The present study showed that electromagnetic radiation did not show any direct effect on the abundance and habitat use of sparrows in Guwahati city. Besides, the sparrows were found to be influenced by other complex ecological variables (e.g., distance to the nearest market place, number of food shops, number of rolling shutter in shops, distance to nearest green patch, percent green cover and plant diversity, percent built-up area and percent green cover). However, the magnitudes and impacts of microhabitat variables mentioned above may vary among the specific needs of the life history traits of both the sparrow species (for details see Nath et al. 2019). It was observed that on spatial and temporal scales, ecological variables contributed significant variation to explain the habitat use of sparrows in the study area.
In general, increasing urbanization has positive association with electromagnetic radiation. The present scenario in most of the cities around the world is clear-cut─ the area with higher developmental activity is also high in electromagnetic radiation because of the growing demand for telecommunication and mobile phone services. Henceforth, the model output of our spatial data set showed a complex interaction of percent built-up and EMR since the percent built-up area partially characterizes the degree of urbanization. The interaction model (House Sparrow ~ Built-up * EMR) showed a positive association, which implies that the sampling locations with more built-up and area with high EMR measures have a large number of House Sparrow sightings (Fig. 5). Balmori and Hallberg (2007) showed that the mean density of sparrows corresponds to the lowest electromagnetic field intensity areas. Despite that, they also come up with the discordant statement that no sparrows would be expected to be found in an area with field strength > 4V/m. However, we recorded House Sparrow in the places where EMR reached up to 7.81 V/m, and even we recorded 1–17 individuals of sparrows (5.0Mean ± 0.87S.E) within 4.00-7.81 V/m range. On the other hand, Tree Sparrow preferred habitat with minimum urbanization (Zhang and Zheng 2010), and its number was significantly influenced by greenness (% green cover, plant diversity) and type of housing structure (primarily residential apartment/building). It was found that EMR had no significant role in governing the habitat use of the Tree Sparrow; even at the different levels of EMR.
In our study, EMR was positively correlated with the degree of urbanization (r = 0.53, p = 0.0001) and negatively with greenness (r = -0.51, p = 0.0002). Subsequently, the negative association of Asian Pied Starling and Spotted Dove could be directly associated with the degree of urbanization. Both the species observed to occupy areas with low level of urbanization in the study area. Sivakumar et al. (2006) also found that the density of Asian Pied starling was more in village edge forest compared to other habitat types in Buxa Tiger Reserve, West Bengal, India. Therefore, solely based on the EMR measures the cause and effect relationship on avifaunal abundance may not be studied in the field condition. Subsequently, our analysis showed that EMR has no direct impact on the distribution and habitat use of urban birds, which collectively depending on the microhabitat needs of the species.
Previously, several studies have examined electromagnetic radiation risk on birds, but outcomes were inconsistent, and contributing factors were incoherent (Cucurachi et al., 2013). Most of the studies carried out on the effect of electromagnetic radiation on birds were laboratory investigation— on chicken (Gallus domesticus) and Japanese quail (Coturnix coturnix subsp. japonica). Cucurachi et al. (2013) review 113 peer-reviewed articles and concluded that two third of reviewed studies, the effects of EMR was reported at low as well as high dosages, the very low dosages can be compared with real field conditions. However, the studies lack of standardization and an inadequate number of observations bounds the prospect of generating results from an organism to an ecosystem level (Cucurachi et al. 2013). Among the 26 peer-reviewed studies on birds, 70% of them concluded possible significant adverse effects of RF-EMF (S5 Fig). However, most of these studies (60%) were carried out in the laboratory either at the embryo or egg stages of development (Cucurachi et al. 2013). On the other hand, only five field studies claim a possible significant effect of RF-EMF on birds’ breeding density, reproduction, or species composition.
The study design and methods used in each of these above mentioned studies have been discussed in details in Table 7. None of these studies, considered possible interaction of electromagnetic radiation with other life history needs (ecological variables) of the studied species. Henceforth, finding out the reliable conclusion is difficult in studies without the other controlling factors (i.e. covariates). For urban birds, the sampling approach, and the intensity of urbanization and its complex association with EMR, could play a major role in the statistically significant cause and effect relationship due to sampling artifact, which has no relevance biologically. Apart from that, most of the studies were carried out in the laboratory showed significant negative impact of EMR. The duration of exposure of EMR was twice as high on average studies, which showed impact of the radiation than that of studies which did not show any effect (Cucurachi et al. 2013).
Moreover, studies that were carried out in the laboratory condition showed the significant negative impact of EMR. The duration of exposure of EMR was twice as high on average in studies, which showed the impact of the radiation than that of studies that did not show any effect (Cucurachi et al. 2013). They have also mentioned, the laboratory studies carried out in the past exposed targets to the high level of MW-EMF, which probably determined the uncontrolled rise in temperature affecting the exposed systems. The information and results on the effects of radiation gathered in laboratory studies may need to be guardedly handled due to the steep nature of the laboratory solutions adopted. The conditions applied in the laboratory studies, in fact, do not always reflect real conditions of exposure, and at times it is essential to carefully evaluate the plausibility that biological systems exposed to RF-EMF could likely translate into ecologically relevant effects.
The WHO confirms that to date the accepted health effects due to high dosage ascribable to RF-EMF are caused by temperature rise (van Deventer et al., 2011). On the contrary, in the present study in Guwahati city, and the other studies which have been carried out in the field underexposure of such a low level of EMR (< 9 V/m) were unable to generate the thermal effect. The measured electric field strength values were far below the required to produce heat as low as 0.5°C (i.e., 10 mW/cm² or ca. 194 V/m; Bernhardt, 1992). However, few studies on the non-thermal effects of electromagnetic radiation provide evidence about the hypothesis of non-thermal effects of electromagnetic radiation on the brain physiology (Cucurachi et al. 2013) Subsequently, they also found that about two-thirds of the reviewed studies on the ecological effects of EMR were reported at high as well as at low dosages. The very low dosages are compatible with actual field situations and could be found under environmental conditions. However, the non-thermal effects on biological tissues still under investigation, although calcium efflux and free radical production are among the candidates of the possible mechanism responsible for the non-thermal effects of EMR. Then again, results are still not conclusive, and there is still some uncertainty about the low dosages and non-thermal effects applied in some studies which did find an effect (Cucurachi et al. 2013).
In conclusion, the studies carried out on the impact of EMR on birds in field condition, is limited. Moreover, the negative correlation strength reported by the previous studies between sparrow density and EMR may simply attribute to sampling artifacts rather than factual causation. In addition, large scale survey through questionnaire across India with the help of citizens did not able to make conclusion on any correlation in the decline of house sparrows with cellphone tower radiation (Rahmani et al. 2013). The present study also emphasized that the complexity of ecological processes involved in the cause and effect relationship should be understood in multidimensional means. Therefore, site covariates need to define and quantify along with EMR measures before concluding remarks on the impact of radiation emitted by cell phone towers.