University of Twente Student Theses


Towards understanding Short-toed Eagle (Circaetus gallicus) Migration: Is meteorology influencing it?

Chaudhary, Nirmal (2011) Towards understanding Short-toed Eagle (Circaetus gallicus) Migration: Is meteorology influencing it?

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Abstract:Migration of birds has been explained and studied in an exclusive manner and a number of different ways of explanations and modelling have been adopted ranging from flight energy, aerodynamics and even on birds communication (calls) to adjust their flights. Some have assessed effect of meteorological variables such as wind and have explained migratory behaviour based on simulation. In this thesis the migratory flight of Short-toed eagle has been studied using the geo-locations based on GPS telemetry (Argos Tracking data system) and meteorological variables that are likely to affect the migratory journey from an idealized route of migration between departing site and destination site. The meteorological variables were extracted from the NCEP/NOMAD’s Global Forecast system in Integrated Data Viewer. Wind related variables in relation to the flight positions- crosswind and headwind were derived. The deviation in migratory journey from the ideal route was calculated for each geo-locations of the flight. The distance covered by the bird was calculated based upon the great circle distance. The assessment of wind related variables were done using Mann-Whitney Test and the deviation has been explained based upon Generalized linear modelling using Gaussian family with Identity function. The Short toed eagle under study is juvenile which migrated for the first time to its wintering ground in Western mid-Africa from Southern Spain. The one way journey is completed in 11 days with 10 stopovers. Maximum distance covered is 131 km in four hours on the 2nd migratory day over Moroccan desert in North Africa. Though the numbers of geo-locations with crosswind from right are more, the speed of crosswind from right and from left are statistically similar (confidence interval of difference contains 0). Similar is with headwind and tailwind (median=3.41 m/s for headwind and 2.88 m/s for tailwind). And the deviation extent from an idealized route is also statistically similar ii (median=104.18 km for deviation to the right and 77.54 km for deviation to the left). The Generalized linear model is based upon crosswind, surface lift index and wind speed. The Surface list index has the highest effect on deviation (coefficient. =-18.85, t value=-9.18, p=1.51e-12), followed by wind speed (coeff.=16.96, t value=3.54, p=0.000) and then surface lift index (coeff.=-5.12, t=-4.39, p=0.000). The alternative hypothesis that the crosswind and surface lift index effects negatively to the deviation so that deviation is to the left is accepted while for wind speed, the alternative hypothesis is rejected (p=0.999). Hypothesis related to headwind could not be tested because this variable is excluded in the stepwise modelling. Theoretically, distribution of wind related opposing factors being equal in magnitude implies it is the bird’s preference to fly through the observed geolocations such that the effect from these is nullified towards the end of the migratory journey. However a separate study of the distribution of these meteorological variables along the migratory course range could bring deeper insight. We conclude that meteorology does determine the migratory route and the observation of flight position is likely to be an adjustment of the bird due to the effect of the meteorological factors. Keywords: Migratory flights, Short-toed Eagle, Headwind, Crosswind, Surface lift Index, Deviation modelling
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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