Red grouse and climate change

Key findings

• Severe weather at the wrong time of year (usually during the early chick period) can reduce grouse productivity.
• We can see the effects of short-term changes in weather in grouse breeding dates.
• Climate change may affect grouse productivity through modifying effects such as parasitism, predation and food quality.

Until recently, observation of the effects of climate change in the uplands has focused on the likely expansion or contraction of some species of conservation concern such as moss campion or dotterel. However, climate change could be affecting red grouse. It has been suggested that wetter conditions could increase the pick-up of gut worm parasites leading to reduced grouse productivity or even that red grouse become heat stressed above the 8ºC mean annual isotherm, and that this isotherm is moving north and uphill. Although unproven, these hypotheses suggest ways in which climate could affect grouse. Chance weather events affect grouse productivity when these occur at critical periods in the lifecycle, such as when chicks are present. But these chance weather events may also be entrained in longer trends in climate. 

We attempted to account for this when we investigated whether grouse have begun to nest earlier. We collected nesting data from radio-tagged red grouse for 15 years and these data suggest that there has been a trend towards earlier hatching in Strathspey (see Figure 1) with earlier and later hatching years being linked in turn to cooler and warmer springs. But by back-calculating hatching dates using chick weight in July, we found no discernible trend over 20 years, even though these two data sets were correlated over the last 15 years (see Figure 2). This correlation, together with sampling periods and chick survival patterns that support such analysis, suggests that this longer term trend is robust.

Figure 1. Hatching dates of observed nests in Strathspey, 1991-2005

Evidence from directly observed nests suggests that grouse clutches have been hatching earlier in Strathspey over the last 15 years. These hatch dates are strongly associated with mean April temperature. Hatch 1 = 1 May; hatch day 32 = 1 June

Figure 2. Predicted hatching day calculated from chick size in July

Evidence obtained by back-calculating from chick size in July suggests that there is actually little discernible pattern in hatch date over the last 20 years. Hatch day 1 = 1 May

However, there are also likely to be indirect effects of climate change: for example through changes that affect tick host number and distribution. In conjunction with milder weather that promotes a longer tick-feeding season, climate change may be one of the forces resulting in an increase in ticks biting grouse chicks in Scotland over the last 20 years (see Figure 3).

Figure 3. Proportion of chicks carrying at least one tick

The proportion of chicks in a grouse brood with at least one tick on it on our study moors has increased by 55% in the last 20 years.

Although there is some evidence that weather events affect grouse, we cannot show that climate change has directly affected grouse biology. We do know that British moors have been subject to varying levels of grazing, predation and grouse parasites, all of which can reduce grouse productivity and that it is possible that climate change may magnify these effects. However, we find that grouse productivity and density can be at their highest on our southern low altitude moors, which are intensively managed, whereas on moors which are neither the highest nor the most northerly within Scotland or England. This is counter to what you would expect if climate was of over-riding importance and suggestive of the continuing importance of intensive moorland management for successful grouse production.