Pigeon homing: sun compass use in the southern hemisphere

Although the sun compass of birds is based on learning the sun's arc during development, it was unclear whether birds can use the sun when its apparent movement is reversed, in particular, whether northern birds that have been introduced into the southern hemisphere can use the southern sun. To answer this question, clock-shift experiments were performed with local homing pigeons in Auckland, New Zealand (37°S). In three fast-shift tests and two slow-shift tests, the experimental birds showed deflections from the untreated controls that were the mirror images of those observed in the northern hemisphere. These results clearly show that homing pigeons in New Zealand use a sun compass that is adapted to the situation in the southern hemisphere. The learning processes establishing the compensation mechanisms thus appear to be free of constraints concerning the direction of the sun's movement. Differences from recent findings with migratory birds, where the direction of celestial rotation proved of crucial importance for establishing the migratory direction, are discussed: the differences may arise from the different orientation tasks, in particular, from the involvement of innate information in establishing the migratory direction. Received: 13 November 1997 / Accepted after revision: 28 February 1998     

When does bearing magnets affect the size of deflection in clock-shifted homing pigeons?

Pigeons whose internal clock is shifted by 6 h show deflections from the direction of untreated controls, yet these deflections are often smaller than predicted. Magnets temporarily disabling the magnetic compass increased these the deflections significantly (R. Wiltschko and Wiltschko 2001), indicating a compromise between sun compass and magnetic compass. – Recently, Ioalé et al. (2006) claim that they could not replicate our findings. The reason lies in a difference in the behavior of the clock-shifted pigeons without magnets: in the study of Ioalè et al. (2006), their deflections was already almost as large as that of our pigeons carrying magnets. This difference is probably caused by the limited experience of the pigeons of Ioalè et al. (2006): Their birds, in contrast to ours, had not used their sun’ compass during extended homing flights at various times of the year and, not having been faced with the necessity to compensate the saisonal changes of the sun’s arc, gave the sun compass more weight than our birds did.A comment to the paper by Ioalè, Odetti and Gagliardo (2006) Behav Ecol Sociobiol 60: 516–521.     

Do bearing magnets affect the extent of deflection in clock-shifted homing pigeons?

When released after clock-shift, homing pigeons fail to orient towards the home direction but display a consistent deflection of their initial orientation due to the difference between the real sun azimuth and the computed azimuth according to the subjective time of each single bird. It has been reported that the size of the observed deflection is frequently smaller than expected and a discussion on the possible factors affecting the size of deflection has emerged. Some authors have proposed that the major factor in reducing the deflection after clock-shift is the simultaneous use of both the magnetic and the sun compasses, giving true and erroneous information, respectively, about the home direction. Therefore, a magnetic disturbance, by impeding the use of the geomagnetic information in determining the home direction, is presumed to increase the size of the deflection up to the levels of the expectation. To test this hypothesis, we released three groups of clock-shifted birds from unfamiliar locations (unmanipulated pigeons, pigeons bearing magnets on their head, and pigeons bearing magnets on their back) together with a group of unshifted control birds. As no difference in the orientation of the three groups emerged, we were not able to confirm the hypothesis of the role of the magnetic compass in reducing the expected deflection after clock-shift.Communicated by W. Wiltschko     

Modeling the combined effect of nutrients and pyrene on the plankton population: Validation using mesocosm experiment data and scenario analysis

A new integrated model that includes a hydrodynamic model coupled with a contaminant fate and effect sub-model and an ecological sub-model is presented and validated using data from mesocosm experiments. The experiments were carried out in the Isefjord (Denmark) and include the combined effects of nutrients and pyrene addition on the lower trophic levels of bacteria, zooplankton and phytoplankton. The model was able to correctly represent the main dynamics observed in the mesocosms during the 11 days of the experiment and thereby confirmed that it is possible to represent short-term changes in the system with a simplified food-web model on a small spatial and temporal scale. Finally, the validated model was used to carry out a scenario analysis to investigate the effects of a contaminant pulse at different pyrene concentrations and different release timings. Results showed that the ecosystem's vulnerability to a pyrene pulse depends on the initial condition of the system. Stronger biomass reduction was observed when the pulse was released during the zooplankton bloom. Conversely, when the pulse was added at low biomass and before the bloom, the system showed a tendency to behave non-linearly.     

On the sudden disappearance of Egeria densa from a Ramsar wetland site of Southern Chile: A climatic event trigger model

Contemporary shallow lakes theory proposes that these ecosystems may experience abrupt regime shifts due to small changes in controlling variables or triggers. So far, these triggers have been related mostly to nutrients as the immediate driver. During May 2004 the río Cruces wetland, a Ramsar site located in Southern Chile, underwent a major regime shift, from a clear water state, vastly dominated by the invasive macrophyte Egeria densa, to a turbid water state. In this article we show, through the analysis of long-term meteorological data that late fall 2004 was anomalous due to the presence of a high-pressure cell that persisted most of the month of May over Southern Chile. This climatic event caused an almost complete absence of precipitations and lower temperatures during this period, including several freezing nights. Eco-physiological experiments showed that 6 h exposure to desiccation kill the macrophyte. We developed a simple-biology dynamic model, under Stella Research 9.1, to show that the climatic anomaly of May 2004, plus the increased sedimentation of the wetland's floodplains, and the associated response of E. densa, explains its sudden disappearance from río Cruces wetland.