the total contamination of fukushima - daiichi nuclear power plant ( f1npp ) accident was about a tenth comparing to that of the chernobyl accident , and a hundred times to that of the windscale accident [ 1 , 2 ] . most of the radioactive materials dispersed to the pacific ocean by the dominant westerly wind occurring at the time of the explosions and release ( vent ) . the rest was deposited mainly towards the northwest terrestrial area of about 1,000 km from the f1npp , on the highland of high biodiversity [ 2 , 3 ] ( fig .  1 ) . ground contamination has been detected as far as 250 km southwest of the chichibu mountain range . vegetation , land use by man , terrain , distance and decontamination activities ,  are all factors related to the f1npp accident that are expected to affect free - living wildlife such as aphids , butterflies , birds , and trees . exposure to free - living wildlife species is likely to be highly variable , depending on their ecology , behavior and by chance . japanese bush warblers ( cettia diphone ) found in the highest contamination district ( akaugi ) had strong contamination on their feathers in 2011 . we have been monitoring the radioactivity level of the environment and of japanese bush warblers in the region . 1.location of f1npp , several cities , towns , and the main survey locations in fukushima and chichibu , japan . the small map indicates the outline of detected radioactively contaminated areas resulting from the f1npp accident . the bottom figure shows the topology profile from f1npp on the pacific coast ( right ) to the west for 55 km , close to abukuma river ( left ) . location of f1npp , several cities , towns , and the main survey locations in fukushima and chichibu , japan . the small map indicates the outline of detected radioactively contaminated areas resulting from the f1npp accident . the bottom figure shows the topology profile from f1npp on the pacific coast ( right ) to the west for 55 km , close to abukuma river ( left ) . the altitude is magnified about five - fold on a relative scale . the most contaminated area , with total radioactive cesium fallout of > 3000 kbq / m , expands 30 km to the northwest from the f1npp through the northern part of the abukuma mountains   ( fig .  1 ) . the landscape consists of a mix of various types of forest , agricultural fields and small residential lands . there are various species of wildlife in the area , including japanese monkey ( macaque fuscata ) , boar ( sus scrofa ) , copper pheasant ( syrmaticus soemmerringii ) , among many other species . forest covers 70% ( 9750 km ) of fukushima prefecture , and is thus the most dominant habitat within the area contaminated by the f1npp accident . the forest and soil have been proved to steadily fix the radioactive cesium and cesium [ 2 , 10 ] . the most contaminated area , with total radioactive cesium fallout of > 3000 kbq / m , expands 30 km to the northwest from the f1npp through the northern part of the abukuma mountains   ( fig .  1 ) . the landscape consists of a mix of various types of forest , agricultural fields and small residential lands . there are various species of wildlife in the area , including japanese monkey ( macaque fuscata ) , boar ( sus scrofa ) , copper pheasant ( syrmaticus soemmerringii ) , among many other species . forest covers 70% ( 9750 km ) of fukushima prefecture , and is thus the most dominant habitat within the area contaminated by the f1npp accident . the forest and soil have been proved to steadily fix the radioactive cesium and cesium [ 2 , 10 ] . japanese bush - warblers were captured with a mist net  in mid august 2011 , and mid july and mid august 20122014 . the net sites were prepared ( by removal of dense bush and grasses ) at akaugi ( 29.631.7 km northwest from f1npp ) , hiruzone ( 20.3 km nw ) , and omaru ( 11.2 km nw ) , namie town , fukushima prefecture , japan . two or more rectrices ( tail feathers ) were removed from each individual for measurement of radioactive contaminants on the feathers . the feathers were placed in contact on a radio - autograph imaging plate ( ms2025 fuji film ) for three ( 2011 ) to seven ( 2013 ) days ( 72168 h ) in a 5c room . the imaging plate was subsequently read using fla5000 ( fuji film ) at a resolution of 100 m . the feather sample dose rates were measured with a germanium semiconductor detector ( gem type , seiko eg&g ) . the air dose rate at breast height around the netting sites was temporally measured with a portable survey meter ( hitachi - aloka ics-311 ) , which indicates the value in sv / h . ten ic recorders ( olympus , ds-750 ) were placed around the area ( the most western point is 20.6 km and the most northwestern point is 32.5 km from f1npp , during late february or early march through september in 20122014 . natural sound was recorded with this equipment early every morning ( starting at around sunrise ) for 23 h. the recorded audio files were manually monitored with headphones , and recording dates and the period of the japanese bush warbler songs and calls were determined . . ltd , es system : providing values in millisievert ) were placed in various microhabitats , including > 15 cm below the ground , on the ground , and at different heights ranging up to 12 m from the ground , at a number of locations ( kamitashiro , yamakiya , kawamata town and tecchiro , akaugi , namie town ) during a period extending from december 2012 to february 2013 ( 12001350 h ) . the geography was a significant factor affecting the habitat and microhabitat contamination , and exposure of wildlife to radiation . overall , the side of mountain peaks and ridges closest to the f1npp received greater fall - out . even structures like a hill or a rock affected the dose rates as measured in situ . for example , the ground measurement at the foot of a small hill facing away from the f1npp , averaged 0.3 sv / h , whereas the ground measurement at the hilltop , facing the f1npp , averaged 4.0 sv / h , in two - month continuous dose - rate observations using glass dosimeter badges in the yamakiya district in 2013 ( fig .  2 ) . the equivalent measurements varied from 12.4 to 30.1 sv / h in the tecchiro district . 
 fig . 2.landscape and representative microhabitat dose rates at kamitashiro , yamakiya , kawamata town , 29 km northwest of the f1npp . the aerial dose rates were measured with glass dosimeter badges ( type es , chiyoda technol . this photo was taken using a motor paraglider , at several hundred meters above the ground ( bizworks corp . typical dose measurement results and microhabitats are indicated in the picture : for example , the minimum and maximum values on the ground , and those close to the average value in air and in several types of forest , agricultural and water environments . landscape and representative microhabitat dose rates at kamitashiro , yamakiya , kawamata town , 29 km northwest of the f1npp . the aerial dose rates were measured with glass dosimeter badges ( type es , chiyoda technol . this photo was taken using a motor paraglider , at several hundred meters above the ground ( bizworks corp . typical dose measurement results and microhabitats are indicated in the picture : for example , the minimum and maximum values on the ground , and those close to the average value in air and in several types of forest , agricultural and water environments . in mid august 2011 , an adult male japanese bush warbler was captured at tsushima district , where the air dose rate measured directly at breast height was 510 sv / h . three other adult males were captured in the akaugi district , where the air dose rates were 2030 sv / h ( fig . the radioactivity of cs and cs was estimated to be above 3000 kbq / m at akaugi and above 1000 kbq / m at tsuhima ( where the birds were captured ) . the contamination of tail feathers ( rectrices ) collected from the three warblers from akaugi were rated as high ( fig . 4 , bird 1 , 1.2 bq of cs and 1.8 bq of cs for 0.015 g feathers ; bird 2 , 2.5 bq of cs and 3.6 bq of cs for 0.026 g feathers ; bird 3 , 12.5 bq of cs , 17.3 bq of cs and 1.4 bq of ag for 0.056 g feathers ) ; these measurements were determined with a germanium semiconductor detector at the university of tokyo . the total feather dose rates were calculated as 117557 bq / g for the three birds . the radioactive material on the feather was strongly fixed , and most of it could not be removed by washing with ethanol or soapy water in the ultrasonic cleaner . feather contamination rates had decreased by 80% ( to about a fifth ) in august 2012 , with further decreases in frequency ( some individuals had no detectable radiocontamination by 2013 ) compared with those collected in 2011 . japanese bush warblers immigrate to this district in late march , as determined by automatic sound recording using ic recorders in 20122014 . usually a male stays in its territory at one location till late summer , and continues to sing and call for the entire day . the feathers collected , which grew the year before , were expected to have remained in a highly contaminated condition for five months . the total external dose for humans was roughly estimated as 50 msv in the akaugi district (= 20 sv / h  24  150 h ) . we need to determine the conversion rate for small birds based on this value to estimate the radiation effects on the birds . one of the three birds captured in akaugi in 2011 had a conspicuous lesion ( that looked like an avian poxvirus lesion ) near its cloaca ; it was not likely to have been a tumor , but more likely was an abscess or a dilatation of cutaneous glands ; histological examination was not performed . similar lesions had been observed on two males of the same bird species at wakayama prefecture , western japan , in summer 2006 ( kumashiro et al . , unpublished data ) . this kind of observation is rare in wild birds in japan , and the probability is currently under research . the observation rate in fukushima ( 1 in 3 ) is significantly higher than that in chichibu mountains during 19892013 ( 0 in 224 adult japanese bush warblers captured by the first author , z = 8.64 , p = 0.013 , exact test  mann it will be necessary to accumulate further comparable data related to this type of lesion . 3.a male japanese bush warbler ( left ) and the lesion observed around the upper tail coverts on a male captured at akaugi on 11 august 2011 . 4.radio images of contaminated rectrices of two male japanese bush warblers captured at akaugi on 11 and 12 august 2011 . strong spots indicate higher contamination on the feathers . a male japanese bush warbler ( left ) and the lesion observed around the upper tail coverts on a male captured at akaugi on 11 august 2011 . radio images of contaminated rectrices of two male japanese bush warblers captured at akaugi on 11 and 12 august 2011 . monitoring of free - living wildlife in their natural habitat in the highly contaminated area of the f1npp accident may have three benefits . first , this may provide helpful information about the real consequences of radioactive contamination in the environment . we assume that the effects of radiation do not independently affect an organism in the contaminated area of the f1npp accident . the behavior of each individual could also change its exposure rate , often by chance , because dose rates vary greatly among the microhabitats . second , the results from studies of wildlife populations may play a role in influencing when and where people decide to return to the homes from which they evacuated immediately following the f1npp accident . wild animals are exposed directly to the environment and may not have any speculation to avoid the radioactive contamination and its influences . we can assume that better conditions for the wildlife , especially vertebrates , should indicate better conditions for human beings , with safety redundancy . a number of species of wildlife , such as warblers , can be indicator species for such speculations . third , there is huge variety among wildlife and in life histories in relation to the environment , intra- and interspecific interactions , etc . we can learn about the various characteristics of radioactivity affecting the organisms in the natural environment . this information has the potential to provide better understanding for future generation of people about ways to deal with radioactivity in the world , which we can not afford to miss . this also means we should study radiation activities , the process and pattern of radiation through organisms in the ecosystem , and how to preserve the biodiversity and the ecosystem services . however , we do not yet know very well enough which wildlife species and how to observe the free - livings . we can search many kinds , according to our own experiences and skills at f1npp accident . the icrp   has listed several wild organisms as candidate reference animals , such as deer , rats , ducks , frogs , bees and earthworms . they also described their standards as provisional ones , and stated that it will be valuable to select other organisms for our references , as the nature of each individual place is unique . the geography was a significant factor affecting the habitat and microhabitat contamination , and exposure of wildlife to radiation . overall , the side of mountain peaks and ridges closest to the f1npp received greater fall - out . even structures like a hill or a rock affected the dose rates as measured in situ . for example , the ground measurement at the foot of a small hill facing away from the f1npp , averaged 0.3 sv / h , whereas the ground measurement at the hilltop , facing the f1npp , averaged 4.0 sv / h , in two - month continuous dose - rate observations using glass dosimeter badges in the yamakiya district in 2013 ( fig .  2 ) . the equivalent measurements varied from 12.4 to 30.1 sv / h in the tecchiro district . 
 fig . 2.landscape and representative microhabitat dose rates at kamitashiro , yamakiya , kawamata town , 29 km northwest of the f1npp . the aerial dose rates were measured with glass dosimeter badges ( type es , chiyoda technol . this photo was taken using a motor paraglider , at several hundred meters above the ground ( bizworks corp . typical dose measurement results and microhabitats are indicated in the picture : for example , the minimum and maximum values on the ground , and those close to the average value in air and in several types of forest , agricultural and water environments . landscape and representative microhabitat dose rates at kamitashiro , yamakiya , kawamata town , 29 km northwest of the f1npp . the aerial dose rates were measured with glass dosimeter badges ( type es , chiyoda technol . this photo was taken using a motor paraglider , at several hundred meters above the ground ( bizworks corp . typical dose measurement results and microhabitats are indicated in the picture : for example , the minimum and maximum values on the ground , and those close to the average value in air and in several types of forest , agricultural and water environments . in mid august 2011 , an adult male japanese bush warbler was captured at tsushima district , where the air dose rate measured directly at breast height was 510 sv / h . three other adult males were captured in the akaugi district , where the air dose rates were 2030 sv / h ( fig . the radioactivity of cs and cs was estimated to be above 3000 kbq / m at akaugi and above 1000 kbq / m at tsuhima ( where the birds were captured ) . the contamination of tail feathers ( rectrices ) collected from the three warblers from akaugi were rated as high ( fig . 4 , bird 1 , 1.2 bq of cs and 1.8 bq of cs for 0.015 g feathers ; bird 2 , 2.5 bq of cs and 3.6 bq of cs for 0.026 g feathers ; bird 3 , 12.5 bq of cs , 17.3 bq of cs and 1.4 bq of ag for 0.056 g feathers ) ; these measurements were determined with a germanium semiconductor detector at the university of tokyo . the total feather dose rates were calculated as 117557 bq / g for the three birds . the radioactive material on the feather was strongly fixed , and most of it could not be removed by washing with ethanol or soapy water in the ultrasonic cleaner . feather contamination rates had decreased by 80% ( to about a fifth ) in august 2012 , with further decreases in frequency ( some individuals had no detectable radiocontamination by 2013 ) compared with those collected in 2011 . japanese bush warblers immigrate to this district in late march , as determined by automatic sound recording using ic recorders in 20122014 . usually a male stays in its territory at one location till late summer , and continues to sing and call for the entire day . the feathers collected , which grew the year before , were expected to have remained in a highly contaminated condition for five months . the total external dose for humans was roughly estimated as 50 msv in the akaugi district (= 20 sv / h  24  150 h ) . we need to determine the conversion rate for small birds based on this value to estimate the radiation effects on the birds . one of the three birds captured in akaugi in 2011 had a conspicuous lesion ( that looked like an avian poxvirus lesion ) near its cloaca ; it was not likely to have been a tumor , but more likely was an abscess or a dilatation of cutaneous glands ; histological examination was not performed . similar lesions had been observed on two males of the same bird species at wakayama prefecture , western japan , in summer 2006 ( kumashiro et al . , unpublished data ) . this kind of observation is rare in wild birds in japan , and the probability is currently under research . the observation rate in fukushima ( 1 in 3 ) is significantly higher than that in chichibu mountains during 19892013 ( 0 in 224 adult japanese bush warblers captured by the first author , z = 8.64 , p = 0.013 , exact test  mann  whitney u test ) . it will be necessary to accumulate further comparable data related to this type of lesion . 3.a male japanese bush warbler ( left ) and the lesion observed around the upper tail coverts on a male captured at akaugi on 11 august 2011 . 4.radio images of contaminated rectrices of two male japanese bush warblers captured at akaugi on 11 and 12 august 2011 . strong spots indicate higher contamination on the feathers . a male japanese bush warbler ( left ) and the lesion observed around the upper tail coverts on a male captured at akaugi on 11 august 2011 . radio images of contaminated rectrices of two male japanese bush warblers captured at akaugi on 11 and 12 august 2011 . monitoring of free - living wildlife in their natural habitat in the highly contaminated area of the f1npp accident may have three benefits . first , this may provide helpful information about the real consequences of radioactive contamination in the environment . we assume that the effects of radiation do not independently affect an organism in the contaminated area of the f1npp accident . the behavior of each individual could also change its exposure rate , often by chance , because dose rates vary greatly among the microhabitats . second , the results from studies of wildlife populations may play a role in influencing when and where people decide to return to the homes from which they evacuated immediately following the f1npp accident . wild animals are exposed directly to the environment and may not have any speculation to avoid the radioactive contamination and its influences . we can assume that better conditions for the wildlife , especially vertebrates , should indicate better conditions for human beings , with safety redundancy . a number of species of wildlife , such as warblers , can be indicator species for such speculations . third , there is huge variety among wildlife and in life histories in relation to the environment , intra- and interspecific interactions , etc . we can learn about the various characteristics of radioactivity affecting the organisms in the natural environment . this information has the potential to provide better understanding for future generation of people about ways to deal with radioactivity in the world , which we can not afford to miss . this also means we should study radiation activities , the process and pattern of radiation through organisms in the ecosystem , and how to preserve the biodiversity and the ecosystem services . however , we do not yet know very well enough which wildlife species and how to observe the free - livings . we can search many kinds , according to our own experiences and skills at f1npp accident . the icrp   has listed several wild organisms as candidate reference animals , such as deer , rats , ducks , frogs , bees and earthworms . they also described their standards as provisional ones , and stated that it will be valuable to select other organisms for our references , as the nature of each individual place is unique . this study was supported in part by a general grant for an agricultural field study by the graduate school of agricultural and life sciences ( gsals ) , the university of tokyo , a 2012 wildlife fund grant provided by wildlife research centre of kyoto university , and a 2013 grant for new technology studies by watanabe memorial foundation for the advancement of new technology . funding to pay the open access publication charges for this special issue was provided by the grant - in - aid from the japan society for the promotion of science ( jsps ) [ kakenhi grant no .