Natures wonders: among birds 'brood parasites' (Introduction)

by David Turell , Sunday, October 06, 2024, 17:22 (16 days ago) @ David Turell

A constant battle in the nests:

https://knowablemagazine.org/content/article/living-world/2024/the-clever-ways-that-bro...

"...some birds manage to forgo caring for their chicks — while still ensuring they’re well looked after. These birds lay their eggs in the nests of other birds that unknowingly adopt the hatchlings, nourishing and protecting them as their own.

"Only about 1 percent of all bird species resort to this sneaky family planning method, called obligate brood parasitism, but it has evolved at least seven separate times in the history of birds and is a way of life for at least 100 species. Since some brood parasites rely on several different bird species as foster parents, more than a sixth of all species in the avian world care for chicks that aren’t their own at some point.

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"Breaching another bird’s nest is the parasite’s first move. Sometimes this is accomplished via deception: The female cuckoo-finch (Anomalospiza imberbis), for example, has evolved to look like a harmless, non-parasitic bird that lives in the same area, allowing it to sneak into host nests unchallenged. The common cuckoo (Cuculus canorus) takes another tack — it has evolved to mimic the look of a predatory raptor, prompting host mothers to fly off in fear and leave their nest unattended.

"But sometimes, parasites get to the party too late and they don’t find nests into which they can plop their eggs; the eggs are almost ready to crack open, or everybody’s already hatched. In this case, they may fling out chirping chicks and sometimes will ransack the nest. This can prompt the hosts to make new nests from scratch, giving the intruder a brand-new nest to parasitize.

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"Host birds, of course, have evolved strategies to defend their nest and brood. Some sit tight and guard their nests, others may attack the intruder — yellow warblers even have a unique bird call that warns nearby birds of a parasite in the neighborhood.

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"When a parasite makes it into another family’s nest, it usually will nudge out one or two of the host’s eggs to make space for its own. And while nonparasitic birds take about 20 minutes to lay an egg, parasites like cuckoos and honeyguides are fast. “They swoop into the nest and they squeeze out an egg in three seconds flat,” says Rosalyn Gloag, an evolutionary biologist from the University of Sydney. Some parasites have also evolved thicker-shelled eggs that are less likely to crack when hastily dropped.

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"And there are parasites that go the extra mile to ensure their eggs aren’t detected and rejected: They mimic the look of the eggs of their hosts. In some birds, such as cuckoo finches, this egg coloration trait is passed from mothers to daughters: Female cuckoo finches mimic the eggs that their mother mimicked. So some cuckoo finches lay blue eggs mottled with brown, while another may specialize in eggs with red speckles, depending on which host nest they grew up in.

"Still, these tricks don’t always work, says ornithologist Mark Hauber of the Graduate Center of the City University of New York. He has long been researching what makes some birds better than others at recognizing intruders’ eggs, and his data suggest that some hosts can eject eggs that are off in terms of color and size.

"Other hosts, like weaver finches and warblers, have perfected laying eggs speckled with signature colors, patterns and squiggles that seem to be impossible to mimic and are easier to tell apart from the intruders’.

“There’s something about the markings, the entropy of the spotting, that is more consistent within the host’s own egg,” says Hauber. It’s so precise that he hasn’t been able to replicate the patterns in his lab despite making hundreds of artificially speckled eggs.

"New research by Hauber suggests that older birds with more breeding experience are better than younger birds at telling foreign eggs from their own. Hauber introduced fake, 3D-printed eggs into the nests of American robins (Turdus migratorius); the eggs were painted robin blue or the beiges and browns of a parasitic cowbird’s egg. Monitoring the robins revealed that hosts can learn and become better at rejecting intruder eggs from their nests, Hauber and colleagues reported in 2023.

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"The relationship between cowbirds and thrushes, honeyguides and swallows, or cuckoos and their various and diverse hosts is one of constant change. Parasites and hosts are waging a war through evolutionary time, such that what scientists see now is just a snapshot in a longstanding battle. And somewhere in there, the birds are finding balance: tactical tradeoffs that allow both parasite and host to persist."

Comment: there are more examples of these bird battles in the article than can fit here. The degree of thought processes suggest that most birds are as bright as the level that crows have demonstrated.