Data on swan arrival, departure, and population size on the Asadokoro tidal flat, Aomori Prefecture, Japan, from 1956 to 2010

The arrival and departure dates and the daily maximum populations of migrating swans (Cygnus cygnus) on the Asadokoro tidal flat, Hiranai town, Aomori Prefecture, Japan, were recorded by elementary school students for more than 50 years between 1956 and 2010. The Asadokoro tidal flat, which lies along the coast of Mutsu Bay, has been designated a National Special Natural Monument, known as “The swans of Kominato and their migration grounds.” This long history of observation unfortunately came to an end with the closure of the elementary school in 2012. If analyzed together with data on environmental factors, such as temperature changes or the effects of avian influenza, this dataset could provide a potentially valuable source of information, and consequently, future secondary use of the data is anticipated.


Specifications
Biological Sciences Specific subject area Ecology, Behavior Type of data

Value of the Data
• Data on fluctuations in migratory bird populations were collected over a period of over 50 years, based on observations performed by elementary school students. • The study site and swans are designated as special natural monuments of Japan [1] . It is argued that a reduction in the number of swans in Hiranai town may have led to a decline in the tourism value of the local area. The data obtained can be used to influence administrative decisions at national and regional levels. • The data are also of potential value for analyzing changes in the number of swans flying to this site in relation to environmental factors, such as temperature [2] , avian influenza [3 , 4] , and human activities [5 , 6] . They may also prove useful for estimating the impact of migratory birds on the surrounding environment, such as vegetation [7 , 8] . For example, the arrival of migratory birds at this site has a substantial effect on the distribution of seagrass [1] .

Data Description
The data presented herein relate to the size of the swan population on the Asadokoro tidal flat and the dates on which the swans arrived at and departed from the study site. Table 1 shows the dates from 1956 to 2009 on which the first swans arrived at the Asadokoro tidal flat in autumn, together with the number of individuals observed on each date, whereas Table 2 shows the dates from 1957 to 2010 (mainly in spring) when the last swans departed from the site, together with the number of individuals observed on each date. Table 3 presents the monthly maximum numbers of observed swans per day from 1960 to 2010, and Table 4 summarizes the timing of data acquisition. The raw data file was deposited in Mendeley data ( http://dx.doi.org/ 10.17632/g9tcw92bgy.1 )
Although the survey was conducted for the final time in the fiscal year 2010, the data for that year were excluded from the dataset presented herein, owing to the lack of certain population data. In March 2012, Asadokoro Elementary School closed down, and the monitoring of swans that had continued for more than 50 years ended. To date, there has been no resumption of similar monitoring.

Ethics Statement
Not applicable.

Declaration of Competing Interest
The authors declare that they have no competing financial interests or personal relationships that influenced or could be perceived to have influenced the work reported in this article.