What do they eat and does it affect their survival?
- Chick diet varied between study sites in the North Pennines and Perthshire.
- In the North Pennines study area, breeding success was better in years when there were more sawfly larvae.
- A mosaic of land management, providing different swards and vegetation structures, may provide the best environment for black grouse brood rearing.
Black grouse used to be common across the UK, but both their numbers and the area in which they live have declined in recent decades. Numbers have dropped by 30% in the last 40 years. Studies have shown that some habitat improvements can help increase numbers, including along moorland fringe.
The diet of black grouse chicks for the first few weeks of life consists mainly of invertebrates (for example beetles, spiders, ants, etc), before their diet changes to consist mainly of berries and shoots as adults. Therefore, the availability of abundant suitable invertebrate food is important in determining chick survival. In the North Pennines, black grouse breeding success has been low, but this has improved with a reduction in grazing pressure. It is thought that improvements in the invertebrate community may have helped chick survival.
What they did
Two areas were studied – one in Perthshire, and one in the North Pennines. In Perthshire, the study area was made up of wet heath moorland, part of which had been afforested 15 years previously. In the North Pennines, the study area was grassland on sheep farms, next to heather moorland. Breeding success is generally higher in Perthshire.
Within these sites, black grouse broods were located and radio-tagged. This allowed roost sites to be identified at night, and dropping samples were collected the next morning after the brood had moved away. From these samples, invertebrate fragments were identified and the relative amounts of each food species eaten were calculated.
The different types of invertebrates available in the environment were studied at each site using sweep nets. For each sample, a large net was swept vigorously through the vegetation 25 times. The contents were emptied into a plastic bag and all invertebrates over 2mm were grouped and counted. The availability of invertebrates in the environment was compared to those making up the chick’s diet, to study which of the available species the chicks were choosing to eat.
Sweep netting was also used to assess the availability of moth caterpillars and sawfly larvae in the North Pennines, to look for a relationship with black grouse breeding success. Breeding success was assessed at the North Pennines sites by searching brood-rearing areas for females with chicks in late July/early August using pointing dogs, and counting the number of chicks per female. By this time, chicks are 6-8 weeks old.
What they found
Some differences were found in the proportion of invertebrate groups making up the chick diet:
||Percent of total invertebrates in dropping samples
The percentage of other invertebrate groups in the chick’s diet did not differ, however diet did vary year to year.
Total invertebrate abundance was the same at both study sites, but there were large differences in the makeup of the invertebrate populations. In Perthshire, both moth caterpillars and ants were 50 times more numerous than they were in the North Pennines, and beetles and spiders/harvestmen were 2-3 times more numerous.
Sawfly larvae are a preferred food, which are selected by black grouse chicks. Although they made up only 3% of the invertebrates found in sweep net samples at both sites, they made up 20% of invertebrates eaten by chicks in Perthshire and 67% at the North Pennines site.
An association was found between the abundance of sawfly larvae and breeding success in the North Pennines. In years when there were more sawfly larvae, black grouse bred more successfully in terms of both brood size, and number of females with a brood.
What does this mean?
This study demonstrates that the availability of certain invertebrate species – both sawfly larvae and moth caterpillars – seems to be beneficial to black grouse chicks. Managing habitats to increase the number of these larvae may help support black grouse population recovery and range expansion. For example, promoting an environment in which the plants they feed on can thrive may increase their number.
Adjustments in grazing intensity may affect the proportion of plants that these larvae prefer. Low-intensity grazing may encourage bog myrtle and bilberry (host plants of moth caterpillars), whereas higher-intensity grazing may increase the coarse grasses and rushes eaten by sawfly larvae. A patchwork of different management strategies may provide the most suitable wider environment for black grouse brood rearing.
Read the original abstract
Baines, D., Richardson, M., & Warren, P. (2017). The invertebrate diet of Black Grouse Tetrao tetrix chicks: a comparison between northern England and the Scottish Highlands. Bird Study, 64: 117-124.