiveness of birds to external stimuli and thus promotes nocturnal restfulness.Abbreviations B corticosterone - VO ₂ rate of oxygen consumption - T ₃ tri-iodothyromine - T ₄ tetra-iodothyronine, thyroxine     

Body frontal area in passerine birds

Projected body frontal area is used when estimating the parasite drag of bird flight. We investigated the relationship between projected frontal area and body mass among passerine birds, and compared it with an equation based on waterfowl and raptors, which is used as default procedure in a widespread software package for flight performance calculations. The allometric equation based on waterfowl/raptors underestimates the frontal area compared to the passerine equation presented here. Consequently, revising the actual frontal areas of small birds will concomitantly change the values of the parasite drag coefficient. We suggest that the new equation     (m²) where m _B is body mass (kg) should be used when a value of frontal area is needed for passerines.     

Postexercise ketosis in night-migrating passerine birds

This study investigated the postexercise metabolism of six species of free-living, night-migrating passerine birds (European robin, pied flycatcher, wheatear, redstart, blackcap, and garden warbler). The birds were caught during autumn migration out of their nocturnal flight, and their metabolism changed from a fasting, highly active state to a fasting, resting state. Concentrations of six plasma metabolites of the fat, carbohydrate, and protein metabolism were measured during up to 10 h of recovery time. The metabolic changes indicated a biphasic pattern: (a) a quick first response to the reduced energy demands during the first 20 min of recovery, suggested by an increase and subsequent decrease of free fatty acid levels, and (b) subsequently, a postexercise ketosis and a reduction of lipolysis and proteolysis, suggested by high beta-hydroxy-butyrate and low free fatty acid, glycerol, triglyceride, and uric acid levels. This metabolic pattern differs from that of humans and rats, in which ketosis starts immediately postexercise or is absent in trained subjects. Since migrating birds are naturally adapted to endurance exercise, it is hypothesized that the high and long-lasting postexercise ketosis does not evoke physiological problems (such as hypoglycemia) but, by contrast, increases the ability of birds to rely on lipids, to a very high extent, during and after flight and decreases the dependence on glucose and glucogenic amino acids. Differences between species in fat stores and metabolic pattern support this hypothesis.     

Moult-related reduction of aerobic scope in passerine birds

It is well established that the nutrient and energy requirements of birds increase substantially during moult, but it is not known if these increased demands affect their aerobic capacity. We quantified the absolute aerobic scope of house and Spanish sparrows, Passer domesticus and P. hispaniolensis, respectively, before and during sequential stages of their moult period. The absolute aerobic scope (AAS) is the difference between maximum metabolic rate (MMR) during peak locomotor activities and minimum resting metabolic rate (RMR_min), thus representing the amount of aerobic power above that committed to maintenance needs available for other activities. As expected, RMR_min increased over the moult period by up to 40 and 63% in house and Spanish sparrows, respectively. Surprisingly, the maximum metabolic rates also decreased during moult in both species, declining as much as 25 and 38% compared with pre-moult values of house and Spanish sparrows, respectively. The concurrent changes in RMR_min and MMR during moult resulted in significant decreases in AAS, being up to 32 and 47% lower than pre-moult levels of house and Spanish sparrows, respectively, during moult stages having substantial feather replacement. We argue that the combination of reduced flight efficiency due to loss of wing feathers and reduced aerobic capacity places moulting birds at greater risk of predation. Such performance constraints likely contribute to most birds temporally separating moult from annual events requiring peak physiological capacity such as breeding and migration.

     

Cross-species amplification of microsatellite primers in passerine birds

Developing species specific microsatellite primers can be avoidedby using existing markers which amplify across species. However,for passerines, such cross-species markers are mostly lackingand few guidelines exist for selecting them from the wide rangeof existing markers. Here cross-species amplification tests of 40microsatellite primers in 13 passerine species show an increasein probability of amplification and polymorphism with decreasingphylogenetic distance. Primers which successfully amplified inmany species had a higher chance to be polymorphic. However,since the amplification success, across a broad range of species,of particular primersets remains difficult to predict it iscrucial to identify such markers empirically. Here we describesuch widely applicable bird (passerines) microsatellite markers.     

The anatomy of the syrinx in passerine birds

The macroscopical structure of the organ of voice in songbirds has long been known, but detailed information on the microscopical anatomy of the syrinx has generally been lacking. Observations based largely on macroscopical evidence have led to a number of erroneous interpretations of function of various syringeal components, and lacking microscopical information, the vocal mechanism of birds cannot be adequately understood.
A wide variety of passeriform bird syrinxes have been studied by means of serial sections. Although there is much less variation in syringeal anatomy amongst songbirds than there is in the other orders of birds, and although all songbird syrinxes conform to the same basic pattern, there is nevertheless marked variation in various syringeal components between different passerine groups. Variations in syringeal structure within families Corvidae ( Corvus corone, C. frugilegus ), Sturnidae ( Sturnus vulgaris, Gracula religiosa ), Turdidae ( Turdus merula, Erithacus rubecula ), Hirundinidae ( Delichon urbica ), Ploceidae ( Passer domesticus ) and Paridae ( Parus major, Aegithalos caudatus ) are described and discussed. The significance of these findings in relation to bird sound production is discussed.     

Genome size and wing parameters in passerine birds

Despite their status as the most speciose group of terrestrial vertebrates, birds exhibit the smallest and least variable genome sizes among tetrapods. It has been suggested that this is because powered flight imposes metabolic constraints on cell size, and thus on genome size. This notion has been supported by analyses of genome size and cell size versus resting metabolic rate and other parameters across birds, but most previous studies suffer from one or more limitations that have left the question open. The present study provides new insights into this issue through an examination of newly measured genome sizes, nucleus and cell sizes, body masses and wing parameters for 74 species of birds in the order Passeriformes. A positive relationship was found between genome size and nucleus/cell size, as well as between genome size and wing loading index, which is interpreted as an indicator of adaptations for efficient flight. This represents the single largest dataset presented for birds to date, and is the first to analyse a distinctly flight-related parameter along with genome size using phylogenetic comparative analyses. The results lend additional support to the hypothesis that the small genomes of birds are indeed related in some manner to flight, though the mechanistic and historical bases for this association remain an interesting area of investigation.     

Sperm morphology and sperm velocity in passerine birds

Sperm velocity is one of the main determinants of the outcome of sperm competition. Since sperm vary considerably in their morphology between and within species, it seems likely that sperm morphology is associated with sperm velocity. Theory predicts that sperm velocity may be increased by enlarged midpiece (energetic component) or flagellum length (kinetic component), or by particular ratios between sperm components, such as between flagellum length and head size. However, such associations have rarely been found in empirical studies. In a comparative framework in passerine birds, we tested these theoretical predictions both across a wide range of species and within a single family, the New World blackbirds (Icteridae). In both study groups, sperm velocity was influenced by sperm morphology in the predicted direction. Consistent with theoretical models, these results show that selection on sperm morphology and velocity are likely to be concomitant evolutionary forces.     

Molecular characterization of malarial parasites in captive passerine birds

Seven of 28 passerine birds that died in captivity were positive for malarial parasites by polymerase chain reaction targeting the mitochondrial cytochrome b (cytB) and apicoplast ribosomal RNA (rRNA) genes. Each bird was infected with a single parasite lineage having a unique genotype. Apicoplast rRNA sequences were present both in Haemoproteus spp. and Plasmodium spp. and had typically high adenosine + thymidine content. Phylogenies for cytB and apicoplast rRNA sequences were largely congruent and supported previous studies that suggest that Plasmodium-Haemoproteus spp. underwent synchronous speciation with their avian hosts, interrupted by sporadic episodes of host switching. Apicoplast phylogeny further indicated that Haemoproteus spp. are ancestral to Plasmodium spp. All the 7 infected passerine birds had histologic lesions of malaria, and malarial parasites may have contributed to the death of at least 4 animals. These findings provide new genetic data on passerine hematozoa, including initial sequences of apicoplast DNA, and emphasize the relevance of parasite prevalence, evolutionary relationships, and host switching to modern management and husbandry practices of captive birds.     

Evolution of sperm structure and energetics in passerine birds

Spermatozoa exhibit considerable interspecific variability in size and shape. Our understanding of the adaptive significance of this diversity, however, remains limited. Determining how variation in sperm structure translates into variation in sperm performance will contribute to our understanding of the evolutionary diversification of sperm form. Here, using data from passerine birds, we test the hypothesis that longer sperm swim faster because they have more available energy. We found that sperm with longer midpieces have higher levels of intracellular adenosine triphosphate (ATP), but that greater energy reserves do not translate into faster-swimming sperm. Additionally, we found that interspecific variation in sperm ATP concentration is not associated with the level of sperm competition faced by males. Finally, using Bayesian methods, we compared the evolutionary trajectories of sperm morphology and ATP content, and show that both traits have undergone directional evolutionary change. However, in contrast to recent suggestions in other taxa, we show that changes in ATP are unlikely to have preceded changes in morphology in passerine sperm. These results suggest that variable selective pressures are likely to have driven the evolution of sperm traits in different taxa, and highlight fundamental biological differences between taxa with internal and external fertilization, as well as those with and without sperm storage.     

An analysis of clutch-size in New World passerine birds

Variation in clutch-size among New World passerine birds was analysed with respect to four variables: body-mass, geographic latitude, the frequency of nest predation, and the structure of the nest. Data were analysed separately by averaging traits at three taxonomic levels: species, genus, and subfamily. Allometric scaling with body-mass did not account for significant variation in clutch-size regardless of the taxonomic level of analysis. Latitudinal effects on clutch-size were highly significant at all taxonomic levels. For species building small-pensile nests and open-cup nests, nest predation had a significant partial effect on clutch-size, with latitude held constant. When nest predation and latitude were held constant, clutch-size was significantly different among species building small-pensile nests, open-cup nests, and domed nests. These results suggest that New World passerine clutch-size is related to at least three variables: latitudinal effects, nest predation, and nest structure.     

Social mating systems and extrapair fertilizations in passerine birds

Two alternative hypotheses have been proposed to explain howsocial and genetic mating systems are interrelated in birds.According to the first (male trade-off) hypothesis, socialpolygyny should increase extrapair fertilizations because whenmales concentrate on attracting additional social mates, theycannot effectively protect females with whom they have already paired from being sexually assaulted. According to the second(female choice) hypothesis, social polygyny should decreaseextrapair fertilizations because a substantial proportion offemales can pair with the male of their choice, and males caneffectively guard each mate during her fertile period. To discriminatethese alternatives, we comprehensively reviewed informationon social mating systems and extrapair fertilizations in temperatezone passerine birds. We found significant inverse relationshipsbetween proportions of socially polygynous males and frequenciesof extrapair young, whether each species was considered asan independent data point (using parametric statistics) orphylogenetically related species were treated as nonindependent (using contrasts analyses). When social mating systems weredichotomized, extrapair chicks were twice as frequent in monogamousas in polygynous species (0.23 vs. 0.11). We hypothesize thatin socially polygynous species, (1) there is less incentivefor females and males to pursue extrapair matings and (2) femalesincur higher costs for sexual infidelity (e.g., due to physical retaliation or reduction of paternal efforts) than in sociallymonogamous species.     

Global warming and body mass decline in Israeli passerine birds

Global warming may affect the physiology, distributions, phenology and adaptations of plants and animals. In Israel, minimum summer temperatures increased by an average of 0.26 degrees C per decade during the second half of the 20th century. Bergmann's rule predicts that, in warm-blooded animals, races from warm regions are smaller than races from cold regions. Numerous studies have reported general correlations between body mass in fossil animals and independently established palaeoclimatic changes from various parts of the world in accordance with this rule. Using museum specimens, I tested the prediction that the body mass and tarsus length of five resident passerine species in Israel declined between 1950 and 1999. The body mass of four species (the graceful warbler Prinia gracilis, the house sparrow Passer domesticus, the yellow-vented bulbul Pycnonotus xanthopygos and the Sardinian warbler Sylvia melanocephala, but not of the crested lark Galerida cristata) declined significantly during this period. Tarsus length also declined significantly during this period for two species (the graceful warbler and the house sparrow). Body condition (body mass-to-tarsus length ratio) decreased in the Sardinian warbler, the yellow-vented bulbul and the crested lark. It is suggested that the above declines in body mass and tarsus length are due to global warming and also in accordance with Bergmann's rule. The above explanation does not exclude the possibility that other factors, such as a decrease in food availability, contributed to the decline in body mass. These declines may have serious implications for community structure and competition among bird species and may affect the survival of small passerines.     

Experiments on clutch size and nest size in passerine birds

Summary Results of experiments on three passerine species suggest that brood size may be constrained by nest size, since the breeding success of pairs provided with large nestcups was greater than that of those provided with small artificial nestcups. These results may have important implications, e.g. to the design of experiments involving manipulation of clutch and brood size. A small nestcup is requisite for successful hatching during the incubation period, but a large one for successful rearing during the nestling period. In nature this difference may select for types of nesting materials that are elastic, such as mosses and lichens. However, experiments showed that such materials rapidly absorb rainwater but only slowly dry out. In addition, because large nests dry out more slowly than small nests, selection will favour small nests among those open-nesting species that have exposed nests. A further possible nest size constraint on open-nesters is nest predation. However, no difference in the predation rate was found in experiments with small and large artificial nests.     

Sexually size dimorphic brains and song complexity in passerine birds

Neural correlates of bird song involve the volume of particularsong nuclei in the brain that govern song development, production,and perception. Intra- and interspecific variation in the volumeof these song nuclei are associated with overall brain size,suggesting that the integration of complex songs into the brainrequires general neural augmentation. In a comparative studyof passerine birds based on generalized least square models,we tested this hypothesis by exploring the interspecific relationshipbetween overall brain size and repertoire size. We found nosignificant association between song complexity of males andbrain size adjusted for body size. However, species in whichmales produced complex songs tended to have sex differencesin overall brain size. This pattern became stronger when wecontrolled statistically for female song complexity by usingsex differences in song complexity. In species with large differencesin song complexity, females evolved smaller brains than didmales. Our results suggest no role for the evolution of extendedneural space, as reflected by total brain size, owing to songcomplexity. However, factors associated with sexual selectionmirrored by sex differences in song complexity were relatedto sexual dimorphism in overall brain size.