The Editor’s Choice for issue 55:3 is by Associate Editor, Des Thompson. The selected article is Reintroducing endangered raptors: A case study of supplementary feeding and removal of nestlings from wild populations by Miguel Ferrer et al.

In his classic book Population Ecology of Raptors (1979), Ian Newton (one of the co-authors of this featured paper) concluded: ‘…in the absence of human intervention almost every aspect of the natural population ecology of a given raptor species can be explained in terms of food: its dispersion over the countryside, its density in different areas and in different years; the extent of its numerical fluctuations; its breeding seasons and breeding rates; and its seasonal movements and dispersals.’

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Two of the authors of the Editor’s Choice article, Ian Newton and Miguel Ferrer. Image courtesy of Ian Newton

It is no wonder, therefore, that some of the most important raptor conservation successes have involved supplementary feeding in order to boost breeding success and survival. Other techniques include captive breeding or rearing, manipulating habitat quality and nest site availability, removing human-related pressures such as disturbance and toxic contaminants, and reintroductions and reinforcements from other populations. Supplementary feeding works best when used judiciously and when mindful of the nutritional and population ecology of the target species (e.g. Ewen et al., 2015). It is typically deployed when and where natural food is scarce and thus limiting populations, or where it provides a safe alternative to what might be available in the wild but contaminated with toxic chemicals and veterinary drugs. However, the technique is not without its detractors. Several studies have pointed to ill-advised uses of the technique where factors other than food are limiting the target species’ productivity or survival, or where artificial increases in food can increase the prevalence of non-breeding ‘floaters’ which can suppress overall population productivity. Indeed, the technique has been included in a list of dogmatic approaches in conservation.

How refreshing then to have an outstanding new study of the effects of supplementary feeding in the Spanish imperial eagle (Aquila adalberti) and bearded vulture (Gypaetus barbatus). Drawing on two long-term data sets and decades of conservation management and research experience, Miguel Ferrer and his team provide key insights to the development of an effective conservation tool.

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Spanish imperial eagle. Image courtesy of Miguel Ferrer

Working on Spanish imperial eagles (one of the world’s rarest eagles) in Sierra Morena and Bearded vultures in Aragon, the team combined field work, careful analyses and modelling to explain population increases of the eagles from 10 pairs in 2001 to 91 pairs in 2015, and 15 territories occupied by vultures in 1988 to 67 in 2012. In both species, supplementary feeding was used over several years to increase productivity. This allowed for the removal of eggs or nestlings for reintroduction programmes.

Their earlier studies have shown that breeding performance is density dependent in relation to habitat variability, with some high quality territories showing consistently high productivity, but others consistently low productivity.  When populations are low, it is mainly the high quality territories that are occupied. However, as numbers rise, more of the poor territories get occupied, resulting in an overall lowering of the production per pair in a density-dependent manner. Differences in quality between territories are due mainly to variation in food availability, degree of human disturbance, and mortality factors.

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Miguel Ferrer and Virginia Morandini attaching a harness and transmitter to a Spanish imperial eagle. Image courtesy of Ian Newton

In this study, Ferrer’s team manipulated food availability as one of the most straightforward techniques for improving the number of young produced in their study populations. They put out supplementary food for the final four years of their study populations (for the eagles, 1-2 rabbits each day were put on ledges several hundred metres from the nest during the breeding season; for the vultures, 15-18 kg of bones from a slaughterhouse were deposited daily more than 1 km from nests during October-March, to improve the physical condition of breeders during the pre-breeding and nesting period, ending some 30 days after egg laying). Financially, the team costed their techniques, and compared them with the budgets for captive rearing programmes for the two species.

The findings are striking and highly important. Supplementary feeding deployed on poor quality territories increased the mean annual productivity by 60% for the Spanish imperial eagles, and by 188% for the bearded vultures. Birds in poor quality territories with low productivity levels had stronger responses to supplementary feeding than birds in territories with higher levels of natural productivity. They concluded that the technique could be deployed most effectively when or where there is an episodic collapse in the main prey (e.g. through disease), and in poor quality territories in a high-density population (often referred to as ‘sinks’), where extra young may then be used for reintroduction or reinforcement programmes elsewhere.

In financial terms, the team found that taking youngsters from supplementary-fed territories was around eight times cheaper than producing young from a captive breeding programme and took ten years less.

This is a superb example of expert scientists understanding the population ecology of their species through painstaking, long-term field studies, and providing pragmatic solutions to a conservation problem.

Read the full Editor’s Choice article, Reintroducing endangered raptors: A case study of supplementary feeding and removal of nestlings from wild populations (free for a limited time) in issue 55:3 of Journal of Applied Ecology.