Grouse chick diet

Key findings

• Grouse populations on more nutritious heather moorland are the most productive.
• Bigger eggs result in better chick survival. 
• Early hatching chicks survive better than late ones.
• Chick diet affects growth rates and this is linked to survival.

Diet affects grouse production in two ways: first during the spring when hens are coming into breeding condition; then during the period when chicks grow prior to fledging. We have been investigating whether improving diet is a potential management tool for increasing grouse breeding success.

We reviewed 20 years of autumn grouse density data from across Scotland. We found that long-term changes in abundance were related to food quality. Moors with high levels of nitrogen in heather are the ones on which grouse numbers have been increasing, whereas the opposite is true for nutrient-poor

Figure 1. Autumn grouse densities compared with nitrogen levels in heather

Autumn grouse densities are increasing (the average annual rate of change over 20 years is greater than 0) in areas where nitrogen levels are highest

Understanding the way this works could lead to better management. Early work in the 1960s and '70s showed that the condition of the hen before breeding affects clutch size, with more eggs produced on moors where the heather has most available phosphorus. We wanted to know whether maternal and chick diet also affected chick survival. To examine this, we conducted field experiments and analysed breeding data from other studies.

Between 1999 and 2004 we monitored grouse hens and chicks on two estates in Strathspey recording hatching date and success from 219 nests and the egg size and fate and weight of chicks from 1,773 eggs. As on many moors, the clutches we monitored hatched over a large range of dates (13 May to 26 June: first clutches only) suggesting that individual hens come into breeding condition at different times. The broods that hatched 10 days earlier than the average for that site survived 10% better than broods that hatched at the normal time. We also found that chick survival increased with egg size. However, any benefits to grouse production may be off-set as larger eggs are also associated with smaller clutches.

Although maternal condition does affect chick survival, it is likely that factors affecting chicks after hatching will be most important in determining survival to fledging. Although predation and disease are important, diet determines the growth rate and survival. For the first month after hatching, chick diet contains 5% invertebrates on average. Where there are more invertebrates chicks grow quicker. This is important because fast-growing chicks survive better than slow-growing ones: models based on the 97 chicks we caught from 23 broods suggest that five-day-old chicks of around 18g had an 18% chance of surviving to 50 days old, whereas ones of 30g had a 45% chance of surviving to this age. As well as invertebrates, nutritious heather is also essential. Grouse chicks choose shoot tips high in nitrogen and phosphorus and their survival is better when heather quality is improved with fertiliser (see Figure 2). A fertile moor also supports more invertebrates.

Figure 2. Average brood size (chicks per hen) at 50 days old on sites where heather has been fertilised and on unfertilised sites

	Fertilised
	Unfertilised

Note: Fertilisation is an experimental technique and we do not suggest this as a management option as there is no persistent improvement in grouse breeding success.

Although many of these effects of diet are small and must be seen in the context of major influences like predation and disease, this work highlights the importance of maximising habitat quality. Future work will focus on best practice for restoring and managing nutritious heather stands and creating invertebrate-rich hotspots.