Interview with Gregorio Moreno-Rueda about the Short-toed Eagle in Spain and it's impact on snake biodiversity

Date of the interview: 20 February 2013

In this interview Gregorio Moreno-Rueda talks about the ecology and conservation of the Short-toed Eagle in Spain and it's impact on snake biodiversity.

Gregorio Moreno Rueda with a big Montpellier Snake

Gregorio Moreno Rueda with a big Montpellier Snake
© Gregorio Moreno-Rueda

Markus Jais: You studied Short-toed Eagles and snakes in the province of Granada. What snake species do occur in your study area and how common is the Short-toed Eagle in that area?
Gregorio Moreno-Rueda: The most common snakes in the study area are the Horseshoe Whip Snake Hemorrhois hippocrepis, the Montpellier Snake Malpolon monspessulanus, the Ladder Snake Rhinechis scalaris, and Natrix snakes (N. natrix and N. maura). Other snakes present in Granada are Smooth Snakes (Coronella girondica and the rare C. austriaca), the False Smooth Snake Macroprotodon cucullatus, and the viper Vipera latasti. With respect to the Short-toed Eagle, it is one of the most common eagles in Granada, where it is during the breeding season.

Markus Jais: What are the most common prey species for the Short-toed Eagle in your study area?
Gregorio Moreno-Rueda: In Granada, the Short-toed Eagle preys mainly on Montpellier Snake, and secondarily on the ladder snake and in minor proportion on the Horseshoe Whip Snake. These snakes constitute the 99% of the snakes consumed, and the 95% of the whole diet. Besides snakes, this eagle may also prey on ocellated lizards, toads and partridges. However, in other places, the preferred snakes may differ. For example, in Greece, the most consumed snake is the Grass Snake Natrix natrix.

Montpellier snake, Malpolon monspessulanus

Montpellier snake Malpolon monspessulanus
© Gregorio Moreno-Rueda

Markus Jais: What are the difficulties studying Short-toed Eagles?
Gregorio Moreno-Rueda: The Short-toed Eagle is easy to see when it is foraging. However, the localization of its nests, in trees, is relatively difficult, especially in comparison with the localization of nests in cliffs as it is common in other species like the Bonelli's eagle.

Markus Jais: How do Short-toed Eagles affect snake biodiversity and species richness?
Gregorio Moreno-Rueda: We suspect that the Short-toed Eagle has a positive role on snake diversity. This eagle preys mainly on the Monpellier Snake. This is a very dominant snake species, which moreover preys on other snakes. Therefore, it is very probable that this snake limits the population size of other snakes, leading to reduced snake diversity. By preying on this species, the Short-toed Eagle is reducing the competition in the community of ophidians (snakes), favoring increased population sizes in other snakes, and thus increased snake diversity. Therefore, in contrast with what one might predict, the Short-toed Eagle very likely has a positive effect on snake biodiversity.

Markus Jais: Does the Short-toed Eagle make difference between poisonous snakes and non-poisonous snakes?
Gregorio Moreno-Rueda: Studies performed in Doñana National Park have shown that raptors avoid preying on plasticine models of snakes with a zigzag pattern pictured in the dorsum, which corresponds with the dorsal pattern of vipers. Moreover, predators tend to attack the head more frequently when facing models mirroring a viper. It should be noted that the most common prey of the Short-toed Eagle, the Monpellier Snake, is venomous, but it is opisthoglyph (rear fanged), and therefore it is very difficult for this snake to inject venom to an eagle trying to kill it.

Ladder Snake Rhinechis scalaris

Ladder Snake Rhinechis scalaris
© Gregorio Moreno-Rueda

Markus Jais: What is the preferred size of the snakes killed by Short-toed Eagles?
Gregorio Moreno-Rueda: This eagle preys on snakes of 700-1000 mm. Small snakes are preyed on less compared to what their availability would suggest, perhaps because they pass unnoticed by the eagle, while the biggest snakes, above one-meter of size, are hunted comparatively less, probably because these snakes are difficult to be caught by the eagle, or even may be dangerous to the eagle.

Markus Jais: How does habit and snake behavior influence prey selection by the Short-toed Eagle?
Gregorio Moreno-Rueda: The capacity of the Short-toed Eagle to catch a snake is strongly influenced by the habitat where the snake lives. Although the Short-toed Eagle nests in trees, it forages on open land, such as grassland and shrubland. In these habitats, snakes are more visible and available for the eagles. And thus, snakes inhabiting these habitats are prone to be eaten by the Short-toed Eagle. Obviously, snakes which are more active at night are less predated on by the Short-toed Eagle.

Markus Jais: Can the Short-toed Eagle live on alternative prey when snakes are rare or absent?
Gregorio Moreno-Rueda: Evidence suggest that it cannot. The Short-toed Eagle may hunt very different snake species across its range. It is a specialist for snakes, but it is not choosy with respect to what snake to eat. However, studies on this eagle strongly suggest that it rarely preys on other things than snakes. Only 0,4% of its prey is not reptile in Granada.

Markus Jais:Does snake species richness affect the breeding success of the Short-toed Eagle?
Gregorio Moreno-Rueda: The abundance of prey undoubtedly affects breeding success in the Short-toed Eagle. However, no study has analyzed the effect of snake species richness. Nonetheless, during the breeding season, snake species richness is related to a higher probability to find a Short-toed Eagle in a place. That suggests that snake diversity probably favors higher breeding success in the Short-toed Eagle, providing a larger availability of prey, especially when the community of snakes is structured in time and/or space.

Lataste's Viper Vipera latastei

Lataste's Viper Vipera latastei
© Gregorio Moreno-Rueda

Markus Jais: Why does the Short-toed Eagle lay only one egg?
Gregorio Moreno-Rueda: That is a good question. There are several species that lay only one egg, but it is rare, because to lay only one egg is a very risky strategy. When the egg is unfertilised or the chick dies as a consequence of diseases, the pair would fail to reproduce that year. A replacement egg would be a good option. But egg production is very costly. Therefore, the most probable explanation is that for this eagle the cost of producing a second egg surpasses the potential benefits of having a replacement chick, because egg production is very costly for this eagle (as a consequence of its specialized diet) or because the probability of failure of the only egg is very low.

Markus Jais: What is the preferred breeding habitat of the Short-toed Eagle?
Gregorio Moreno-Rueda: The Short-toed Eagle nests in trees, but it needs open lands for foraging. Consequently, this eagle needs open countries with forest patches. The optimal habitat in southeastern Spain is open land (mainly pastures or scrublands) with 10% of the surface covered by forests.

Searching for eagles in a Mediterranean landscape

Searching for eagles in a Mediterranean landscape
© Gregorio Moreno-Rueda

Markus Jais: Do Short-toed Snake eagles compete with other eagles like Spanish Imperial Eagles or Golden Eagles for habit, food and nesting places?
Gregorio Moreno-Rueda: In Granada there are no imperial eagles. But where the two eagles coexist it is improbable that they compete. Competition for food is absent, and nesting places (trees) probably are abundant. I studied competition in the case of golden eagles and Bonelli's eagles, which compete for cliffs for nesting and show a marked diet overlap. In spite of this situation, both eagles coexisted when prey diversity was high. Therefore, competition between imperial eagles and Short-toed Eagles should be much more reduced. However, I would not discard intraguild predation.

Markus Jais: Are snakes still persecuted and does this negatively affect the Short-toed Eagle?
Gregorio Moreno-Rueda: Although legislation protects snakes, the truth is that they are still commonly persecuted by rural people. There are no sanctions for killing snakes, and thus impunity is absolute. Consequently, the law is absolutely unsuccessful. Education is the best preventive measure to protect snakes. Perhaps, at this moment, the main threat for snakes, besides of habitat destruction, is road killing, which is exacerbated by the behavior of snakes, which frequently move to roads in order to warm up. One may see a lot of dead snakes on the roads. The fact is that most snakes (except, perhaps Monpellier Snake) are in decline in southeastern Spain. The precarious situation of snakes is presumably having negative consequences for the Short-toed Eagle. However, given that this eagle is a long-lived animal, the problem might be detected at the level of the eagle when it will be very late.

Markus Jais: What should be done to protect snake species and Short-toed Eagles?
Gregorio Moreno-Rueda: Mainly, to protect their habitats. To stop habitat destruction would be the first step. Direct persecution should be stopped by educational programs. The law should be applied. A law that is not applied is meaningless.

Short-toed Eagle, Spain

Short-toed Eagle, Monfragüe, Spain.
© Markus Jais

Markus Jais: How do you see the future of the Short-toed Eagle in Spain?
Gregorio Moreno-Rueda: I am very pessimistic about the future of most of nature in Spain, as Spanish politicians have no interest in protecting our wildlife (they only are interested in growing their bank accounts). In the case of the Short-toed Eagle, one should consider that it is a very specialist animal. It eats almost exclusively snakes! Specialist organisms are especially prone to extinction, and in the case of this eagle, its prey (snakes) are top predators. Top predators are found in low densities and for this reason they are also prone to extinction. Moreover, snakes are in decline, a situation contributed to by social and cultural persecution. The future of snakes is black, and thus, the future of the sort-toed eagle is black too. In Spain, while I have to fill hundreds of requests and reports for any study with wild animals, the truth is that the killers of snakes and eagles enjoy an absolute impunity. Moreover, habitat destruction in Spain is catastrophic. The global economic crisis is the best thing for the protection of nature in Spain, as it has stopped habitat destruction. It is very painful for me go to an old place, and find new buildings that I didn't remember seeing before. And finding that where I saw such a strange or beautiful animal, now I only can see a block of concrete.

Markus Jais: What was your most amazing experience with Short-toed Eagles?
Gregorio Moreno-Rueda: Gregorio Moreno-Rueda: To be in the field, look to the sky and see a flock of Short-toed Eagles going to Africa. Bye, bye, see you the next year, babies!

More information

See Gregorio's paper Snake species richness and the distribution of the Short-toed Eagle (Circaetus gallicus) in the book Ecology and conservation of European forest-dwelling raptors.