Wind turbines can affect bird populations by causing mortality when birds collide with turbine blades or displacement when the individuals move permanently to other areas. While mortality is well documented, displacement has only been studied more extensively in the last decade, and it is important to summarise the current knowledge and research trends.
The exponential expansion of wind energy production in recent decades, driven by decarbonization goals and financial incentives by some governments, has promoted multiple interactions between human activities and ecological systems 
. Mortality is probably the most well-documented effect on wildlife, occurring when flying birds and bats collide with wind turbines 
. Additionally, when placed in natural or seminatural habitats, infrastructures associated with the wind industry modify the landscape, promoting habitat loss and fragmentation, and may alter species behaviour 
, potentially leading to multiple ecological impacts 
, and ultimately population-level effects.
Besides habitat alterations associated with wind farms, the visual intrusion caused by the turbines, the rotating blades, noise and vibration resulting from turbine operation, and human or vehicle circulation due to the construction or maintenance activities, may cause disturbance to birds, during both the construction and operational phases 
. Such activities may trigger an avoidance response that can occur at three spatial scales: macro-avoidance when birds avoid the wind-power plant area as a whole, meso-avoidance if turbine arrays or single turbines are avoided, and micro-avoidance, which consists in last-second evasive movements of the rotor blades 
. Depending on the study design and wind farm layout (with clustered/random turbines) it may not be possible to fully disentangle macro and meso-avoidance.
Here, we consider displacement as the reduced density of birds occurring near wind turbines, due to long-term disturbance leading to functional habitat loss, i.e., the joint effect of macro-avoidance and meso-avoidance 
. Conversely, we define attraction as an increase in bird density within or near the wind farm. Attraction has been described mostly in the offshore industry, as infrastructures associated with wind energy production may provide favourable roosting conditions or act as a reef, increasing food resources 
. However, an attraction effect is not necessarily positive for birds, as it increases their risk of collision with turbines.
Current Knowledge on Bird Displacement by Wind Turbines
We used the data from 68 of the 71 studies to assess the current knowledge on bird displacement, with a total of 286 trials. We excluded three studies from this analysis 
, whose information was duplicated, at least partially, in other works. We stress that, as the considered studies implemented quite variable methodological approaches (see previous section), comparisons across studies and taxa should be conducted with caution.
Overall, negative effects (i.e., displacement) were recorded in 40.6% of the trials, with offshore wind farms presenting a slightly higher frequency of displacement (43.8%) when compared to onshore wind farms (39.3%) (Figure 1
). Still, attraction effects were recorded in 7.7% of the trials, being observed 15.0% at offshore wind farms, contrasting with only 4.9% at onshore environments (Figure 1
). Such difference confirms previous findings that offshore wind farms may act as attractive locations for some species, by providing roosting locations and enhanced food resources as a result of an ‘artificial reef effect’ 
. The lack of effects of wind turbines on birds’ abundance and space use was reported in 49% of the trials (Figure 1
Figure 1. Relative frequency of trials (n = 286) reporting attraction, displacement or no effects of wind turbines on bird space use or abundance, per wind farm type. The bar width represents the number of experimental trials performed per wind farm type.
There is evidence of displacement, but also attraction, for the most studied groups. Displacement effects were reported for the majority of the trials involving Gaviiformes (100%), Anseriformes (68.2%), Suliformes (66.7%), Accipitriformes (48.7%) and Falconiformes (50.0%). Although not prevalent, attraction effects were observed mostly in Falconiformes (12.5%), Charadriiformes (11.6%) and Accipitriformes (10.3%); but never observed in Anseriformes, Gaviiformes, Pelecaniformes and Suliformes. On the other end, the lack of effects was prevalent in Pelecaniformes (75%), Passeriformes (66.2%) and Charadriiformes (55.1%) (Figure 2). Galliformes had a similar frequency of trials with evidence of displacement (48.5%) and with the absence of effects (45.5%). We highlight that Falconiformes, Gaviiformes and Pelecaniformes were studied in a low number of trials/studies, so their results should be considered preliminary.
Figure 2. Relative frequency of trials (n = 286) reporting attraction, displacement or no effects of wind turbines on bird space use or abundance, per birds’ group. The bar width represents the number of experimental trials performed per each birds’ groups.
The spatial extent of the effects of wind turbines on birds’ space is highly variable across taxa and studies. Birds could be displaced from the turbines proximity up to mean distances (± standard deviation) of 116 ± 64 m in Anseriformes, 248 ± 103 m in Passeriformes, 474 ± 213 m in Accipitriformes, 2517 ± 5560 m in Charadriiformes, 4557 ± 4340 m in Galliformes, and 12,062 ± 6911 m in Gaviiformes. Displacement effects also resulted in lower abundances near the wind energy facilities, with mean relative reductions of 40 ± 24% in Passeriformes, 54 ± 15% in Anseriformes, 56% in Galliformes, 59 ± 18% in Charadriiformes, 59% in Falconiformes, 59 ± 18% in Accipitriformes, 61 ± 24% in Suliformes and 71 ± 20% in Gaviiformes. It is however important to note that, although the magnitude of the variations may seem relatively similar across taxa, the limits of the wind farm area, i.e., the area assumed to be under turbine influence and for which the variation was estimated, varied greatly across studies.
The negative effects of wind farms on bird abundance and space use are frequently reported in the literature, affecting species from different habitats, guilds and phylogeny. However, a large number of studies found no effects or even attraction effects, to a smaller extent, even within the same taxa. The lack of consistency and clear patterns regarding the effects across and within birds’ groups suggests that displacement is probably a species-specific issue and dependent on birds’ age and life-cycle, as well as local features and on the wind farm characteristics 
, similarly to what happens with birds’ collisions with wind turbines 
. Still, the wide variability of methodological protocols applied in the studies may also explain the wide variability of the results.
Research on birds’ displacement is fairly recent and it is steadily increasing. However, we detected geographical, taxa and temporal biases. This research topic is still overlooked in regions where the wind industry is already implemented, such as South America, Asia and some parts of Africa, namely South Africa. Still, a small number of bird groups are the more frequently targeted by researchers (Accipitriformes, Galliformes, Charadriiformes, Anseriformes and Passeriformes), particularly during the breeding period. Sensible life-cycle periods as migration are still understudied.
In terms of the design of future research, we believe long-term studies are crucial, as only 14% of the studies sampled 10 or more years after the beginning of operation. It is possible that disturbance caused by wind farms, or attraction effects, are temporary, as continuous exposure over time may increase tolerance or reduce risk perception, causing habituation to the infrastructure 
. Furthermore, we consider that a better understanding of this topic may be achieved with an investment in the quality of the experimental design of future studies. Robust experimental designs are still a minority among the literature and relevant data is frequently missing, such as a description of the wind farm characteristics or the quantification/extent of the displacement effect.