Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk

We used radiotelemetry to investigate the time of migratory flight initiation relative to available celestial orientation cues and departure direction of a nocturnal passerine migrant, the reed warbler, Acrocephalus scirpaceus, during autumn migration. The study was carried out at Falsterbo, a coastal site in southwest Sweden. The warblers initiated migration from times well after local sunset and well into the night, corresponding to sun elevations between - 4 degrees and - 35 degrees, coinciding with the occurrence of stars at night. They departed in the expected migratory direction towards south of southwest with a few initiating migration in reverse directions towards northeast to east. Flight directions under overcast conditions (7-8/8) were more scattered than under dear sky conditions (0-4/8). There were fewer clouds on departure nights than on nights when the birds did not initiate migration. For birds staying longer than one night at stopover the horizontal visibility was higher and precipitation was less likely on departure nights than on the previous night. The results show that the visibility of celestial cues, and stars in particular, are important for the decision to initiate migration in reed warblers. However, cloud cover, horizontal visibility and precipitation might be correlated with other weather variables (i.e. wind or air pressure) that are also likely to be important for the decision to migrate. (C) 2001 The Association for the Study of Animal Behaviour

No evidence of genetic differentiation between lesser redpolls Carduelis flammea cabaret and common redpolls Carduelis f. flammea.

The remarkable variation in plumage and morphological characters in the redpoll complex Carduelis flammea-hornemanni has puzzled taxonomists for several decades. In contrast, molecular studies have not revealed any genetic differentiation among the phenotypic redpoll forms. This could either be a result of high present-day gene flow or morphological differentiation following a rapid and recent population expansion. We sequenced a major portion (960 bp) of the mitochondrial control region in individuals of the two taxa Carduelis flammea flammea and C. f. cabaret. Birds were sampled on autumn migration in southern Sweden (n = 30) and on breeding areas in southern Norway (n = 11). We found 22 variable sites defining 26 different haplotypes, of which most (22/26) were singletons. The level of haplotype and nucleotide diversity was low in the two taxa and we found no evidence of genetic differentiation. A mismatch distribution was very similar to that expected from a sudden population expansion model. Our estimates suggest that the redpoll population expanded during the last glaciation episode from a small population to a long-term effective population size of 230 000 females. The findings in our study suggest that the morphological differentiation between the two taxa occurred rather recently but after the population expansion

Nocturnal migratory flight initiation in reed warblers Acrocephalus scirpaceus: effect of wind on orientation and timing of migration

We used radio-telemetry to study autumn migratory flight initiation and orientation in relation to wind and air pressure in a nocturnal passerine migrant, the reed warbler Acrocephalus scirpaceus at Falsterbo, southwest Sweden. The majority of the reed warblers departed in the expected migratory direction towards south of southwest, while a low number of the birds took off in reverse directions between north and east. Flight directions at departure correlated with wind directions. These correlations were particularly prominent at higher wind speeds but were absent at wind speeds below 4 m/s. Birds departing in the expected migratory direction compensated completely for wind drift. The reed warblers preferred to depart during nights with tailwinds and when air pressure was increasing suggesting that reed warblers are sensitive to winds and air pressure and select favourable wind conditions for their migratory flights. Since air pressure as well as velocity and direction of the wind are correlated with the passage of cyclones, a combination of these weather variables is presumably important for the birds' decision to migrate and should therefore be considered in optimal migration models

Effects of low-dose X-irradiation on mouse-brain aggregation cultures

Biochemical and morphological differentiation in reaggregating mouse-brain cell cultures after low-dose radiation (0.5 Gy) in vitro was studied. Cells were irradiated on culture day 2, corresponding to embryonic day 15-16, and different glial and neuronal markers were followed through development to postnatal day 40. The shape and size of irradiated aggregates were more irregular and smaller compared with controls. Total amounts of DNA and protein were significantly lower in irradiated aggregates than in controls between days 8 and 20. After 30 days in culture activities of the glial markers glutamine synthetase (GS) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) were lower in X-irradiated aggregates than in controls. However, after 40 days the CNP activity in irradiated aggregates increased to levels above those of the controls. Irradiated and control aggregates did not differ significantly in neuronal marker enzyme activities, i.e. choline acetyltransferase (ChAT), acetylcholine esterase (AChE) and glutamic acid decarboxylase (GAD) measured on a per mg protein basis. On days 20 and 30 the amount of nerve growth factor (NGF) was two-fold higher in irradiated aggregates compared with non-irradiated ones, suggesting that, after irradiation, surviving cells in culture were induced to produce more NGF. After 40 days the amount of NGF in irradiated aggregates had decreased to the level found in the control aggregates