Bird communities wintering in the mosaic-like farming landscapes of SW Poland were studied to assess drivers of intra-seasonal changes in numbers and habitat associations of birds. During two severe winters (November–March) the complete area search method was applied to count birds and link their occurrence with weather and habitat in six plots (320 ha in total). A modelling approach was used to test birds' responses to environmental factors at community and species levels. These farmlands were inhabited by diverse and dense populations of wintering birds, including a significant proportion of species of conservation concern. Bird numbers revealed decreasing trends, with winter- and species-specific fluctuations, affected in particular by snow cover. The lowest population indices were recorded in mid-winter (February) and remained low until mid-March. Field margins (6.6% of the total area) supported 35.6% of individuals and 55.0% of flocks. The preference for field margins over other farmland habitats was particularly prominent during severe weather conditions. Our results suggest that during severe winters, complex farmlands are important areas for birds, enhancing their known importance to breeding birds. Intense snowfall in mid-winter rather than temperature drops lead to immediate population declines, aggravating the effects of natural food depletion. Adverse weather keeps populations at low levels until mid-March, despite the influx of spring migrants. Behavioural adjustments to winter conditions elicit the characteristic habitat distribution of birds, exposing the crucial importance of non-cropped landscape elements. In particular, the variety of field margins significantly contributes to the persistence of internationally important populations of farmland birds in Poland.