The aim of the program is to register the seasonal changes in bird population. The collected data can be used primarily in the study of bird migration and in the monitoring of bird population. 

The first phenological observation network was established by the Estonian Naturalists’ Society in the early spring of 1922. Until 1948 only early spring observations were registered, but later on the project was widened to cover the observations during the whole year. Today, the observers collect data about the arrival of the birds in spring and about their departure in the autumn, about the arrival and departure of the winter birds, but also about that of the cranes, geese, brant geese, as well as about other phenological phenomena. 

The observation data has been systematised and published 14 times in the publication series ‘Abiks Loodusvaatlejale (Nature Observer’s Aid)’. A. Lint, I. & L. Rootsmäe and H. Veroman have contributed most energy to the compilation of these publications. Some of these can still be obtained from the EOS office. 

A. Ader and J. Keskpaik analysed the observation data from the point of using them in the monitoring projects, and the results suggested optimism. Thanks to the consistent sequence of the observations, this project enables to observe the long-term alterations in the Estonian avifauna. In 1994–1998, the results of the project were used in the Estonian national program of biological monitoring. 

Over the course of the project of ornitho-phenological  observations, the number of people participating in it has varied a lot. In the beginning, about 60 consistent observers participated. The number rose drastically, achieving the participation of 234 people in 1978. At present, the Society receives the annual observation results from approximately one hundred observers. The methodology of ornitho-phenological observations consists a considerable amount of freedom – there are no strict rules about when and where one should go outdoors. The collecting of such data requires only one thing – regular trips to nature.