For many species, breeding performance changes as an individual ages. So in order to properly monitor and manage animal populations, it is important to understand how different species age. But this is easier said than done! A recent paper published in the Journal of Animal Ecology used a unique dataset from a reintroduced population of white‐tailed eagles in Scotland to study age‐ and sex‐specific trends. Lead author Dr Megan Murgatroyd (FitzPatrick Institute of African Ornithology) is here to tell us about sea eagle aging, raptor-gurus, reintroduction work, and ‘toy boy’ relationships! 

Despite being revered in early history and once widespread in Britain, like many other predators White-tailed Eagles have a long history of persecution and human conflict. In 1918 the last known British White-tailed Eagle was shot rendering them extinct. Today, 100 years on the story has fortunately changed. Unique partnerships between multiple agencies, landowners and Government have seen this species come back home.

The reintroduction of White-tailed Eagles began in 1975, when four chicks were taken under permit from nests in Norway and released on the island of Rum. From the outset these releases were only possible due to the dedication of a small team of people. Sir Ian Newton, John Love and Roy Dennis played a major role and were assisted by the cooperation of multiple organizations, not least NCC and RSPB, but also the Norwegian Air Force, who transported the chicks back to Rum. This was before I had even entered the world but John Love tells the details of the first efforts in his book The return of the Sea Eagle and all of the names above now hold a raptor-guru status to me.

These releases and the ones that were to follow can be broadly split into two phases. During the first release phase (1975 – 1985) 82 birds were released on the island of Rum and the second release (1993 – 1998) saw 58 birds being released on mainland Wester Ross. All of the eagles that were released and many of those which fledged in the wild following the first successful breeding attempt in 1985 were marked in some way; either colour rings or patagial tags. Monitoring of this population of marked birds commenced immediately and we owe thanks to many eager fieldworkers, volunteers and members of the Scottish Raptor Study Group for the follow up of these birds. Now, the RSPB have curated a unique long-term dataset of the breeding performance of known age and known origin eagles through their lives. This dataset has already been used in several studies, including examining the breeding dispersal, population demography and future range modeling of the species. Until recently much of this was spearheaded by Richard Evans. Although he sadly died before the completion of our latest research, we have no doubt that he would have been thrilled to see the continued improvement of our understanding of White-tailed Eagles.

Age-specific breeding performance in long-lived animals is usually difficult to measure owing to the timescale and effort involved in the marking and long-term monitoring of individuals. In long-lived birds, this has been achieved most often for seabirds whose natal philopatry and breeding site fidelity aid data collection. I’m not saying that the seabird guys have got it easy, in fact they have often shown extreme dedication to monitoring of isolated island populations, just take a look some of the work on albatrosses and you will see what I mean (e.g. Froy et al. 2017). Nevertheless the literature available on age-specific breeding performance in long-lived raptors is relatively sparse. The only similar example I know of for a long-lived raptor is work by Millon et al. 2011 on Tawny Owls, which demonstrates how early-life experiences, as measured by vole abundance in the year of birth, can shape first year survival and also that the rate and onset of reproductive senescence can differ between males and females.

Thus the data collected from the reintroduced and subsequent wild-bred eagles has given us a unique opportunity to investigate how breeding performance changes through an individual’s lifetime and how these changes might be impacted by early-life experiences and inter-sex differences. Early-life experiences are known to affect development and performance in later life for a range of species, including humans. In our example we wanted to understand if the rearing and release conditions affected age-specific breeding performance as this information could have important implications for population viability in reintroduction programs. We also wanted to understand sex-related differences in breeding performance, which might impact population structure and possible mate changes in later life.

It is fortunate that the early-life experiences of White-tailed Eagles, as measured by their origin (i.e. which reintroduction phase they came from or if they were wild-bred), did not appear to impact the rate that breeding performance increased in early-life, thus suggesting that rearing techniques were not detrimental to this early-life performance. Nor did this rate of increase in performance appear to differ between male and female eagles. However, after peaking around middle age, male breeding performance began to rapidly decline, while female performance did not. Why so? Well, possibly male eagles sustain more wear and tear than females during competitive interactions to secure breeding opportunities and defend the territory. Males are also responsible for most of the food provisioning during the breeding season which is likely to incur costs.

In fact, in our study no chicks were ever produced by a male eagle more than 23 years old. While female eagles were still able to produce chicks at 26 years old. Therefore it follows that these old females must be paired with males at least four year younger than themselves. Indeed there was an overall trend for older females to be paired on average with younger males. Cougars! That’s all I have to say 😉

Today western Scotland has a viable population of breeding White-tailed Eagles and they can also be found in eastern Scotland and Ireland. Public perception of them has largely changed and words like magnificent and majestic are usually used to describe them. Although persecution of raptors remains a problem in parts of the UK and new threats like wind turbines are emerging, the stage has been set for British White-tailed Eagles to stay in the bird guides not the history books. We can only hope that we can now continue to work together to provide a landscape in which eagles can safely make their way across their former breeding range as far as southwest England.

More Info:

Murgatroyd, M., Roos, S., Evans, R., Sansom, A., Whitfield, D. P., Sexton, D., Reid, R., Grant, J. and Amar, A. (2018) Sex‐specific patterns of reproductive senescence in a long‐lived reintroduced raptor. Journal of Animal Ecology. DOI: 10.1111/1365-2656.12880