Bird Atlas has long been used as an effective tool worldwide to monitor the condition of bird population. It is a grid-based sampling method to collect information of numbers, species and breeding status of birds. Information obtained could be a good reference for:

• Identifying species and areas of conservation importance,
• Understanding habitat preferences of a particular species, as well as
• Monitoring temporal changes of population of a species and species diversity of a habitat when comparing with the results of previous atlas surveys.

It has been long since the last breeding atlas in 1993-96 and winter atlas in 2001-05 in Hong Kong, and the results were published on The Avifauna of Hong Kong and on website, respectively. A three-year atlas surveys with both breeding atlas and winter atlas were planned to be carried out in 2016-19.

To be comparable to the past atlas surveys, Hong Kong was divided into 1,591 1 km² grids within the territory with the same grid reference. Each grid was surveyed once in breeding season (mid-March to mid-July) and once in winter season (December to February) during the study period. A surveyor had to:

• Conduct an atlas survey in early morning when birds are usually more active
• Survey a grid in around 2 hours’ time, while survey time would be shorten proportionally at coastal grids with less land coverage
• Plan a feasible route so that all major habitats in grid would be visited
• Record his/her name, date, grid ID (i.e. location) and survey time
• Record all birds seen and heard with their species and abundance specified
• Record breeding evidence if any was observed, according to guidelines below:

Code	Breeding Evidence
CONFIRMED
NB	Nest building or adult carrying nesting materials.
NE	Nest building or adult carrying nesting materials.
NY	Nest with young seen or heard.
NU	Used nest or eggshell found.
FY	Recently fledge young or downy young.
FS	Adult carrying fecal sac.
FC	Adult carrying food for young.
AE	Adults leaving or entering, indicating occupied nest.
DD	Distraction display or injury feigning.
PROBABLE
S	Singing males.
D	Display (between a male and a female or two males), courtship (including courtship feeding) and copulation.
A	Agitated behavior or anxiety calls of an adult.

Remarks: The survey period of breeding season 1993-96 was longer from March to September and the only occurrence and breeding status of each species were marked (i.e. without accurate abundance).