2 Subproject status
2.4 White-tailed eagle (WTE)
2.4.4 WTE behaviour inside and outside the wind-power plant area
Subproject responsibility: Pernille Lund Hoel, Kjetil Bevanger, Hans Chr. Pedersen, Eivin Røskaft, Bård Stokke
Objectives: Observation of WTE behaviour inside the wind-power plant area and in an adjacent control area, to collect data on possible behavioural differences as a response to the wind-power plant.
Photo 10. Pernille Lund Hoel elaborates on her findings at the Annual Meeting in March. Photo:
Kjetil Bevanger.
2.4.4.1 Activities and findings
In order to investigate behavioural differences related to the distance from the turbines, data on flight activity (moving flight, social behaviour and soaring) and flight height (below, in and above the rotor zone) were collected at 12 vantage points, 6 from inside the wind-power plant area and 6 from control areas close to the power-plant area. In order to investigate possible differences in-side the wind-power plant versus the control area, observations from the 12 vantage points have been separated into two groups (6 vantage points in each) in some of the analysis and named control area (CA) and wind-power plant area (WPA).
First, possible explanatory variables that could account for variation in general activity were inves-tigated. Second, variables that could account for variation in flight activity and flight height were investigated, and third variables that could explain differences between the flight activities and flight heights were investigated.
Any variation in the two response variables (flight activity and flight height) could possibly be ex-plained by several different explanatory variables. In order to investigate which variables that in-fluence on variation in the response variables, data were collected on distance to nearest turbine, distance to nearest active nest, number of individuals observed together, date, time of day, weather (precipitation, temperature and wind speed), and age of individuals. Data collected from 144 observation hours, during mid-March to the end of May 2008, were analyzed using ANOVA, Chi-square tests and multinomial logistic regression.
The only explanatory variables that showed a statistically significant effect on the general activity were week number and distance to nearest active nest. The results showed a statistically signifi-cant difference in activity among the weeks. The general activity peaked in April (week 15 and 17) which is the first part of the breeding period for the WTE (Figure 28)
.
Moreover, more activity was observed at distance 0-500 m than further away from the nearest active nest. There was more activity at 0-500 m than further away from the nest, which probably could be due to defending ter-ritories and delivering food to the nest (Dementavicius & Treinys 2009).
Neither distance to near-est turbine, nor the locations seem to have any effect on the general activity.Figure 28. Observed flight activity (%) of WTE of total observation time pr. observation week. N=number of two-hour observation periods.
The results showed, furthermore, a statistically significant difference in age distribution between the two locations, with a higher percentage of adults in the CA and a higher percentage of subadults in the WPA (Figure 29). There was also a statistically significant difference between the age categories in the different observation weeks with more adults than subadults represented throughout the whole study period.
Figure 29. Age distribution (%) of WTE in the control area and the wind-power plant area (NCAadult=182, NCASubadult=79, NWPAadult=135, NWPAsubadult=106).
Regarding flight activity, the results from the multinomial regression analyses showed a statisti-cally significant difference between moving flight and soaring in number of individuals observed together, with more individuals observed together in moving flight than in soaring (Figure 30). This could possibly be caused bypairs of individuals performing moving flight when moving back and forth between territories and feeding areas (Dementavicius & Treinys 2009).
Furthermore, there was a statistically significant difference between social behaviour and the two other activity categories in number of individuals observed together, with more individuals ob-served together in social behaviour than in moving flight and soaring. There was also a significant difference between social behaviour and the other two activities related to age, with more adults than subadults in social behaviour than in moving flight and soaring. Since adults are reproduc-tively active in contrast to subadults, and therefore more likely to participate in social behaviour in order to increase their fitness, this result is as expected.
Soaring was statistically significant different from the two other activity categories in relation to flight height, with soaring only occurring in – and above the rotor height, while moving flight and social behaviour occurred in all three flight-height categories. One reason for this is that the WTE can climb in altitude during sustained soaring and in this way gain high altitudes.
Figure 30. Observed age distribution (%) of WTE in the different flight activity categories.
Furthermore, a statistically significant difference in age distribution between the two locations ap-peared, with a higher percentage of adults in the CA and a higher percentage of subadults in the WPA. This finding could indicate that the adults are either behavioural displaced away from the WPA, or that there are a higher percentage of adults than subadults killed inside the WPA. One possible reason for this difference could be connected to social behaviour. A much higher per-centage of adults than subadults are involved with this type of behaviour, and this can possible impose a greater risk due to decreased awareness of the surroundings, or/and the fact that there are more activity in the air during the period were social behaviour are most important for pair-bounding, as also the analyses of the effect of week number on the general activity suggest.
Because of low sample sizes due to low breeding densities, raptors are among the most difficult group of birds to demonstrate effects of disturbance, thus more long-term studies are needed. In order to test the assumption about social behaviour imposing greater risk to collision than the other flight activities, it is therefore necessary to conduct more long-term studies. More studies will also give larger sample sizes, which can give the opportunity to distinguish between more types of flight behaviour (e.g. more types of social behaviour).
Other studies (Henderson et al. 1996) indicate that moving flight could impose a greater risk than the flight activities because adults are flying more frequently under or between man-made struc-tures in order to reduce their journey time when rising young. The present study show that moving flight is the activity that is most observed both in the WPA and the CA, and in both age categories.
One alternative explanation for the high amount of adults found killed could therefore be that mov-ing flight in relation to parental care could impose a higher risk for the adults than the subadults.
These results indicate that the WTE on Smøla does not have any behavioural responses to the wind-power plant construction. It may, however, contribute to explain why the WTE is vulnerable to collisions with the turbines and the number of killed individuals recorded within the power-plant area. The results may also contribute to explain the high percentage of adults found killed in the
WPA. The WTE has a peak activity during the start of the breeding period, which can be fatal to both adult individuals and thus also the nestlings. This should be carefully considered when look-ing at the possible long-term effects of the wind-power plant on the WTE population on Smøla.