The Avian Brood Parasites

The Avian Brood Parasites

Brood parasitic birds lay their eggs in the nests of other birds (the “hosts”) who then raise them as their own. Examples of parasitic birds includes the cuckoo, cow birds, widow (“whyda”) birds, honeyguides, and even the South American Black-headed Ducks. Brood parasitism is virtually a world wide phenomenon.

Many interspecific brood parasites are obligate for this strategy … this is the only way they raise their own young. There are many variants (beyond the scope of this post). Intraspecific parasitism is known in many colonially nesting birds.

The Red Queen effect is a concept now widely known by aficionados of biology. The phrase is from Alice Through the Looking Glass. While the Red queen and Alice are discussing chess, the following dialog and events ensue:

Just at this moment, somehow or other, they began to run.

Alice never could quite make out, in thinking it over afterwards, how it was that they began: all she remembers is, that they were running hand in hand, and the Queen went so fast that it was all she could do to keep up with her: and still the Queen kept crying `Faster! Faster!’ but Alice felt she COULD NOT go faster, thought she had not breath left to say so.

The most curious part of the thing was, that the trees and the other things round them never changed their places at all: however fast they went, they never seemed to pass anything. `I wonder if all the things move along with us?’ thought poor puzzled Alice. And the Queen seemed to guess her thoughts, for she cried, `Faster! Don’t try to talk!’

Not that Alice had any idea of doing THAT. She felt as if she would never be able to talk again, she was getting so much out of breath: and still the Queen cried `Faster! Faster!’ and dragged her along. `Are we nearly there?’ Alice managed to pant out at last.
`Nearly there!’ the Queen repeated. `Why, we passed it ten minutes ago! Faster! And they ran on for a time in silence, with the wind whistling in Alice’s ears, and almost blowing her hair off her head, she fancied.

`Now! Now!’ cried the Queen. `Faster! Faster!’ And they went so fast that at last they seemed to skim through the air, hardly touching the ground with their feet, till suddenly, just as Alice was getting quite exhausted, they stopped, and she found herself sitting on the ground, breathless and giddy.

The Queen propped her up against a tree, and said kindly, `You may rest a little now.’

Alice looked round her in great surprise. `Why, I do believe we’ve been under this tree the whole time! Everything’s just as it was!’

`Of course it is,’ said the Queen, `what would you have it?’

`Well, in OUR country,’ said Alice, still panting a little, `you’d generally get to somewhere else — if you ran very fast for a long time, as we’ve been doing.’

`A slow sort of country!’ said the Queen. `Now, HERE, you see, it takes all the running YOU can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!’

Lewis Caroll Alice Through The Looking Glass

This story is a metaphor for a phenomenon in evolution that is also analogized as an “arms race.” Typically, two species are engaged in some sort of competition, or one is parasitic on the other. Over time, Natural Selection shapes the relevant features of one species in the context of the countervailing features of the other species, and visa versa. Since the relevant features of both species are dynamic, the selection is dynamic, and all else being equal, the relationship of the two species (or their relevant features) stays roughly the same while the entire system shifts in its intensity.

The system of parasitic birds is an excellent example of numerous evolutionary phenomena, including:

• The Red Queen effect
• Mimicry
• Exploitation of Parental Investment (PI)
• The adaptation of developmental strategies by Natural selection
• Species interaction and competition

Let’s start with PI. Parental Investment can be thought of as the flow of energy of adult individuals to pre-adult individuals (anything from zygote to sub adult). This energy flow is considered to be Parental Investment if it influences the direct fitness of the recipient in a positive way and influences the direct fitness of the provider in a negative way. The concept has always been understood by biologists in some manner, but was formalized by Robert Trivers in Parental Investment and Reproductive Success (conveniently reprinted in “Natural Selection and Social Theory” which happens to be this month’s “Book Link” … see sidebar).

The indirect fitness of both parent and offspring depends in part (sometimes in LARGE part) on this parental investment. Deep and significant differences among animal taxa often derive from the pattern of parental investment. That mammalian females carry their young internally and lactate, and mammalian young typically have very little investment from the male parent, fundamentally shapes mammalian socioecology, which in turn is very different from birds which lay eggs, do not lactate, and typically have significant PAternal investment (instead of mainly MAternal investment).

Brood parasitism is perhaps the ultimate example of co-opting someone else’s parental investment.

Developmental strategies are shaped by selection in order to make this work. Chicks typically do not have well developed legs during their early days in the nest. This keeps them from standing up and wandering off, falling from the tree, etc. But parasitic bird chicks often have very well developed legs, allowing them to do a little squirmy dance thing during which they push the host’s eggs and (if already hatched) chicks out of the nest to their ultimate doom. Parasitic bird eggs develop faster than the host’s eggs. This is essential because the parasites can’t lay their eggs in an empty nest … the host bird will typically eject them. The parasitic eggs are laid among pre-existing host eggs. The more rapid development of the parasitic bird eggs ensures that they hatch either at the same time as the host’s eggs or before.

Even within some species of intraspecific brood parasites, the parasitic eggs may develop faster (this has been observed in swallows).

Mimicry comes in where signals are important in avoiding parasitism. Adult birds recognize their own eggs … at the level of species anyway … by spots or other markings or by a color cast. A parasitic bird cannot easily exploit a host if the parasitic egg looks different form the host eggs. This is thought to be the very reason why bird eggs have evolved such a variety of design and decoration. When you see eggs that are colored, striped, and otherwise “decorated” you are looking at the evolutionary legacy of brood parasites. There are brood parasites that are essentially locked in to exploiting a particular species because the parasite lays eggs that have been selected to look like the hosts eggs, and thus, these eggs will not easily fool birds of other species.

This is where the Red Queen effect comes in. Hosts that are more easily fooled to think a parasitic egg is their own will have lower fitness than those who are not easily fooled. Parasitic birds that lay eggs that are not as close in appearance to the host will have their eggs more often ejected … minor variants in egg appearance that make the parasitic egg look more like the host are very strongly selected for. Thus over time, the ability to detect egg-fraud increases and the ability to produce counterfeit eggs increases.

Thumbnail from The National Watch and Clock Museum

Neither species is allowed to “rest” (this is a metaphor .. there is no consciousness or planning here in the natural setting) else the other “runs” past. Thus the Red Queen analogy.

Species interaction comes into play as already described: A parasite may be locked into a particular host. However, there are other more subtle effects as well. Birds that lay their eggs in enclosed nests, such as hollows in trees or in hanging baskets (like oriels and weavers) can be very easily fooled by interspecific parasites. However, because the actual parasites are typically larger than the hosts … which they may need to be to produce high-energy fast brooding offspring … the enclosed nests are easier to defend. So the “parasite” may be a scientist checking to see if there are anti-parasitic adaptation in place with the closed-nest bird (and there typically are not). Intraspecific brood parasitism does happen in birds with enclosed nests, such as swallows.

A man who unwittingly raises a child fathered by another man (for example, the milkman or Fuller Brush Man of yore) is called a cuckold, a word derived from the cuckoo bird. The origin of the cuckoo clock (in Germany) is a little obscure and there is debate over who invented this annoying device. However, clocks of the 17th and 18th century were peddled by traveling clock salesmen. Coincidence? No way.

Faithful husbands: Beware the traveling clock salesman.