The 7th Hong Kong Sparrow Census organized by The Hong Kong Bird Watching Society has recorded a 36% rebound in the population of Eurasian Tree Sparrows (sparrows) in Hong Kong, totaling around 287,000 individuals. While the upturn may bring us a sigh of relief, we are yet to figure out the possible reasons behind and it is still premature to identify the long-term population trend. Hence, it is imperative that we perform continuous monitoring to gather more data for analysis.
Rebounding population, with recreational land reporting substantial growth
As in previous years, the 2022 census was held in May and included 88 one-kilometer routes covering all 18 districts in Hong Kong. According to the latest estimation, there are about 287,000 sparrows, a rebound from last year's record-low of 211,000. Recreational, residential and village zonings recorded more sparrows, with recreational land (i.e. parks) showing notable growth in particular. Sham Shui Po continues to host the highest density of sparrows out of all districts, followed by Tuen Mun and Wong Tai Sin.
Four more common urban bird species were also included in this year's census. We have recorded 68,000 Spotted Dove, 23,000 Oriental Magpie-Robin, 80,000 Red-whiskered Bulbul and 3,000 Red-billed Blue Magpie. While the numbers of Spotted Doves and Red-billed Blue Magpie remain stable, populations of Oriental Magpie-Robin and Red-whiskered Bulbul show signs of increase.
With population trend unknown, natural fluctuations cannot be ruled out
Sparrow population was numbered at around 300,000 when the census was first taken in 2016. This common bird species underwent population fluctuations over the past seven years and hit a record low of 210,000 individuals in 2021, before recovering this year. Weather, food availability and disease are some of the most basic factors impacting population sizes of bird species. Trying to establish a rationale for the fluctuations, we've looked into the temperature, humidity and rainfall both one week before and during the survey period year-over-year. However, no significant correlation was found between the weather data and the population figures. That is to say, the cause of rebound is still unknown.
Eurasian Tree Sparrow is listed as "Least Concern" on the IUCN Red List. However, trends between 1980 and 2013 showed that populations in Europe underwent a moderate decline. BirdLife International therefore estimated that the global population trend of the Eurasian Tree Sparrow is decreasing. While in Hong Kong, the Sparrow Census is still in its early stages and there is insufficient long-term data to detect the population trend. Ups and downs of the figures in recent years may indeed be attributable to natural fluctuations.
Gathering data through continuous monitoring to figure out the population trend
Continuous monitoring is a must if we want to understand species' population trend. Easton Write, a quantitative ecologist, once examined populations of more than 800 vertebrate species and found that over 70% of those species required at least 10 years of continuous monitoring in order to detect significant trends. Long-term monitoring also offers insights into the impact of environmental changes on bird species. Study has shown that global warming causes birds to lay eggs earlier than usual, potentially leading to the asynchronism between nestling period and the period of high prey availability, and thus reduces the survival rates of nestlings.
Bird research in Hong Kong is still at a very preliminary stage in comparison with other countries and regions. Hence we shall continue to keep an eye on the population trends of sparrows and other common urban birds. As the Sparrow Census plays a part in promoting citizen science and enriching public understanding of urban ecology, it is our wish that the COVID-19 pandemic will soon wind down so that open recruitment of surveyors can be resumed next year.
