The Imitation Game -- Cuckoo Style

The Common Cuckoo (Cuculus canorus) might be known for its iconic call, but its true hallmark is the tactics it uses to shirk parenting duties

During the European spring and summer, the cuckoo's familiar call echoes through the air. 

'CUCK-oo' 'CUCK-oo' 'CUCK-oo'  

Like many birds, common cuckoos journey from their southern wintering grounds to exploit the bounty that comes with warm weather further north. And like many birds, spring heralds that time of year when 'the birds and the bees' are in full flight and buzz. But while caring parents like wrens, robins, and common redstarts busily build their nests, female cuckoos sit and wait. 

And wait.

And wait.

All the meanwhile stuffing themselves on the myriad bugs. Until finally, a nest is complete -- someone else's nest, that is. 

A small Eurasian reed warbler that also made a long northern migration set up camp in the marsh reeds. Strung among a handful of reeds sits its conical nest with three or four cream-colored eggs with brown splotches. As she sits, diligently warming her eggs, a sudden call pierces the air.

'kwik-kwik-kwik-kwik-kwik...'

The warbler lifts her head, alarmed by the threat.

'kwik-kwik-kwik-kwik-kwik...' the call repeats

A Eurasian sparrowhawk -- a predator and close by.

She hurriedly scans the area. The call picks up again. 

'kwik-kwik-kwik-kwik-kwik...'

Not wanting to take any chances for her own life, the warbler hops out of her nest, heading away to other reeds where she can scan the sky better. While away, though, a female cuckoo swoops right near the nest. Forcing herself into the nest, the cuckoo settles for a minute. At twice the size of the warbler's nest, the cuckoo looks like an adult person stuck in a toddler swing. She lays an egg -- cream-colored with brown splotches, just like the warblers. And just before vacating the nest, she lets out a final call.

'kwik-kwik-kwik-kwik-kwik...'

And takes off. 

Clever Cuckoos

As all parents know, it takes a lot of time and energy to raise kids. For animals, the challenge is even greater -- find food, catch the food, feed the kids, avoid the predators, stay alive. It's tough. So for animals, they score big if they can avoid parenthood and hand the reins to someone else. And cuckoos epitomize this technique.

About 58 species of cuckoo have mastered some degree of stealth, cleverness, and imitation in order to fool other birds into raising their young. Its a technique biologists call brood parasitism. It's where cuckoos lay one of their eggs in another bird's nest -- possibly at the expense of one of the eggs already in there -- and let the other bird raise the chick for them. If the cuckoo egg is accepted, the cuckoo flies free, and the other bird ends up raising a chick about twice the size of itself.

A reed warbler feeds a cuckoo chick -- the warbler's chicks having died because the cuckoo chick rolled them out of the nest before they hatched

But why does the other bird tolerate this? And even more so, how does it not recognize the chick looks nothing like it? Cuckoos have a way of coercing other birds to care for their chicks. Sometimes they ransack the nest or harass the bird until it agrees -- or loses all of its own eggs because the cuckoo destroys the nest. But sometimes, the bird won't even recognize that an imposter egg has been placed, and even when born, the warbler responds mostly to the chick's behavior, the size difference aside.

This is because cuckoos are masters of imitation, and for some, the degree of imitation reaches incredible proportions -- eggs that look identical to their host's; feathers that nearly match those of common predators; and now, a recent study published last week in Nature Ecology and Evolution showed that common cuckoos even imitate the sounds or chuckles of hawks to ward off would be vigilant parents.

"[F]emale [cuckoo] chuckles play an important role in a suite of specialized female traits associated with a brood-parasitic lifestyle," wrote authors Jenny York, an associate research professor at the University of Cambridge, and Nick Davies, a professor of behavioral ecology also at the University of Cambridge. Those traits include stealth skills while sneaking up on a host nest, feather plumage that looks much like a hawk's, and incredible spatial memory skills to recall where their victim's nest is. These chuckles, though, unify their toolbox of deceptive skills because it misdirects the host's attention away from the nest.

Various cuckoo species have evolved astounding imitations, such as egg colors that almost perfectly match their host's

(Source: http://fsc.fernbank.edu/Birding/parasitism.htm)

"Female cuckoos rarely call," wrote the authors, a point that differs diametrically from male cuckoos that announce their position with the famous 'CUCK-oo' call. But oddly enough, the time when female cuckoos do call is while monitoring a host's nest and immediately after parasitizing the nest, both of which open a slew of possible problems for the cuckoo.

"Previous studies have shown that hosts of the common cuckoo...defend against parasitism by mobbing the adult cuckoos," noted the authors, "and by rejecting eggs that differ from their own." Cuckoos compensate for some of this through egg mimicry and stealthy operation. But announcing their position just as they plan to swoop in on the nest still seems odd.

The clue that tipped York and Davies came from the similarity between a cuckoo's chuckle and a Eurasian sparrowhawk's call, the latter a common predator of the area.

"[The cuckoo's] chuckle call -- a rapidly repeated 'kwik-kwik-kwik-...' -- is similar in fundamental frequency and rate to the 'kii-kii-kii...' of Accipiter hawks..." On top of it, common cuckoo's can look a lot like the sparrowhawk.

The similar feather plumage between a common cuckoo (left) and a Eurasian sparrowhawk (right) is uncanny but serves an important purpose in helping the cuckoo fool others to raise its chicks.
(Sources: http://www.birdforum.net/opus/images/thumb/0/0c/Common_Hawk_Cuckoo.jpg/500px-Common_Hawk_Cuckoo.jpg
and
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEizdI6bq7ukOMVWS12ETwK7_ZEkAwiZ7c8d99hvn57TGgHAeis7f8zzNkt677zqDLGd1aQIofDiu0Ubq6RIMDHgV_HIAtKQepUQlRSLkJszyYjpYuQNAZI71KESvve9jL12x00Qjf1ZdAgF/s1600/Sparrowhawk.jpg)

York and Davies put their idea to the test by playing multiple bird sounds through speakers placed near reed warbler nests. Some bird sounds were from harmless neighbors, like the coos of collared doves and 'cuck-oo' of male cuckoos, whereas the others came from potential threats, like the Eurasian sparrowhawk call and the female cuckoo's chuckle.

They found that reed warblers increased their vigilance and scanning when they heard either the sparrowhawk call or cuckoo's chuckle but remained imperturbed by the other calls. This was true even when they tested calls on great tits and blue tits, birds in the area not parasitized by cuckoos.

But even if cuckoos distract the warblers, that doesn't mean the cuckoo egg laid in the nest will be accepted. So to really nail how effective this tactic is, York and Davies manipulated the nests with dummy eggs, painting a warbler's egg brown so it looked like the nest had been parasitized by a cuckoo and then stuck a wooden cuckoo next to the nest with a speaker behind it.

Though warblers invariably attacked the models, it turned out that the probability of warblers accepting the "parasitic" egg after a female cuckoo chuckle was nearly 80 percent. This seems to make sense to York and Davies.

"If hosts respond to a cuckoo call as if it were a hawk, they are less likely to reject a cuckoo egg, but if they fail to respond to a hawk call they may lose their life."

The trade-off for the warbler is clear: Protect your young or protect your life. It's a stunning discovery that shows how female cuckoos increase their chances of success and adds yet one more skill to the cuckoo's ever-growing resume of manipulation and imitation.

"[T]he female cuckoo might have 'the last laugh' in this particular battle between host defense and parasite trickery."

Check out the original article here in Nature Ecology and Evolution and listen to the chuckle call of the female common cuckoo!

Ideas in Conservation: The Case of Declining Bumblebee Populations

News has been all abuzz about declining global bee populations for several years now. From talk of cell phone radiation disorienting bees to overexposure to pesticides and, particularly, insecticides threatening populations, bees have made headlines for their growing threatened status. But while bee populations, in general, are on the decline, a new study published this past week in Proceedings of the Royal Society, B delved a bit deeper into the story using bumblebee species and found nearly one-third of species examined were declining--and all of them closely related.

 Results from this new study show that individuals from the older Thoracobombus (top) subgenus are significantly more susceptible to extinction than those from Pyrobombus (bottom), which currently shows almost no population decline.
Photo sources: https://www.flickr.com/photos/giaa/5073361366/ and https://www.flickr.com/photos/29697818@N03/7304148034/

Bumblebees are fuzzy, flying juggernauts of insects, the largest of which lives in Chile and reaches a body length of 1.6 inches - "a monstrous, fluffy, ginger beast," as British scientist David Goulson described it in his book A Sting in the Tale. With 260 species, bumblebees populate almost every continent around the globe (Antarctica and Australia being the exceptions), preferring higher altitudes and latitudes where temperatures are cooler, which raises concern. As the global climate continues to rise, this penchant for cooler climate may have dire consequences for many bumblebee species and possibly explain their declining populations. But that might be only one factor. Habitat loss and fragmentation of land repurposed for agriculture, increased use of pesticides and insecticides, and transmission of pathogens between species all may play a part, but nobody is certain.

"[T]he global picture of bumblebee decline is still fragmentary," wrote Marina Arbetman and colleagues, the authors of the study. "[N]o previous study has evaluated worldwide patterns of bumblebee decline as well as their potential predictors within a phylogenetic framework." 

Traditional methods of determining conservation status stem from collecting information pertaining to an organism's biology: where it lives; how its populations are distributed across a town, a country, a continent; what habitat it prefers; what the major threats are to that habitat; what its ecological role is in that habitat. From this information, a conservation status is determined: least concern, near threatened, endangered, critically endangered. 

That's exactly what the International Union for the Conservation of Nature (IUCN) did for bees in Europe back in 2014. They found about eight percent of nearly 2000 bee species in Europe had declining populations, which seems small, but that's because 79 percent of species had insufficient population data to draw any conclusion -- they were only seeing the tip of the iceberg. Even so, bumblebees, making up less than four percent of bee species in Europe, were almost entirely accounted for and over a quarter were threatened or near-threatened.

Traditional conservation status methods, however, exclude what some argue is an important puzzle piece: the phylogenetic diversity of a group. Phylogenies--the evolutionary relations of organisms--act as an excellent proxy for the functional diversity of organisms, which in turn helps scientists grasp what role those species play in an ecosystem and what services they perform for both the ecosystem and for humans. 

"[T]he conservation of higher phylogenetic diversity is likely to safeguard the evolutionary legacy of bumblebees," wrote Nicolas Vereecken, an assistant professor at the Université Libre de Bruxelles in Belgium, earlier this year and who was not an author on this paper. He believes that the conservation of phylogenetic diversity of bumblebees may protect their potential to face environmental change and ultimately play a pivotal role in the maintenance of ecosystem processes and services. "The time is now ripe for the incorporation of phylogenetic diversity as an alternative biodiversity metric into conservation planning to avoid worst-case losses of long branches from the bumblebee tree of life."

Using a New Tool

Arbetman and colleagues looked into the global phylogenetic diversity of bumblebees, asking if the species that were declining fell into specific lineages that play critical ecological roles as well as what factors may contribute to their declining populations, such as range size, presence of certain parasites, and even tongue length, a characteristic that can indicate the bee's specialization to feeding on specific flowers.

The study found that roughly one-third of the bumblebee species evaluated were declining in populations, and the highest proportion of these fell within three subgenera of bumblebees rather than being scattered randomly across the bumblebee tree. One of those subgenera, the Thoracobombus, is the second largest bumblebee subgenus and also one of the oldest.

"[Twenty-two] of the 52 recognized species in [Thoracobombus]...have a well-established extinction risk status," wrote the authors. "From them, almost two-thirds of these 22 species...are declining." The authors also concluded that the loss of this as well as the subgenus Cullumanobombus, another of the subgenera severely threatened, would result in higher losses of phylogenetic diversity than would be predicted from random extinctions of bumblebees. 

"Thus, from a phylogenetic perspective, species of these subgenera deserve the highest conservation priority," concluded the authors.

In striking contrast, bumblebees from the largest subgenus Pyrobombus showed almost no indication of population decline or threatened status. "Out of the 50 species belonging to this subgenus, 32 species (64%) had well-established extinction risk status...with only two species...being threatened," wrote the authors. 

The difference may stem from the other correlates that they found: bumblebees with small ranges, long tongues, and almost no pathogens--all indications of specialization to some degree--showed higher susceptibility to extinction. This becomes even more worrying considering that global commercial exchange and transportation of bee colonies has increased over the past decade, giving prime opportunity for species to invade and potentially out-compete these vulnerable bumblebee lines; the process that has already begun in Chile, where Bombus dahlbomii -- the largest bumblebee species in the world and a member of the declining Thoracobombus subgenus -- is threatened by commercial bees with pathogens previously unknown to the area.

Broader Questions

Overall, this study provides a precise method of determining organisms to prioritize in conservation efforts as well as a key to determining the species crucial to an ecosystem or the preservation of a lineage, an otherwise daunting challenge considering that many inhabitants are declining simultaneously. 

But why should this matter to us?

"Bumblebees are an essential component in our agroecosystem," wrote Vereecken. "[T]heir decline represents a major threat for the sexual reproduction -- and hence survival -- of wild flowers and several pollinator-dependent crops alike." 

Bumblebees play not only a crucial role in natural ecosystems but also our own crop productions through what conservationists call "ecosystem services." We ultimately depend on pollinators like bumblebees, bats, hummingbirds, and various other insects to help produce our food from year-to-year, which in turn save companies, farms, and consumers millions of dollars. This dramatic impact has led some conservation organizations to shift their focus from traditional "preservation of biodiversity" conservation campaigns to ecosystem service campaigns when discussing the need for conservation. People seem to understand money better. 

But Arbetman and colleagues employ a method and present results that hinge on the principle of preserving diversity. From a market's view, though, only a species that optimally performs the desired service is necessary--the others are theoretically expendable. This raises concerns for some conservationists: Focus on a species' ecosystem service neglects the complete picture of natural interactions. After all, nothing in nature is actually independent of the other parts. 

So, is there a way we discuss both economic benefits to a market and people and still emphasize biodiversity preservation? A group of environmental scientists from the United States and France believes they have found a way.

I'll talk about that in the next post.