Avian Biology

Some Prodigious, Hot-Off-the-Press Publications about Birds

Past 'Recent Publications':

October 2009

September 2009

Birds vs. bats -- Birds and bats have evolved powered flight independently, which makes a comparison of evolutionary ‘design’ solutions potentially interesting. Hedenstrom et al. (2009) examined similarities and differences with respect to flight characteristics, including morphology, flight kinematics, aerodynamics, energetics and flight performance. Birds’ size range is 0.002–15 kg and bats’ size range is 0.002–1.5 kg. The wingbeat kinematics differ between birds and bats, which is mainly due to the different flexing of the wing during the upstroke and constraints by having a wing of feathers and a skin membrane, respectively. Aerodynamically, bats appear to generate a more complex wake than birds. Bats may be more closely adapted for slow maneuvering flight than birds, as required by their aerial hawking foraging habits. The metabolic rate and power required to fly are similar among birds and bats. Both groups share many characteristics associated with flight, such as for example low amounts of DNA in cells, the ability to accumulate fat as fuel for hibernation and migration, and parallel habitat-related wing shape adaptations.

Reference:

Hedenstrom, A., L. C. Johansson, and G. R. Spedding. 2009. Bird or bat: comparing airframe design and flight performance. Bioinspiration & Biomemetics 4: 1-13.

Impacts of climate change on the annual cycles of birds -- Organisms living today are descended from ancestors that experienced considerable climate change in the past. However, they are currently presented with many new, man-made challenges, including rapid climate change. Migration and reproduction of many avian species are controlled by endogenous mechanisms that have been under intense selection over time to ensure that arrival to and departure from breeding grounds is synchronized with moderate temperatures, peak food availability and availability of nesting sites. The timing of egg laying is determined, usually by both endogenous clocks and local factors, so that food availability is near optimal for raising young. Climate change is causing mismatches in food supplies, snow cover and other factors that could severely impact successful migration and reproduction of avian populations unless they are able to adjust to new conditions. Resident (non-migratory) birds also face challenges if precipitation and/or temperature patterns vary in ways that result in mismatches of food and breeding. Predictions that many existing climates will disappear and novel climates will appear in the future suggest that communities will be dramatically restructured by extinctions and changes in range distributions. Species that persist into future climates may be able to do so in part owing to the genetic heritage passed down from ancestors who survived climate changes in the past.

Reference:

Carey, C. 2009. The impacts of climate change on the annual cycles of birds. Philosophical Transactions of the Royal Society B, online early.

Birds beam antipredator calls to predators and conspecifics -- Animals in many vertebrate species vocalize in response to predators, but it is often unclear whether these antipredator calls function to communicate with predators, conspecifics, or both. Yorzinski and Patricelli (2009) evaluated the function of antipredator calls in 10 species of passerines by measuring the acoustic directionality of these calls in response to experimental presentations of a model predator. Acoustic directionality quantifies the radiation pattern of vocalizations and may provide evidence about the receiver of these calls. The authors predicted that antipredator calls would have a lower directionality if they function to communicate with surrounding conspecifics, and a higher directionality and aimed at the receiver if they function to communicate with the predator. The results support both of these functions. Overall, the birds produce antipredator calls that have a relatively low directionality, suggesting that the calls radiate in many directions to alert conspecifics. However, birds in some species increase the directionality of their calls when facing the predator. They can even direct their calls towards the predator when facing lateral to it—effectively vocalizing sideways towards the predator. These results suggest that antipredator calls in some species are used to communicate both to conspecifics and to predators, and that birds adjust the directionality of their calls with remarkable sophistication according to the context in which they are used.

Reference:

Yorzinski, J. L., and G. L. Patricelli. 2009. Birds adjust acoustic directionality to beam their antipredator calls to predators and conspecifics. Proceedings of the Royal Society B, online early.

Spotted Antbird

Inactive lifestyle of a tropical bird -- Birds in the lowland tropical rain forest are expected to have low energy turnover. Steiger et al. (2009) used heart rate telemetry to estimate nighttime resting metabolic rate (RMR), daily energy expenditure (DEE), and locomotor activity of a small, long-lived tropical rain forest–understory bird, the Spotted Antbird (Hylophylax naevioides). Heart rate was linearly related to oxygen consumption in respirometry measurements that encompassed 96% of heart rates measured in wild birds. Heart rates in the wild ranged from 260 beats/min at night to 824 beats/min during the day, with a mean of 492 beats/min. Compared with temperate-forest birds of similar body mass, wild Spotted Antbirds had a low DEE, only 51% of the expected value. Such low metabolism was achieved mainly by being locomotively inactive for 35% of the daytime (i.e., 0 hops or flights/min). On average, Spotted Antbirds exhibited 1.6 hops or short flights/min during the daytime. In addition, they decreased nighttime RMR in the wild (at ambient temperatures below their thermoneutral zone [TNZ]) to levels equivalent to nighttime RMR in the laboratory at temperatures within their TNZ. This suggests that wild birds reduce their body temperature every night. These data confirm and extend previous studies showing that tropical passerines have low metabolic rates.

Reference:

Steiger, S. S., W. W. Cochran, and M. Wikelski. 2009. Low metabolism and inactive lifestyle of a tropical rain forest bird investigated via heart-rate telemetry. Physiological and Biochemical Zoology 82: 580-589.

Passerine song volume -- Songs of passerines are generally complex, long-range acoustic signals, and are highly diverse across species. This diversity must nevertheless be shaped by the capabilities of the avian vocal physiology. For example, within species, loudness has been shown to trade-off with aspects of song complexity. Cardoso (2009) examined the question of whether trade-offs with loudness influenced the evolutionary diversification of song among passerines. Comparing perceived song loudness across > 140 European and North American species showed that loudness is positively related to body size and to singing with simple trilled syntax, and negatively related to aspects of syllable complexity. Syntax and syllable phonology together explained more variation than body size did, indicating that the acoustic design of songs is an important factor determining loudness. These results show for the first time that loudness covaries with, and possibly limits, song complexity across species, suggesting that a trade-off with loudness shaped the evolutionary diversification of passerine song.

Reference:

Cardoso, G. C. 2009. Loudness of birdsong is related to the body size, syntax and phonology of passerine species. Journal of Evolutionary Biology, online early.

Hypothesized pattern of diversity for Pterosauria through the Mesozoic. Note the hypothesized peak in diversity at the Jurassic/Cretaceous boundary, followed by a decline in diversity occurring throughout the Cretaceous.

Did birds competitively exclude pterosaurs? -- Pterosaurs were the first flying vertebrates and formed important components of terrestrial and marginal marine ecosystems during the Mesozoic. They became extinct during the latest Cretaceous (latest Maastrichtian), at, or near, the Cretaceous/Paleogene boundary, following an apparent decline in diversity in the Late Cretaceous. This reduction in species richness has been linked to the ecological radiation of birds in the Early Cretaceous and the proposal that birds competitively excluded pterosaurs from many key niches. However, although competition is often posited as a causal mechanism for many of the clade-clade replacements observed in the fossil record, these hypotheses are rarely tested. Butler et al. (2009) provide a detailed examination of pterosaur diversity through time, including both taxic and phylogenetically corrected diversity estimates and comparison of these estimates with a model describing temporal variation in the number of pterosaur-bearing formations (a proxy for rock availability). Both taxic and phylogenetic diversity curves are strongly correlated with numbers of pterosaur-bearing formations, suggesting that a significant part of the signal contained within pterosaur diversity patterns may be controlled by geological and taphonomic megabiases rather than macroevolutionary processes. There is no evidence for a long-term decline in pterosaur diversity through the Cretaceous, although a reduction in morphological, ecological, and phylogenetic diversity does appear to have occurred in the latest Cretaceous. Competitive replacement of pterosaurs by birds is difficult to support on the basis of diversity patterns.

Reference:

Butler, R. J., P. M. Barrett, S. Nowbath, and P. Upchurch. 2009. Estimating the effects of sampling biases on pterosaur diversity patterns: implications for hypotheses of bird/pterosaur competitive replacement. Paleobiology 35: 432-446.

The blood-gas barrier: a unique avian solution -- Two separate selective pressures determine the structure of the blood-gas barrier in air breathing vertebrates. The first pressure which has been recognized for 100 years is to facilitate diffusive gas exchange. This requires the barrier to be extremely thin and have a large area. The second pressure has only recently been appreciated. This is to maintain the mechanical integrity of the barrier in the face of its extreme thinness. The most important tensile stress comes from the pressure within the pulmonary capillaries which results in a hoop stress. The strength of the barrier can be attributed to the type IV collagen in the extracellular matrix. In addition, the stress is minimized in mammals and birds by complete separation of the pulmonary and systemic circulations. Remarkably the avian barrier is about 2.5 times thinner than that in mammals and it is also much more uniform in thickness. These advantages for gas exchange come about because the avian pulmonary capillaries are unique among air breathers in being mechanically supported externally in addition to the strength that comes from the structure of their walls. This external support comes from epithelial plates that are part of the air capillaries and the support is available because the terminal air spaces in the avian lung are extremely small owing to the flow-through nature of ventilation in contrast to the reciprocating pattern in mammals.

Reference:

West, J. B. 2009. Comparative physiology of the pulmonary blood-gas barrier: the unique avian solution. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, in press.

Check this video (at http://ajpregu.physiology.org/content/vol0/issue2009/images/data/00459.2009/DC1/video1.avi) showing an electron tomography study of chicken lung.

Nocturnal life of diurnal birds -- Songbirds are generally considered diurnal, although many species show periodic nocturnal activity during migration seasons. From a breeding-range perspective, such migratory species appear to be diurnal because they are observed to nest and feed their young during the day. But are they really exclusively diurnal? Muhkin and Grinkevich (2009) examined how a passerine long-distance migrant, the Eurasian Reed Warbler (Acrocephalus scirpaceus), schedules movements during the breeding period by tracking birds in two experimental situations: 1) Birds experienced simulated nest loss and were monitored during their search for alternative locations, and 2) birds were translocated to reed beds at distances from 2 to 21 km and tracked during homing. The simulated unpredictable events disrupted normal breeding, forced birds to move over relatively long distances, and triggered rapid change in diel activity. In all but 1 case, birds resorted to nocturnality to find their way home and to search for new places to breed. Nocturnality during the breeding season indicates that songbird schedules are far more flexible than previously assumed. The reasons for nocturnal movements are poorly understood. Among the presumed advantages, the reduced predation pressure at night stands out because it is advantageous for movements on local as well as global scales. Predation may be particularly relevant for inhabitants of fragmented habitats, which encounter unfavorable conditions when crossing gaps in their preferred habitat. Therefore, similar selection pressures around the year may have favored the evolution of a general circadian mechanism for switches to nocturnality. Furthermore, the novel finding of homing and dispersal at night may give leads toward understanding the still enigmatic navigational abilities of songbirds.

Reference:

Muhkin, A., and V. Grinkevich. 2009. Under cover of darkness: nocturnal life of diurnal birds. Journal of Biological Rhythms 24: 225-231.

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