METHODS The status of India’s birds was assessed primarily using data uploaded to eBird, with an end date of 31 May 2019. Birdwatchers upload their observations to eBird in the form of checklists, which may be marked ‘complete’ or ‘incomplete’. A complete checklist is a list of all species observed during the period of birding, and therefore implicitly includes information about which species were absent (or, more accurately, undetected). Using this, it is possible to calculate an index of abundance, termed ‘frequency of reporting’ for any species. This frequency of reporting is the proportion of all complete checklists in which a species was reported. By following changes in frequency of reporting over several years, an index of trend in abundance can be estimated.
This report presents three indices of status for each species for which there is sufficient data: 1. Long-term Trend: the proportional change in frequency of reporting in 2018 when compared with the frequency before the year 2000; 2. Current Annual Trend: the average annual change in frequency of reporting during the last 5 years (2014 to 2018); 3. Range Size: the area covered by all those 25 × 25 km cells that are occupied by a species within the country during the last 5 years. A year is considered to be a ‘migratory year’ from 1 June to 31 May the next year because this spans a single migratory season for many bird species in India.
The statistical model for index of trend in abundance
Frequency of reporting
median list length
Frequency of reporting
For each time period, the frequency of reporting of (a) a checklist of median effort was (b) averaged across all grid cells and (c) averaged across all months
Month
List Length (number of species in the list)
(a) a checklist of median effort 40
(b) averaged across all grid cells
(c) averaged across all months
