Recent research has shown that the decline in yellow wagtail (Motacilla flava flavissima) populations may be partly attributed to reduced soil penetrability.

Yellow wagtail populations declined by 65% between 1972 and 2006, resulting in the species being placed on the red list of birds of conservation concern in the UK. Research was conducted examining the relationship between the distribution of these birds and a variety of habitat features across 14 intensively cultivated farms in the UK.

The results found that yellow wagtail numbers were most strongly linked to the penetrability of the soil in fields planted with crops. Fields with more penetrable soils supported a greater number of nesting colonies of the birds.

Interestingly, yellow wagtails themselves do not feed by probing the ground for food, instead catching food in flight or picking insects off the surface. However, it is thought that hard soil may reduce the food source of the yellow wagtail’s prey, and therefore have a knock-on effect on yellow wagtail populations.

Another possible reason for the correlation is that yellow wagtails construct their nests by scraping a hollow out of the ground, a practice which could be more difficult in less penetrable soils. Compacted soils may also have worse drainage qualities, decreasing offspring survival rates.

Modern, intensive-farming practices are largely responsible for this degradation of soil quality, with heavy machinery in particular compacting soils and reducing their penetrability. The researchers recommend that soils be protected from further degradation. Key measures include reducing the intensive use of heavy machinery, autumn sowing and irrigation, all of which would lead to benefits for both biodiversity and agriculture. Further research on the relationship between soil permeability and other farmland bird populations is also required.