How many birds are there?

Attempts to assess the magnitude of global biodiversity have focused on estimating species richness. However, this is but one component of biodiversity, and others, such as numbers of individuals or biomass, are at least as poorly known and just as important to quantify. Here, we use a variety of methods to estimate the global number of individuals for a single taxon, birds. The different methods yield surprisingly consistent estimates of a global bird population of between 200 billion and 400 billion individuals (1 billion=109). We discuss some of the implications of this figure.     

Why are female birds ornamented?

Sexual selection is now widely accepted as the main evolutionary explanation of extravagant male ornaments. By contrast, ornaments occurring in females have received little attention and often have been considered as nonadaptive, correlated effects of selection on males. However, recent comparative evidence suggests that female ornaments have evolved quite independently of male showiness. Also, new theoretical models predict that both male mate choice and female contest competition will occur under certain circumstances. This is supported by recent experimental studies. Thus, selection acting on females might be a widespread cause of female ornaments.     

Why don't birds lay more eggs?

Fifty years ago David Lack put forward a key hypothesis in life-history theory: that avian clutch is ultimately determined by the number of young that parents can provide with food. Since then, a plethora of brood manipulations has shown that birds can rear more young than the number of eggs they lay, and prompted a search for negative effects of increased effort on future reproduction. However, recent studies have shown that the demands of laying and incubating eggs generally omitted from experiments, could affect parental fitness. Lack's hypothesis, and the tests of its validity, need to be extended to encompass the full demands of producing and rearing the brood.     

Do birds possess brown adipose tissue?

• 1.1. Fluorescence and electron microscopy were used to visualize differences between avian adipose tissue (AAT) collected from clavicular and abdominal regions of the great tit, the willow tit, the house sparrow and the Japanese quail, and interscapular brown adipose tissue (BAT) obtained from the Djungarian dwarf hamster.
• 2.2. Multilocular fat cells were found in AAT. The prerequisite for multilocularity, however, was not simply winter acclimatization [short photophase 4L:20D and low ambient temperature (< −20°C in January in Oulu)] or cold-acclimation (−25°C). Multilocular adipocytes were found during autumn and in unacclimated control birds as well. Mitochondria in the AAT were fewer and about one-sixth the length of those in BAT. This finding was associated with low cytochrome oxidase (COX) activity in the tissue homogenate and isolated mitochondrial fraction of the AAT (< 5.2% of that in BAT).
• 3.3. Catecholamine fluorescence was seen only around arteries in the AAT. Signs of sympathetic parenchymal innervation were found neither in winter- nor in cold-acclimated birds, but typically, sympathetic nerve fibers forming a basket-like network around every cell were seen in the brown fat of the hamster.
• 4.4. Our results show that AAT in the adult birds resembles white adipose tissue more than brown. Multilocularity of adipocytes may improve lipolysis to deliver fatty acids for muscle fuel of shivering or NST.

     

Coevolution of mistletoes and frugivorous birds?*

Small frugivorous birds that feed largely on the fruits of stem-parasitic mistletoes have independently evolved in various parts of the world. Local populations of mistletoes may be dispersed almost exclusively by these birds. Four attributes of mistletoe dispersal systems may have enhanced the evolution of reciprocal dependence between mistletoes and specialized dispersers: (1) Safe sites for mistletoe seeds (i.e. the young branches of a compatible host) are precisely defined in space and time. (2) The viscidity of mistletoe seeds induces smaller dispersers to deposit seeds in safe sites. (3) Frugivores differ markedly in the efficiency with which they disperse mistletoe seeds to safe sites. (4) Relatively large viscid fruits and adaptive fruiting displays exclude or deter most members of the potential disperser guild. Some birds have anatomical adaptations as a result of dietary specialization on mistletoe fruit, and some mistletoes have fruiting displays that target specialized birds or a narrow disperser spectrum. Coevolution between guilds of mistletoes and specialized dispersers is therefore probable. The uncoupled selective pressures that might have driven their coevolution are the mistletoes’ provision of fruit crops that are unavailable to more generalized frugivores, in return for seed dispersal to the small stems most suitable for infection. As in other mutualistic seed dispersal systems, phylogenetic, ecological and life history factors constrain the evolution of monophyletic interdependence, resulting in varying degrees and patterns of reciprocal specificity between mistletoes and dispersers.     

Animal navigation: birds as geometers?

New experiments on a bird species able to remember the sites of thousands of cached seeds have revealed how a site can be specified by combining distance information from several landmarks.     

Mesozoic birds of China—a synoptic review

A synoptic review of the discoveries and studies of Chinese Mesozoic birds is provided in this paper. ⁴⁰Ar/³⁹Ar dating of several bird-bearing deposits in the Jehol Group has established a geochronological framework for the study of the early avian radiation. Chinese Mesozoic birds had lasted for at least 11 Ma during about 131 Ma and 120 Ma (Barremian to Aptian) of the middle and late Early Cretaceous, respectively. In order to further evaluate the change of the avian diversity in the Jehol Biota, six new orders and families are erected based on known genera and species, which brings the total number of orders of Chinese Mesozoic birds to 15 and highlights a remarkable radiation ever since the first appearance of birds in the Late Jurassic. Chinese Early Cretaceous birds had experienced a significant differentiation in morphology, flight, diet and habitat. Further examination of the foot of Jeholornis suggests this bird might not have possessed a fully reversed hallux. However, the attachment of metatarsal I to the medial side of metatarsal II does not preclude trunk climbing, a pre-adaptation for well developed perching life of early birds. Arboreality had proved to be a key adaptation in the origin and early evolution of bird flight, and the adaptation to lakeshore environment had played an equally important role in the origin of ornithurine birds and their near-modern flight skill. Many Chinese Early Cretaceous birds had preserved the direct evidence of their diet, showing that the most primitive birds were probably mainly insectivorous and that specialized herbivorous or carnivorous (e.g., piscivorous) dietary adaptation had appeared only in later advanced forms. The only known Early Cretaceous bird embryo fossil has shown that precocial birds had occurred prior to altricial birds in avian history, and the size of the embryo and other analysis indicate it probably had a short incubation period. Leg feathers probably have a wide range of distribution in early birds, further suggesting that leg feathers had played a key role in the beginning stage of the flight of birds. Finally, the Early Cretaceous avian radiation can be better understood against the background of their unique ecosystem. The advantage of birds in the competitions with other vertebrate groups such as pterosaurs had probably not only resulted in the rapid differentiation and radiation of birds but also the worldwide spreading of pterosaurs and other vertebrates from East Asia in the Early Cretaceous. Selected from Vertebrata PalAsiatica 2006, 44 (1): 74–98     

Pheromones in birds: myth or reality?

Birds are anosmic or at best microsmatic… This misbelief persisted until very recently and has strongly influenced the outcome of communication studies in birds, with olfaction remaining neglected as compared to acoustic and visual channels. However, there is now clear empirical evidence showing that olfaction is perfectly functional in birds and birds use olfactory information in a variety of ethological contexts. Although the existence of pheromones has never been formally demonstrated in this vertebrate class, different groups of birds, such as petrels, auklets and ducks have been shown to produce specific scents that could play a significant role in within-species social interactions. Behavioral experiments have indeed demonstrated that these odors influence the behavior of conspecifics. Additionally, in quail, deprivation of olfactory inputs decreases neuronal activation induced by sexual interactions with a female. It seems therefore well established that birds enjoy a functional sense of smell and a fast growing body of experimental evidence suggests that they use this channel of olfactory communication to control their social life. The unequivocal identification of an avian pheromone is, however, still ahead of us but there are now many exciting opportunities to unravel the behavioral and physiological particularities of chemical communication in birds.     

Are orientation mechanisms among migratory birds speciesospecific?

The mechanisms by which migratory birds find their way from breeding grounds to winter quarters and back have been the subject of intensive research during the past four decades. Birds are equipped with genetic information about the migratory direction, and they can use the earth's magnetic field, star patterns and the sun and/or skylight polarization patterns as compass references. Studies on a number of North American and European species have suggested possible species-specific differences in the relative role of the compass mechanisms. This may be largely the result of divergent experimental designs, which make results difficult to compare. Comparative studies with identical methods are needed to see how much species-specific variation exists in basic orientation mechanisms.     

Can experienced birds select for Mullerian mimicry?

Field experiments have shown that avian predators in the wildcan select for similarity of warning signals in aposematic prey(Müllerian mimicry) because a common signal is better protectedthan a signal that is novel and rare. The original theory ofMüllerian mimicry assumes that the mechanism promotingmimicry is predator learning; by sharing a signal, the comimicspecies share the mortality that is due to sampling by inexperiencedpredators. Predation events have not been observed in the wild,and learning experiments with naive bird predators in a laboratoryhave not unambiguously shown a benefit of a uniform signal comparedwith different signals. As predators in the field experimentsare likely to be more experienced compared with previous laboratoryexperiments, we studied selection by experienced predators ona novel imperfect mimic. We trained great tits Parus major toavoid artificial aposematic models and subsequently introducedperfect and imperfect mimics at different frequencies. Birdswith prior experience on the models selected against the imperfectmimics that were at a disadvantage also in a memory test conducteda week after their introduction. Selection against the imperfectmimics was antiapostatic. However, the imperfect mimics alsobenefited from some signal generalization to the models andpossibly gained protection because the birds were familiar withthe alternative cryptic prey that was also present. Our resultssuggest that experienced predators might be more important tothe evolution of mimicry than the learning-based theory assumes.     

Can birds count eggs in their nests?

Sensory mechanisms controlling avian clutch size have diversified into distinct types, according to the nature of the input that is used to disrupt the growth of ovarian follicles and hence halt egg‐laying. In an article on brood parasitism, Lyon (2003) claimed that female American Coots Fulica americana can reduce their clutch size on the basis of visual cues in response to eggs laid in their nests by other females; in this species, therefore, egg counting would be used to control clutch size. After a close examination of the physiological determination of clutch size in American Coots, I show that seven of 17 parasitized clutches were smaller than the range controlled through the mechanism using an input to disrupt follicular growth (7–10 eggs per clutch). My reanalysis suggests that American Coots are incapable of adjusting clutch size via counting and re‐asserts that a species that can count eggs has yet to be found among birds that rely upon their own body heat for incubation.     

Are predatory birds effective secondary seed dispersers?

We have studied the unusual phenomenon of secondary seed dispersal of Lycium intricatum seeds on a small oceanic Atlantic island (Alegranza, Canarian Archipelago) in which a small frugivorous lizard ( Gallotia atlantica ) and two different predatory birds participate, a shrike ( Lanius excubitor ) and a kestrel ( Falco tinnunculus ). Endemic lizards that are common prey of both bird species consume Lycium fruits. Lizard remains were significantly matched with the presence of Lycium fruits in the regurgitation pellets of the two predatory birds. Seeds were found in 7.3% of the lizard droppings, 31.0% of kestrel pellets and 55.7% of shrike regurgitations. The mean number of seeds per dropping or pellet was 4.8 ±4 in lizard, 20.2 ±34.5 in shrike and 6.7 ±8.1 in kestrel. The percentage of viable seeds showed significant differences among all four treatments, decreasing in the following direction: seeds collected directly from plants (98.0%), shrikes (88.0%), lizards (72.3%), and kestrels (31.7%). Seeds from Lycium fresh fruits and shrike pellets showed significantly higher germination rates than those from lizard droppings and kestrel pellets. While lizards and shrikes are effective seed dispersers, kestrel gut treatment decreases seed viability. Seed viability is always higher than seed germination in each of the four treatments. In this island environment, Lycium seeds are under an important random influence during the seed dispersal process. Secondary seed dispersal seems to acquire a relevant dimension in small and remote insular environments or isolated continental systems where interactions among the different elements involved are intense, all of them are abundant native residents, and they have been coexisting for a long time. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 345–352.     

Does organic farming benefit farmland birds in winter?

The generally higher biodiversity on organic farms may be influenced by management features such as no synthetic pesticide and fertilizer inputs and/or by differences in uncropped habitat at the site and landscape scale. We analysed bird and habitat data collected on 48 paired organic and conventional farms over two winters to determine the extent to which broad-scale habitat differences between systems could explain overall differences in farmland bird abundance. Density was significantly higher on organic farms for six out of 16 species, and none on conventional. Total abundance of all species combined was higher on organic farms in both years. Analyses using an information-theoretic approach suggested that both habitat extent and farm type were important predictors only for starling and greenfinch. Organic farming as currently practised may not provide significant benefits to those bird species that are limited by winter food resources, in particular, several declining granivores.     

Do birds of a feather disperse plants together?

1. Dispersal of propagules by waterbirds is thought to be important for wetland plants because of the abundance of birds and their frequent movements among aquatic habitats. Differences in bird characteristics (size, movement, feeding ecology) were expected to lead to different outcomes for plant dispersal. 2. We investigated heterogeneity in plant dispersal by ducks (Anas superciliosa, Anas gracilis, Anas castanea). We calculated the probability of transport of viable seeds by germinating propagules retrieved from feathers and feet (epizoochory) and the contents of the oesophagus, gizzard and lower gut (endozoochory). 3. The abundance and richness of seeds carried internally and externally did not differ among sympatric bird species. We used estimates from the literature of movements of Anas species to approximate dispersal kernels for the transport of plant propagules. 4. Heterogeneity in the abundance and movement ecology of disperser species will result in differing patterns and degrees of connectivity for wetland plant metacommunities. Sedentary waterfowl are likely to have an important role in replenishing propagules and connecting aquatic metacommunities over small distances. Nomadic waterfowl may facilitate long‐distance dispersal. We discuss the implications of differences between duck species in movement patterns for connectivity of aquatic plant metacommunities across landscapes.     

Malarial parasites as geographical markers in migratory birds?

We tested the hypothesis that malarial parasites (Plasmodium and Haemoproteus) of black-throated blue warblers (Dendroica caerulescens) provide sufficient geographical signal to track population movements between the warbler's breeding and wintering habitats in North America. Our results from 1083 warblers sampled across the species' breeding range indicate that parasite lineages are geographically widespread and do not provide site-specific information. The wide distribution of malarial parasites probably reflects postnatal dispersal of their hosts as well as mixing of breeding populations on the wintering range. When compared to geographically structured parasites of sedentary Caribbean songbirds, patterns of malarial infections in black-throated blue warblers suggest that host-malaria dynamics of migratory and sedentary bird populations may be subject to contrasting selection pressures.