Nocturnal bird surveys
When should this method be used?
Thermal imaging is a powerful tool for recording how birds use habitats at night, but watching those behaviours is difficult in low light environments. Under the surveys of breeding birds methodology, we recommend using one of the six survey visits to visit during the evening, at which time detectability may be different to first thing in the morning (for example, for grey partridge or barn owl).
However, this visit/s may not be enough to quantify how birds are using habitats after dark (for example, golden plover or woodcock on farmland).
Here we suggest some consideration for using thermal imaging to survey bird behaviour on specific habitats at night as a supplementary survey visit to compliment a more general standard breeding, or non-breeding survey schedule.
When nocturnal surveys may be required
The circumstances in which a nocturnal survey might be required, and indeed the specific methodology, should be informed by the behaviour of priority species, and habitats present, on a survey site. For example:
- During non-breeding surveys on arable land, where inland wintering waders (golden plover, woodcock, redshank, curlew etc.) have been recorded on the land, either as part of survey efforts or as identified at desk study;
- In areas where migratory geese are likely to roost and forage in arable fields overnight, for example, pink-footed geese in the Northwest of England, or barnacle geese in parts of Scotland etc;
- Where nightjar, or certain species of owl, quail or stone-curlew have been identified on the survey area, either by survey efforts, a desk study, or where habitats and location indicate likely occupancy;
- On coastal and maritime environments which may hold breeding populations of procellariforms (Manx shearwater, European or Leech’s storm petrel), which visit breeding colonies at night.
Survey effort will be determined based on the priority species being targeted. When determining the survey effort, the surveyor should ensure that the survey effort chosen will be justifiable at the reporting stage.
A single visit during an appropriate period may be appropriate for some situations (such as determining if an arable field that is not associated with any designated sites supports wintering woodcock), or several visits may be required if nocturnal usage is a major consideration for the scheme assessment (for example if determining if some arable fields play a regular role in supporting qualifying features of a Special Protection Area at night).
The time at night and the conditions on site will both markedly effect the mechanical capability of any imager, but also, will influence the behaviour (and subsequent detectability) of organisms using land at night.
Timing should therefore be when nocturnal species are likely to be active (often well after dusk, but variable between species) but also, when ground/air temperature has dropped to a point at which detection increases.
During winter months, this is likely to be of negligible importance, but during the summer, thermal surveys should take place in the early hours of the morning, rather than immediately after dusk.
Planning survey dates
The number of survey visits should reflect data collected at desk study, and priority species potentially present on site.
In many cases, one visit may suffice in determining the relative importance of a site for a species (for example, woodcock) given that survey coverage (that is, assuming surrounding fields of similar habitat are also covered is extensive.
Conversely, several visits may be required if nocturnal usage is a major consideration for the scheme assessment (for example if determining if some arable fields play a regular role in supporting qualifying features of a Special protection Area at night) or if a potential priority species could be present over an extended period (e.g. autumn and spring migration).
Prior to starting the survey, record the date, start time, time of sunrise / sunset, temperature (°C), wind (Beaufort scale 0–12), cloud cover (Okta 0–9) and rain (as a brief description e.g. dry, light drizzle). The end time and closing weather conditions (if there has been a substantial change during the survey), should also be recorded.
A transect should be walked with pre-determined vantage point locations that allow open areas to be scanned.
The number and approximate distribution of priority species will be recorded on a map (typically a 1:25,000 scale map), using the BTO two-letter codes .
Additional consideration should be given to any hazards identified in the site-specific risk assessment. Under no circumstances should observers enter into a hazardous situation in order to complete a survey. Instead, surveys of inaccessible or hazardous habitats should be undertaken from a vantage point or other safe distance, and any limitations should be recorded in the report, with justification and a consideration of potential impacts. Where appropriate, these hazards should be reported and any site specific risk assessment changed in light of this new information.
Care should also be taken to avoid causing damage to arable crops or disturbance to livestock present within a survey area.
Surveyors should be mindful of the zone of influence of the development scheme (i.e. the distance beyond the redline boundary that a scheme may still have an impact on breeding birds, either during construction or operation). Surrounding habitat (outside of the redline boundary) should also be surveyed to a reasonable distance – particularly land immediately surrounding the site itself, and any nearby potentially valuable habitat which is likely to be impacted by construction and/or operation.
All species encountered on the site or adjacent land should be reported.
The approximate locations of priority species should be plotted on a site map together with behavioural notation where appropriate. Counts of secondary species should be recorded separately and based on the highest number of each species in a distinct location, being careful to avoid repeat counting of individuals.