next message in archive
next message in thread
previous message in archive
previous message in thread
Index of Subjects
Index of Subjects
Thanks James, interesting review in a field that as usual I had strayed into innocently, but that is obviously professionally well-worked. Some take-home points:
> When faced with attempted brood parasitism by cowbirds, there are 'accepter' host species that succumb, and 'rejecter' host species that fight back. If successful, brood parasitism is always very expensive for the hosts. The only reason advanced for an accepter host having not (yet) developed defenses is that the parasitism has developed too recently to have triggered an evolutionary response, that I'd thought was a lame idea. The argument is plausible though, because 'recent' can mean a few millenia or much longer -- the long time it takes to evolve all the steps in a complex rejection behaviour.
> Unsurprisingly, many hosts actually can detect the difference visually between their own eggs and those of a cowbird. No clear case was cited where egg mimicry is so good that the host could not detect it, although apparently there is a good match for Northern Cardinals and cowbirds except for egg shape. Egg detection can be based on egg colour, egg size and 'maculateness' which I think means how relatively white the eggs are, as opposed to spotted.
> The most interesting conclusion is that in several species somehow geographically isolated and so currently not exposed to cowbird parasitism, when challenged experimentally nonetheless reveal a strong egg rejection behaviour. The explanation advanced is that these species were parasitized in the past, evolved defenses back then, and that these evolved abilities have remained coded within the species' genomes for huge amounts of time without decaying, well after the brood parasite's challenge disappeared -- 800,000 years is mentioned for one grackle species. The authors even suggest that this may have happened far back in a common ancestor, before the development of the modern species.
Thanks -- well worth reading and not technical.
Steve
________________________________________
From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on behalf of Gerald [naturens@zdoit.airpost.net]
Sent: Tuesday, September 9, 2014 2:24 PM
To: naturens@chebucto.ns.ca
Subject: Re: [NatureNS] Brood parasites
James,
Thanks for the link to the paper. Fascinating subject with lots of ways
to analyze the data.
--
Gerald
On 9/9/14 11:59, James Churchill wrote:
> hi folks,
>
> Here is a recent paper summarizing host defense against cowbird
> parisitism in North America:
>
> HOST DEFENSES AGAINST COWBIRD (MOLOTHRUS SPP.)
> PARASITISM: IMPLICATIONS FOR COWBIRD MANAGEMENT
>
> Ornithological Monographs, 2005
> http://www.biosci.missouri.edu/avianecology/courses/avianecology/readings/Peer_BD_2005.pdf
>
> Note, this opens a pdf.
>
> cheers,
>
>
>
>
> On Tue, Sep 9, 2014 at 12:38 AM, Stephen Shaw <srshaw@dal.ca
> <mailto:srshaw@dal.ca>> wrote:
>
> An interesting question is why host species at least in some cases
> do not take countermeasures to turf out the egg(s) of the parasite.
> At least in one case, it is not a problem of clever cryptic
> coloration having been used to evade detection. The single cuckoo
> species we used to see in UK often lays an egg in the nest of the
> hedge sparrow (dunnock). The single cuckoo egg is much larger than
> those of the dunnock, is often white with brown spots versus always
> plain bright blue for the dunnock. Some birds, perhaps most, have
> good colour vision so there should be no problem in detecting an
> alien egg that is also twice the size. The newly hatched cuckoo
> throws out all the dunnock's eggs, so the dunnock ends up not
> rearing any offspring of its own for that breeding cycle. This
> should be a strong evolutionary incentive to develop a defense.
>
> Presumably there must be some disadvantage attached to developing a
> simple defense of detecting then removing an alien egg? I could see
> it if the parasite's eggs looked very similar to those of the host
> -- the defending host bird then might remove some of its own eggs by
> mistake, a disadvantage. At least for the UK cuckoo this is not the
> case: the eggs are easily distinguished from those of most host
> species'.
>
> Is there a plausible explanation for this, and is it a general
> phenomenon? If parasitism of the dunnock by the cuckoo were a very
> recent development, it could be argued that the dunnock has not yet
> had time to evolve countermeasures, but this sounds a bit lame.
> Have any N. American brood parasites (cowbirds?) developed eggs
> that mimic those of their hosts -- is there a general rule for
> this, where the UK cuckoo is an exception? My guess is that 97% of
> the folk on NatureNS are birders of some ilk, so someone out there
> must have an answer.
>
> As a related afterthought, bird books in the UK in the 50s-60s used
> to describe the nests, eggs and nesting habits of birds, not just
> their plumage. I haven't seen this here in the Sibley, Petersen etc
> recent era in Canada/USA or I could probably have answered the
> cowbird question myself. I presume the main (and valid) reasons
> are now to discourage any interest in egg-collecting or
> nest-disturbing, by simply not giving out any useful information?
> Steve (Hfx)
> ________________________________________
> From: naturens-owner@chebucto.ns.ca
> <mailto:naturens-owner@chebucto.ns.ca>
> [naturens-owner@chebucto.ns.ca
> <mailto:naturens-owner@chebucto.ns.ca>] on behalf of Randy Lauff
> [randy.lauff@gmail.com <mailto:randy.lauff@gmail.com>]
> Sent: Monday, September 8, 2014 2:03 PM
> To: NatureNS
> Subject: Re: [NatureNS] Brood parasites
>
> Our own Black-billed Cuckoo normally builds its own nest, but will
> sometimes brood parasitize other species.
>
> They avoid wiping themselves out in the same way carnivores do...too
> many carnivores, not enough prey, many carnivores starve, prey
> rebounds, carnivores increase. This is a basic explanation...there's
> a lot to this.
>
> Randy
>
> _________________________________
> RF Lauff
> Way in the boonies of
> Antigonish County, NS.
>
> On 8 September 2014 13:49, Gerald <naturens@zdoit.airpost.net
> <mailto:naturens@zdoit.airpost.net><mailto:naturens@zdoit.airpost.net <mailto:naturens@zdoit.airpost.net>>>
> wrote:
> I hope brood parasites is the