Anyone who has watched birds for several years in a woodlot or other local habitat becomes aware of variations. Some species are present in some years, absent in others, and populations change in abundance between years. Other populations vary about a long-term average that seems to be relatively stable. The abundance of wintering Stonechats (Saxicola torquata) or tits in Europe, for example, changes from year to year, apparently in response to weather and food conditions during the previous winter that affect overwinter survival and the reproductive output of the birds during the summer. Long-term trends in abundance are not evident, however (Dhondt 1983, van Balen 1980, Klomp 1980). In other cases, variations are more episodic. The winter of 1962–63 was the harshest in southern England since 1740, and populations of the Wren (Troglodytes troglodytes) were reduced by 75%, the Song Thrush (Turdus philomelos) by 50% (Williamson 1975). A prolonged drought in the Sahel (sub–Saharan Africa) between 1968 and 1974 devastated populations of wintering palearctic migrants, leading to sharp reductions in their breeding numbers. Abundances of Whitethroats (Sylvia communis), for example, decreased by as much as 77% in some parts of England between 1968 and 1969, and similar reductions were recorded elsewhere in its breeding range (Winstanley et al. 1974, Berthold 1973). When the drought broke in 1975, Whitethroat abundances rebounded, increasing by 60% in 1976 (Batten and Marchant 1977).

Over a longer time period, populations may show systematic trends in abundance.