Sunday, December 16, 2012

How to protect your kids from brood parasites: Talk to them!

Let's do some science, shall we?

Lots of birds have problems with brood parasites, such as the cowbirds of the new world and cuckoos of the old world. Most brood parasites have adapted a faster incubation period and a faster growth period, so that they can hatch before the parasitized bird's actual eggs and either push them out of the nest, or just take all the food so the other chicks starve. However they do it, there are various solutions that birds have adapted to avoid this problem. Some kill the parasite's egg, some abandon their nest, but by far one of the most unique solutions are that of the Fairy-wrens, a fascinating group of songbirds native to Australia that could be described as chickadees plus hummingbird-level color.
Internet photo of Superb Fairy-wrens, male and female

A recent study has revealed that fairy-wren mothers begin communicating with their offspring before they have even hatched. During incubation, female Fairy-wrens give a sort of incubation call that, when the chicks hatch, will become a vocal password for food. Without this vocal password, the chicks will not be fed. She gives this call repetitively late in her offspring's incubation period, and is embedded in their brains when they hatch. This vocal password, then, is their begging call and is unique to each nest, so mothers always know which chicks are actually theirs, or at least which chicks they incubated. Mothers use these

Let's go through this:
internet photo of a Horsfield's Bronze-cuckoo
The female lays and and begins incubating her eggs. The eggs typically take about 15 days to hatch, so the female waits until day 10 to start giving her incubation calls, both so that her embryonic offspring can actually hear her, and so that she doesn't spend the entire incubation period giving her nest away to predators. Timing is where the tables are turned for the early hatching bronze-cuckoos' (see left) chicks that frequently parasitize fairy-wrens' broods: bronze-cuckoos hatch after 12 days rather than 15, so they don't get nearly as much time within the egg to learn the vocal password. Without the correct vocal password, the fairy-wren parents will not feed the chick, so the brood parasite never gets fed, while the actual fairy-wren chicks, hatching three days later with the correct begging calls, are given all the food. Case closed, end of story. Suddenly, the early hatching habits of the bronze-cuckoo that were once an adaptation become a HUGE disadvantage. Cool stuff, right?

This discovery leads to a few questions, too. First off, if Fairy-wrens learn their begging calls during incubation, do they have any begging calls genetically programmed? The answer, it turns out, is no; Fairy-wren chicks develop with a blank slate, and learn everything in the egg. The significant implication here is well put by one of the researchers:

     "These findings demonstrate that traits that appear to be innate, such as nestling begging calls, may actually be learned. Knowing this adds to our understanding of evolution and also has many practical implications, particularly for captive breeding and conservation biology."

This also means that if a mother's actual offspring are replaced by unrelated offspring during incubation, the mother's actual offspring will not know the correct call to be fed by their actual mother. It all depends on learning the right call. Another obvious question: how do males learn the calls? They don't participate at all in the incubation, but they still know the vocal password, so the female must communicate it in some way. The researchers in Australia had similar questions, and they found that females share the vocal password with their mates through a so-called "solicitation song". This way, there are no loop-holes in the vocal password system. Finally, we'll end with a prediction in the next step in what has now become an "acoustic arms race" between brood parasite and Fairy-wren: in order to survive, Bronze-cuckoos will have to adapt to their smaller window for learning the vocal password, by, well, learning it faster.

Whatever the cuckoo's "counter-solution", it seems the relationship between parasite and parasitized always produces fascinating discoveries. The big question now is if any species here in North America have adapted a similar system. The encroachment of farmland onto what were once eastern woodlands safe from brood parasitism has brought birds like Red-eyed Vireos, a species completely without adaptation against brood parasitism, face to face with Brown-headed Cowbirds, who like open areas more than forests. It will be interesting to see where evolution takes this conflict, and this discovery on these diminutive, color-blasted Australian songbirds gives us new insight into this process.

Have a great day, everyone.

Saturday, December 1, 2012

Nutting's Flycatcher: a Big Year Birder's Dream

A Nutting's Flycatcher--Myiarchus nuttingi--recently made an appearance in the desert oasis of Bill Williams National Wildlife Refuge for the second time this year. This makes Arizona a pretty lucky state: Nutting's Flycatcher is a Code 5 (a.k.a. accidental) species in the ABA Area, meaning that the species has been recorded five or fewer times in the North American continent, and yet it has been recorded twice in the same spot in the same year! It even seems to have overwintered in this wildlife refuge, as it was first found in December of 2011, and was last found March 25th, this year. Basically, it's one of those really rare birds that make ANY Big Year that much better, including Sandy Komito's record setting Big Year of 1998 after which both the book and the movie titled "The Big Year" are based. Nutting's Flycatcher was the first rarity that Komito started with. Image that. January 1st; you start your year with a Code 5. That's good birding man.

But beyond the birding aspect of it, let's look into the biological aspects of this sighting. We'll start with a perennial birding favorite: the range map.
From this fine map--found on Cornell's Neotropical Birds--we can see that Nutting's Flycatchers are not migratory, but can infer that they probably disperse from their breeding territories when the season's brood is raised. We can also see that their range stretches up the western half of Mexico almost to Arizona, so it's not much of a stretch, one would think, for Nutting's Flycatchers to occur in Arizona. Their nonmigratory habits would account for their Code 5 status, though, because the birds just don't move around enough to make them common outside of their range. So what would draw the bird (if it is the same individual) to this wildlife refuge twiceBill Williams National Wildlife Refuge is described as a rare desert habitat with some of the last stands of native cottonwood-willow forest along the Colorado River. This creates a rich, even lush riparian habitat that would appear attractive to neotropical migrants, and even neotropical rarities. But why didn't the bird just stay where it was? Wouldn't the bird have plenty of habitat in its actual range? There's not an easy answer to these questions as we don't know much about this individual other than its occurrence north-of-range, but the answer may be yes. During the nonbreeding season, a hierarchy develops that dictates which songbirds get which qualities of habitats, with older adult males at the top, younger males and females just below, and young females at the bottom with the lowest quality habitat. As habitat destruction and degradation continues, this hierarchy is pressured for all sorts of species, and the birds at the bottom are getting it worse and worse. This could push some of the individuals on the bottom of the scale out-of-range just to survive, so it would be interesting to see if this individual is a young female. We'll see how long this bird sticks around--we might get some experts on the job.

With every rarity, there are always tons of questions, even beyond identification. IDing Nutting's Flycatchers is a challenge in of itself; they even used to be considered the same species as the more common Ash-throated Flycatcher. Luckily, they give their distinctive callnotes frequently. But beyond that, why the bird is occurring out of range raises all sorts of questions, such as how it behaves differently when in and out of range. Sadly, the likelihood that most of these questions will be answered is slim, but that's one of the best things about birds, and nature as a whole: it puts you in a constant state of inquiry, and that's a big reason why I'll never stop.

To read more about the bird, check here and here.

Internet Photo of the Nutting's Flycatcher. Found here.