PARTRIDGE wildflower blocks increase local linnet population at Diemarden demonstration site, Germany
As the English and German names imply, the linnet or Hänfling, has a strong preference for seeds rich in oil, such as linseed and hemp. A linnet’s diet is predominantly made up of seeds; it is one of the few passerines that feed their nestlings an exclusively herbivorous diet.
Linnet in a PARTRIDGE wildflower block at demonstration site Diemarden (Photo: Lisa Dumpe)
Linnets feed on a vast variety of plants commonly found on uncultivated field margins, fallow plots, and grassland. Shifts in land use and agricultural intensification have led to the loss of uncultivated land and semi-natural grassland, habitats which traditionally provide seed-bearing plants and therefore food for linnets. Sadly, among finches, linnets are most dependant on these open landscapes, and the loss of these habitats has caused drastic declines in linnets across Europe, with populations declining by approximately 60% since the 1980’s.
Coinciding with the decline in linnet numbers has been an alarming decline in the variety of plant species in linnet nestling diets, pointing to the need for farmland habitat that provides diverse plant species to help linnet populations recover (Moorcorft et al., 2006, Newton, 1967).
In the PARTRIDGE project, our habitat measures, tailored to the grey partridge, aim to provide year-round cover and chick-food insects for partridges. This blog looks at this partridge-tailored landscape from the perspective of a seed-eating herbivorous farmland bird.
Increase in linnets at the Diemarden demonstration site.
Despite the negative trend across Europe and Germany, at the Diemarden demonstration site our monitoring shows that the number of linnet territories has clearly increased (see Figure 1). The actual number of linnet territories are likely to be higher than that recorded, as the transects used for monitoring only cover a proportion of the demonstration site.
Figure 1: Number of breeding territories of linnets from 2017 to 2022 at the PARTRIDGE demonstration site Diemarden (green) and its reference site Bilshausen (yellow)
Masters student, Sophie Groos, investigated whether habitat measures implemented for the grey partridge are a key factor in the increase in linnets at the site. Sophie looked closely at the habitat use of linnets at the Diemarden demonstration site. She walked transects across the demonstration site, repeating them weekly, for a total of 16 times from late March to early July in 2022. All linnet observations were mapped, then separated into foraging/non foraging.
PARTRIDGE wildflower blocks were the preferred habitat type for linnets on the transects where data was recorded.
The observations of foraging linnets across the landscape are clearly concentrated at the PARTRIDGE wildflower blocks (magenta, Figure 2). The wildflower blocks were found to be the most attractive habitat type for foraging linnets, whilst cereal fields were strongly avoided.
Figure 2: Observations of foraging linnets (circles, colours describing the habitat of foraging) are concentrated at PARTRIDGE flower blocks (magenta) and a few weedy fields (strawberry, red) and oilseed rape (yellow). Habitat types were recorded along the transects
Looking at the phenology of habitat use, wildflower blocks were preferred consistently throughout the observation period, whilst other habitats were only used at certain times. In June and July, the oilseed rape fields became an important food resource as well. Half-ripened rapeseeds, specifically, attracted linnets for a few weeks in June. During this period, when rapeseeds are abundant, linnet food sources were not limited. However, when rapeseed is not abundant, linnets must search further for food often foraging up to 2 km around the nest. The critical time for linnet chicks is in late spring before rapeseed becomes available and again in July for late broods. Linnets prefer to feed their nestlings with half ripened seed, so through the summer a succession of different plant species that set seeds at different times provides the best food resource.
The PARTRIDGE wildflower blocks, with a wide variety of plants, provided food sources for linnets for the longest period compared to other habitat types. This makes PARTRIDGE blocks crucial to nestling survival and the recovery of linnet on farmland.
What plant species do linnets favour?
We were not able to identify the plant species which the linnets visited most often. It was extremely difficult to observe directly where the birds foraged. Either they foraged partly on the ground, searching for seeds dropped from the seed heads or they were hidden by plants when harvesting seed. Sophie did manage to identify the plant species where linnets had been observed feeding. Sophie was able to collate a list of plants important for linnets during the breeding season (Table 1) by combining her observations with a literature review of the plant species that linnets prefer. This showed that weeds contribute more to the diet of linnets than the plants sown in PARTRIDGE wildflower blocks.
Table 1. Plants that linnets prefer. In general, linnets prefer plants in either the daisy (Asteraceae) or brassica (Brassicaeeae) families
Dandelions (Taraxacum spp.), Thistles (Cirsium and Carduus spp.), Cleavers (Galium aparine), Shepherd’s purse (Capsella spp.), Sowthistle (Sonchus spp.), Chickweed (Stellaria media), Docks (Rumex spp.), Plantains (Plantago spp.)
PARTRIDGE Wildflower Block sown
Teasels (Dipsacus fullonum. -likely to be dropped seeds in early spring), Cornflower (Centaurea cyanus)
Interestingly, linnets were observed feeding on the seeds of elm trees (Ulmus spp.). Elms are the tree with the earliest seed setting in the year, during May, when seeds of weeds are sparce. This underlines the need for alternative food sources to later maturing plants even more important.
Our conservation lesson
This study highlights just how valuable weeds are in the farmland landscape for seed-eating birds like the linnet. With agricultural intensification being largely responsible for the decimation of farmland bird populations it is more important than ever before to be tolerant of weeds!
Although weeds were preferred by linnets, the species planted in PARTRIDGE wildflower blocks were important food sources for linnets. The tolerance of some weed species in the blocks provided food resources beyond just the species sown in the blocks, with the result that PARTRIDGE flower blocks were a significant factor in increasing linnet territories at the Diemarden demonstration site.
Wildflower block management approaches recommended by the PARTRIDGE project also make the habitat more favourable to linnets. Annual cultivation of just half of a wildflower block supports annual and biennial plants (investing more in seed production than perennials), whilst leaving the previous years seed heads in the untouched part of the block – ideal for linnets and other seed-eating farmland birds. This partial cultivation developed during the PARTRIDGE is obviously perfect for linnets!
PARTRIDGE wildflower block at the Diemarden demonstration site. See half has been cultivated whilst the sencond half (back) has been left, providing food for seed-eating birds (Photo: Francis Buner)
To find out more about the Diemarden demonstration site visit this interactive StoryMap:
Moorcroft, D., Wilson, J.D., & Bradbury, R. B. (2006) Diet of nestling Linnets Carduelis cannabina on lowland farmland before and after agricultural intensification. Bird Study, 53, 156-162.
Newton, I. 1967. The adaptive radiation and feeding ecology of some British finches. Ibis 109: 33–98.
Written by Echard Gotschalk, University of Göttingen, edited by Beth Brown, PARTRIDGE placement student.