Report Reveals Key Factors Behind Hen Harrier Recovery

Starting around 2018, the Hen Harrier population in England experienced a sudden and rapid increase. This resurgence coincided with the trial of a new management approach, sparking a crucial question: What exactly is driving this change?

A recently commissioned report by the British Trust for Ornithology (BTO) for Natural England looked into this puzzle using the power of population modelling. This blog post looks at the key findings, revealing the most likely explanations for the Hen Harrier's encouraging recovery.

Jump to Section

Time For a New Approach
What Could Be Driving the Increase?
What the Models Revealed
Putting It All Together: A Multifaceted Recovery
Looking Ahead
Listen to Audio Summary

Time For a New Approach

Efforts to help the Hen Harrier population recover have been ongoing for years, but without significant success until recently. This led to the consideration of alternative approaches with two key methods trialled in England:

Diversionary Feeding: Providing alternative food sources to breeding Hen Harriers to reduce their need to prey on Red Grouse chicks. While Harriers readily take this food and grouse predation can be reduced, its impact on grouse populations hasn't been proven, limiting its appeal to moor managers.
Brood Management: Under license, eggs or chicks are removed from nests in areas with high Harrier densities and reared in captivity before being released back into the wild in the same general area. This aims to reduce the number of grouse taken by parent Harriers feeding their young. This approach has been controversial but was trialled over seven years (2018-2024).

Interestingly, the number of recorded Hen Harrier breeding attempts in England almost quadrupled during this brood management trial period, from 14 in 2018 to a peak of 54 in 2023, before declining to 34 in 2024. This dramatic increase is what prompted the BTO to investigate the underlying causes.

What Could Be Driving the Increase?

The report authors considered several potential mechanisms that could explain this rapid population growth:

Increased Productivity: Could Hen Harriers be breeding more successfully? Perhaps due to the direct effects of brood management (more chicks fledging from managed nests) or diversionary feeding, or even indirect effects like reduced disturbance.
Increased Survival: Are more Hen Harriers, especially young birds, surviving? This could be due to reduced illegal killing linked to the presence of brood management, or other factors like better environmental conditions.
Increased Settlement: Are more breeding-age birds present in England attempting to breed and being detected? This could involve a larger overall population, a higher proportion of birds choosing to breed (less "skipping"), or even more breeding attempts surviving long enough to be recorded.

To explore these possibilities, the researchers built population models, essentially sophisticated simulations that project how a population might change under different scenarios. These models used various pieces of evidence, including detailed records of Hen Harrier nests in England from 2018 to 2024 and a recent study on Hen Harrier survival in Britain.

What the Models Revealed

Productivity

The models suggest that the direct impact of brood management and diversionary feeding on overall population growth was relatively limited. While brood-managed nests were successful, and diversionary feeding provided an alternative food source, the study found that a large proportion of the chicks fledging from these nests likely would have done so even without the interventions.

The difference in average annual productivity with and without interventions was small, suggesting only about one extra chick fledged per five breeding attempts due to these efforts.
In the peak year of 2023, even with 24 chicks released from brood management and 14 fledged from diversionary fed nests, the models estimated this resulted in only about 10-11 additional fledglings across England.
Given the lower survival rates of young Harriers, this translated to a projected difference of only a single breeding pair by 2023-2024, far less than the observed increase.

The report highlights that headstarting (similar to brood management) is typically most effective for species with very low nest survival rates, which isn't the case for Hen Harriers in the absence of interference.

While there might have been indirect benefits from the trial, such as a potential reduction in disturbance at nests due to increased observer presence, the models didn't support a significant productivity uplift as the primary driver of the population increase. It's also important to note that some nest destruction was still reported during the trial.

Survival

While the study couldn't definitively determine if overall survival rates have actually improved in England during the trial, the modelling strongly indicates that improvements in survival, even partial ones, are likely to be a key factor in the observed population increase, possibly in combination with other changes.

Settlement

The concept of settlement, which in this study encompasses the rate at which breeding-age birds start recorded breeding attempts and the survival of these attempts to the point of detection, offers another compelling explanation. This is where the idea of "skipping" breeding and early, undetected nest failures comes into play.

Hen Harriers are known to sometimes skip breeding in a given year, particularly if conditions are poor.
It's also possible that some breeding attempts fail very early on and are never detected by surveyors.

The models explored a range of hypothetical "skip rates" and found that changes in settlement could significantly influence population growth.

The researchers suggest that a reduction in disturbance during the pre-breeding period could lead to more birds attempting to breed and more of these early attempts surviving to be detected. The availability of brood management might have inadvertently reduced the incentive for such pre-breeding disturbance.
The models also showed that a higher rate of subadults (one-year-old birds) breeding could lead to faster population growth.

Given that the English Hen Harrier population is measured by the number of observed breeding attempts, an increase in settlement rates could lead to a rapid rise in recorded numbers, even if the overall population size wasn't increasing as dramatically. The substantial decline in breeding attempts between 2023 and 2024 also hints at the potential for a high skip rate in response to adverse conditions.

The report concludes that increased settlement rates, potentially linked to reduced disturbance, could have contributed to the observed population increase, but this likely requires some improvement in survival rates and the presence of non-breeding birds or immigration to fully explain the rapid growth.

Putting It All Together: A Multifaceted Recovery

The population modelling undertaken in this study provides valuable insights into the recent Hen Harrier trends in England. The key takeaway is that the rapid increase in breeding attempts is unlikely to be solely due to the direct effects of brood management on productivity.

Instead, the most plausible explanations point towards improvements in either survival rates or settlement rates, or a combination of both.

It's important to note that the lack of a similar increase in neighbouring populations like Wales and southern Scotland suggests that large-scale environmental factors are unlikely to be the primary driver in England.

Looking Ahead

While the recent increase in Hen Harrier breeding attempts in England is encouraging, the report also highlights important caveats. There's still a lack of direct data on crucial factors like survival rates, adult skipping, and subadult breeding within the English population during the trial period.

The significant drop in breeding attempts in 2024 serves as a stark reminder that the recovery remains vulnerable to both natural and human-linked adverse conditions.

The findings of this study underscore the complex interplay of factors influencing Hen Harrier populations and the ongoing challenges in balancing conservation goals with other land management practices. Further research will be crucial to ensure the continued positive trajectory of this iconic bird of prey in the English landscape.