The population of Tree Sparrows on the floodplain of Afon Tywi is estimated at 100–400 pairs. This is a minimum of 30% of the Welsh popualtion but probably closer to 60–70% of this red–listed species. It is the only population of more than 50 pairs remaining in Wales. Between 60 and 80 pairs nest every year in nest boxes. Pairs make between one and three attempts between April and August. Productivity is slightly lower than in the rest of UK (3.43 young raised per successful attempt compared to 4.28) but the population seems stable.

The tie between the Tree Sparrows and the flood plain has been investigated and is believed to be the invertebrates associated with the standing and slow flowing water found in ditches and ox–bow lakes. The Tree Sparrows feed their pulli on insects. An analysis of faecal samples showed that 23% of the diet of the pulli whilst in the nest is mosquitoes; a further 9% is midges and dragonflies.Work on two groups of nest boxes on and off the floodplain shows that when the boxes are 13 metres vertically and 50 metres horizontally away from the floodplain, breeding success drops dramatically.

All pulli are ringed and recently have been colour–ringed. The limited recapture and resighting data gathered to date suggests that individuals are highly sedentary, with the longest distance travelled so far being 1.8km.

A winter feeding programme is carried out at several sites along the floodplain and is important in maintaining the level of the popualtion. In addition householders close to the river who feed their garden birds are important in supporting the Tree Sparrows.