Recent publications in Avian Biology

Are birds stressed during long-term flights? -- During endurance flight most birds do not feed and have to rely on their body reserves. Fat and protein is catabolised to meet the high energetic demands. Even though the hormonal regulation of migration is complex and not yet fully understood, the adrenocortical hormone corticosterone crystallizes to play a major role in controlling physiological traits in migratory birds during flight. However, results from field studies are partially equivocal, not least because data from birds during endurance flight are hard to get and present mostly a momentary shot. A wind-tunnel experiment offered the possibility to measure repeatedly under controlled conditions the effect of long flights on the stress hormone corticosterone. In a long-distance migrating shorebird, the Red Knot (Calidris canutus), Jenni-Eiermann et al. (2009) measured plasma concentrations of corticosterone within 3 min and after a restraint time of 30 min directly after 2-hour and 10-hour non-stop flights, respectively, and during rest. Baseline corticosterone levels were unchanged directly after the flights, indicating that endurance flight did not affect corticosterone levels. The adrenocortical response to restraint showed the typical rise in birds during rest, while birds after a 2- or 10-hour flight substantially decreased plasma corticosterone concentrations. These results suggest that the negative adrenocortical response to restraint after flight is part of the mechanism to reduce the proteolytic effect of corticosterone to save muscle protein and to avoid muscle damaging effects.

Reference:

Jenni-Eiermann, S., D. Hasselquist, A. Lindstrom, A, Koolhaas, and T. Piersma. 2009. Are birds stressed during long-term flights? A wind-tunnel study on circulating corticosterone in the Red Knot. General and Comparative Endocrinology 164: 101-106.

Photo source: http://www.flickr.com/photos/74287342@N00/3556753985/

European Robin (Photo by H. Mouritsen)

Vision-mediated magnetic compass in European Robins -- Magnetic compass information has a key role in bird orientation, but the physiological mechanisms enabling birds to sense the Earth's magnetic field remain one of the unresolved mysteries in biology. Two biophysical mechanisms have become established as the most promising magnetodetection candidates. The iron-mineral-based hypothesis suggests that magnetic information is detected by magnetoreceptors in the upper beak and transmitted through the ophthalmic branch of the trigeminal nerve to the brain. The light-dependent hypothesis suggests that magnetic field direction is sensed by radical pair-forming photopigments in the eyes and that this visual signal is processed in cluster N, a specialized, night-time active, light-processing forebrain region. Zapka et al. (2009) found that European Robins (Erithacus rubecula) with bilateral lesions of cluster N are unable to show oriented magnetic-compass-guided behaviour but are able to perform sun compass and star compass orientation behavior. In contrast, bilateral section of the ophthalmic branch of the trigeminal nerve in European Robins did not influence the birds' ability to use their magnetic compass for orientation. These data show that cluster N is required for magnetic compass orientation in this species and indicate that it may be specifically involved in processing of magnetic compass information. Furthermore, the data strongly suggest that a vision-mediated mechanism underlies the magnetic compass in this migratory songbird, and that the putative iron-mineral-based receptors in the upper beak connected to the brain by the trigeminal nerve are neither necessary nor sufficient for magnetic compass orientation in European Robins.

Migratory double breeding by Neotropical migrant birds -- Neotropical migratory songbirds typically breed in temperate regions and then travel long distances to spend the majority of the annual cycle in tropical wintering areas. Using stable-isotope methodology, Rohwer et al. (2009) provided quantitative evidence of dual breeding ranges for 5 species of Neotropical migrants, including Yellow-billed Cuckoos, Cassin's Vireos, Yellow-breasted Chats, Hooded Orioles, and Orchard Orioles. Each is well known to have a Neotropical winter range and a breeding range in the United States and Canada. However, after their first bout of breeding in the north, many individuals migrate hundreds to thousands of kilometers south in midsummer to breed a second time during the same summer in coastal west Mexico or Baja California Sur. They then migrate further south to their final wintering areas in the Neotropics. This discovery of dual breeding ranges in Neotropical migrants reveals a hitherto unrealized flexibility in life-history strategies for these species and underscores that demographic models and conservation plans must consider dual breeding for these migrants.

Are birds that vocalize at higher frequencies preadapted for urban areas? -- Urban environments have become an increasingly important part of the world's ecosystems, and the characteristics that enableanimals to live there are not fully understood. A typical urbancharacteristic is the high level of ambient noise, which presentsdifficulties for animals that use vocal communication. Urbannoise is most intense at lower frequencies, and, therefore,species vocalizing at higher frequencies may be less affectedand thus better able to inhabit urban environments. Hu and Cardoso (2009) testedthis hypothesis with within-genera comparisons of the vocalizationfrequency of 529 bird species from 103 genera. They found thatspecies occurring in urban environments generally vocalize athigher dominant frequency than strictly nonurban congenericspecies, without differing in body size or in the vegetationdensity of their natural habitats. In most passerine generawith low-frequency songs, which are more subject to maskingby noise, minimum song frequency was also higher for urban species.These results suggest that species using high frequencies arepreadapted to inhabit urban environments and that reducing noisepollution in urban areas may contribute to restore more diverseavian communities.

(Left) The size and estimated age for all 10 Archaeopteryx specimens are depicted. The growth curves are based upon age and size estimates (diamonds)
for the eight specimens where femoral length is known. The dashed line represents the best fit for the unconstrained statistical analysis with hatchling and adult
size undefined. The solid line represents the best fit when hatchling and adult size are constrained. (Right) The maximal growth rates from these analyses
(1.87–2.2 g/day; hollow diamond) fit expectations (1.83–1.87 g/day) for same-sized non-avialan dinosaurs (solid line) – animals that grew like slow
growing endotherms, here compared to marsupials (M). The Archaeopteryx estimates are three times lower than typical rates for extant precocial land birds
(5.7 g/day; P], 15 times lower than alticial land birds (28.6 g/day; A), and four times higher than typical rates for extant reptiles (0.46 g/day; R).
Specimens designations: Ei = Eichstäat, Mu = Munich, 8^th = 8^th Exemplar, Te = Teyler, Th = Thermopolis, Be = Berlin, Ma = Maxberg,
O&S = Exemplar der Familien Ottmann & Steil, Lo = London, So = Solnhofen.

Archaeopteryx grew slower than living precocial birds -- Archaeopteryx is the oldest and most primitive known bird (Avialae). It is believed that the growth and energetic physiology of basalmost birds such as Archaeopteryx were inherited in their entirety from non-avialan dinosaurs. This hypothesis predicts that the long bones in these birds formed using rapidly growing, well-vascularized woven tissue typical of non-avialan dinosaurs. Erickson et al. (2009) report that Archaeopteryx long bones are composed of nearly avascular parallel-fibered bone. This is among the slowest growing osseous tissues and is common in ectothermic reptiles. These findings dispute the hypothesis that non-avialan dinosaur growth and physiology were inherited in totality by the first birds. Examining these findings in a phylogenetic context required intensive sampling of outgroup dinosaurs and basalmost birds. These results demonstrate the presence of a scale-dependent maniraptoran histological continuum that Archaeopteryx and other basal-most birds follow. Growth analysis for Archaeopteryx suggests that these animals showed exponential growth rates like non-avialan dinosaurs, three times slower than living precocial birds, but still within the lowermost range for all endothermic vertebrates. The unexpected histology of Archaeopteryx and other basalmost birds is actually consistent with retention of the phylogenetically earlier paravian dinosaur condition when size is considered. Thus, Archaeopteryx was very primitive, similar to their non-avialan dinosaur precursors. In other words, Archaeopteryx was simply a feathered and presumably volant dinosaur. The first birds were simply feathered dinosaurs with respect to growth and energetic physiology. The evolution of the novel pattern in modern forms occurred later in the group's history.

Digital still images obtained from three cameras mounted on Black-browed Albatrosses. A: a ‘featureless’ sea, B: an iceberg encountered,
C: a killer whale breaking the ocean surface, apparent from its dorsal fin (white arrow) and three black-browed albatrosses attracted to the whale,
D: two albatrosses flying in association with the camera-mounted bird, E: a fisheries vessel in the distance (white arrow) with an aggregation of birds,
F: a bright light source during the night, possibly a vessel or the moon.

Bird-Borne Camera Shows an Association between Albatrosses and a Killer Whale -- Albatrosses fly many hundreds of kilometers across the open ocean to find and feed upon their prey. Despite the growing number of studies concerning their foraging behaviour, relatively little is known about how albatrosses actually locate their prey. Sakamoto et al. (2009) deployed a combined animal-borne camera and depth data logger on free-ranging Black-browed Albatrosses (Thalassarche melanophrys). The still images recorded from these cameras showed that some albatrosses actively followed a killer whale (Orcinus orca), possibly to feed on food scraps left by this diving predator. The camera images together with the depth profiles showed that the birds dived only occasionally, but that they actively dived when other birds or the killer whale were present. This association with diving predators or other birds may partially explain how albatrosses find their prey more efficiently in the apparently ‘featureless’ ocean, with a minimal requirement for energetically costly diving or landing activities.

Reference:

Sakamoto, K. Q., A. Takahashi, T. Iwata, and P. N. Trathan. 2009. From the eye of the albatross: a bird-borne camera shows an association between albatrosses and a killer whale in the Southern Ocean. PLoS one, online.