This week I was interested to see what sounded like some really cool new science reported in the Daily Mail, with the print headline of “Seagull gangs ‘are pricking on women.’” From briefly skimming the article it seemed that the much maligned blight of British seafronts had developed a novel foraging tactic, with “targets more likely to drop food when pecked.” The eponymous words of Jurassic Park rang through my head “clever girl.” But on a closer read it seems that this new revelation, reported as if it were fact, is just the opinion of two lifeguards, Ash and Becca. So is Mail is just reporting non-fact as fact once more?

 

On the face of it yes, as there doesn't appear to have been any research into this and it’s just someone’s view. But seagulls are smart and other smart birds have the ability to remember the faces of humans for decades, so it seems perfectly sensible to suggest that seagulls could tell men and women apart. But what about using some crazy foraging behaviour?

 

It turns out that there is actually precedent for targeting a subset of a population to steal food from. Fork-tailed drongos, amazingly acrobatic birds that have a dual impact on other species: they can act as a sentinel system, allowing others to spend more time foraging, and alerting them when predators are near but they can also make false alarms and steal food. So, there is a both a cost and a benefit to having these duplicitous birds around. I’ve blogged about some of the cool interactions of drongos before. Drongos follow pied babblers, with smaller babbler groups paying more attention to their sentinel behaviour than larger ones. The interesting thing is that within pied babbler groups some individuals respond more to drongo alarm calls than others, with juvenile birds more likely to respond. Drongos will actually preferentially target these juvenile birds, making their stealing more efficient. This is really similar to what the Mail suggests is happening on British beaches.

 

However, it still needs to be researched. So if anyone wants to fund me to go to the beach to collect data and run experiments…..

 

The above research is published in Ridley & Child (2009) Behav Ecol Sociobiol 63:1119-1126

 

Other cool drongo papers:

 

Flower & Gribble (2012)

Flower et al. (2014)

Flower (2011)

So after finishing my PhD I went back the kalahari to help Tom Flower out with some cool work investigating deception tactics and learning in fork-tailed drongos. While I was there I also had a chance to help his honours student, Bruce, out with his research investigating the relationship between drongos and sociable weavers. The kalahari is full of cool inter-species interactions (I'll put a couple of papers below). Bruce's work has just been published in Proceedings of the Royal Society, I have put the abstract and a link to the paper below. The work was really fun to do and highlights how conflict and cooperation have to coexist in both and ecological and evolutionary setting, and that this can lead to cool things evolving.

Baigrie, Thompson & Flower (2014) Interspecific signalling between mutualists: food-thieving drongos use a cooperative sentinel call to manipulate foraging partners. Proc R Soc, 281:20141232
Interspecific communication is common in nature, particularly between mutualists. However, whether signals evolved for communication with other species, or are in fact conspecific signals eavesdropped upon by partners, is often unclear. Fork-tailed drongos (Dicrurus adsimilis) associate with mixed-species groups and often produce true alarms at predators, whereupon associating species flee to cover, but also false alarms to steal associating species' food (kleptoparasitism). Despite such deception, associating species respond to drongo non-alarm calls by increasing their foraging and decreasing vigilance. Yet, whether these calls represent interspecific sentinel signals remains unknown. We show that drongos produced a specific sentinel call when foraging with a common associate, the sociable weaver (Philetairus socius), but not when alone. Weavers increased their foraging and decreased vigilance when naturally associating with drongos, and in response to sentinel call playback. Further, drongos sentinel-called more often when weavers were moving, and weavers approached sentinel calls, suggesting a recruitment function. Finally, drongos sentinel-called when weavers fled following false alarms, thereby reducing disruption to weaver foraging time. Results therefore provide evidence of an ‘all clear’ signal that mitigates the cost of inaccurate communication. Our results suggest that drongos enhance exploitation of a foraging mutualist through coevolution of interspecific sentinel signals.
http://rspb.royalsocietypublishing.org/content/281/1791/20141232.abstract

Other cool interspecies interactions:

Pied babblers and scimitarbills:
Ridley, Wiley & Thompson (2014) The ecological benefits of interceptive eavesdropping. Functional Ecology, 28: 197-205

Drongos and pied babblers:Flower (2011) Fork-tailed drongos use deceptive mimicked alarm calls to steal food. Proc R Soc, 278:1548-1555

I know this ‘coverage’ is a bit late but I was on a remote Scottish Island when this paper came out and as Tom is the first of my friends to get a paper in Science I obviously had to write about it. And as you will now know, I know the authors and so my slant on this paper will probably be biased in a positive way.

So the paper is:

Flower et al (2014) Deception by flexible alarm mimicry in an African Bird. 344: 513-516

Firstly you’ll need a bit of background on fork-tailed drongos (Dicrurus adsimilis), hereafter drongos. They are extremely agile birds that live in Southern Africa and are very adept at hawking small flying insects. In addition to their aerobatics they are vocal mimics that can copy the calls of other species, and they use these calls to make false alarm calls (i.e. when there are no real predators) to scare individuals from other species (host species) when they have food, then they fly down and steal the food item. Drongos primarily spend time with sociable weavers (Philetairus socius), pied babblers (Turdoides bicolor) and meerkats (Suricata suricatta). Previous work has investigated some of these associations, and I have put the papers at the end.

The problem with using these false alarm calls to gain food from other species, usually food that drongos are unable to gain themselves, is that if you are deceptive too often then your hosts will stop responding. However, these host species don’t just have their food stolen by drongos, they actually use drongos as part of their vigilance system as drongos will alarm at actual predators. Thus there is an interesting ‘mutualism’ that exists within this system. This investigation sought to understand how drongos maximize their gain from these repeated interactions.

Natural observations of this wild population showed that drongos exclusively use mimetic alarm calls in 42% of false alarms and a combination of mimetic and drongo specific calls in a further 27%. To investigate these patterns the researchers carried out a playback experiment using four drongo generated call types:  1) a control territorial call 2) drongo-specific alarm 3) mimic of glossy starling (Lamprotornis nitens) alarm and 4) mimic of pied babbler alarm. The experiment was carried out on groups of habituated pied babblers, a host species for the drongos. Pied babblers were slower to return to foraging after mimetic alarms than drongo specific ones, thus showing that mimetic alarm calls produce a stronger response. They then carried out a second experiment where they played back three calls in one of four treatments: 1) All drongo-specific alarms 2) All mimetic starling alarms 3) two drongo followed by a starling and 4) two starling followed by a drongo alarm. They found that pied babblers habituated to treatments of all one type but not to the treatments that varied. Showing that varying the calls you use has benefits for the drongo. These experimental results fit with the natural data that showed in instances of repeated attempts at theft on the same individual that drongos changed the false alarm call that they used on 74% of occasions. Drongos were more likely to change if their previous attempt had failed.

This paper shows that drongos avoid host species habituating to false alarm calls by varying the mimetic false alarms that they use and thus avoiding frequency dependent constraints.

This paper has a great blend of natural observation and carefully designed experiments. The results speak for themselves and tell an interesting story of how birds in the wild adjust to problems that are familiar to many people. It shows the value of working at a research site with multiple habituated species that associate and interact in interesting ways, and thus the value of long-term research stations.

Some of the media coverage seems to have sensationalized this work, going much further with their assertions than the authors did. Some have even suggested that these birds have ‘theory of mind’. Sciencemag noted that Tom (Dr Flower) is doing some current work to further investigate drongos tactics and learning, work that I assisted with in the Kalahari. From my observations while performing these experiments the drongos seemed to use simple rules and were not as clever as we had hoped. However, that does not detract from the brilliance of their behaviour: many complex behaviours and structures are made using simple rules, e.g. spider’s webs and wasp nests. Even some seemingly complex human behaviours are governed by simple rules. The amazing adaptability of evolution is shown by such seemingly complex behaviours. In fact the authors use examples such as the changing of cell surface proteins by influenza as an analogue to drongo behaviour, and I’m pretty sure that virus’s don’t have theory of mind!

Here are some links to other media coverage:

http://news.sciencemag.org/biology/2014/05/african-bird-cries-wolf-steal-food?rss=1

http://whyfiles.org/2014/deceptive-bird-lies-to-steal-food/

http://www.nbcnews.com/science/weird-science/african-bird-uses-sound-effects-bamboozle-other-species-n94256

More drongo papers:

Flower et al (2013) The ecological economics of kleptoparasitism: pay-offs from self-foraging versus kleptoparasitism. Journal of Animal Ecology 82:245-255

Flower & Gribble (2012) Kleptoparasitism by attacks versus alarm calls in fork-tailed drongos. Animal Behaviour 83:403-410

Ridley & Child (2009) Specific targeting of host individuals by a kleptoparasitic bird. Behavioral Ecology & Sociobiology 63: 1119-1126.