Abundance of Bangladesh's bird-life makes it an ornithologist's paradise. Of the 525 recorded species, 350 are residents. Among them are bulbul, magpie robin, common game birds, cuckoos, hawks, owls, crows, kingfishers, woodpeckers, parrots and myna. A wide variety of warblers are also found. Some of them are migrants and appear only during winter. The migratory and seasonal birds are predominantly ducks.

The average annual rainfall varies from a maximum of 5,690 mm in the northeast of the country to a minimum of 1,110 mm in the west. The groundwater, however, provides adequate storage to compensate for annual variations in rainfall and stream-flow. The average annual discharge from the three major rivers - 35,000 cumec - is second only to the Amazon. The rivers earn' billions of tons of sediment-load every year.

The word 'Monsoon' is derived from 'Mousum', the Arabic word for season. It can also mean 'weather'. The monsoon is a seasonal wind, which starts blowing from the Indian Ocean from early June and continues till mid-August.15 June corresponds to the first of Ashar in the Bengali calendar, which is the day the Rainy season is supposed to commence.