next message in archive
next message in thread
previous message in archive
previous message in thread
Index of Subjects
defense of detecting then removing an alien egg?=A0 I could see it Hi Gerald, others, Yes, some birds and many insects are known to have UV photoreceptors, but no, that doesn't supply the explanation suggested. These animals are not 'monochromats' with only one type of visual receptor, like the rods-only vision of certain deep sea fish, such that they can't see colours. They mostly have trichromacy -- 3 overlapping visual pigments, so they can see colour well but their visual spectrum is just shifted to shorter wavelengths (better below the violet than us and worse at the red end of the spectrum than us). Some fish are tetrachromatic and one stomatopod crustacean has ~11 visual pigments. The definition of where 'infrared' (IR) starts is a bit fuzzy, but generally any wavelength longer than ~800 nm (nanometers) is considered IR. No animal has IR photoreceptors in this range or beyond for a simple reason: the energy of an IR photon has become too small to cause the necessary chemical rearrangement (isomerization) in the visual pigment required to start vision, which requires a certain minimum energy captured. The energy E associated with one photon is E = h*c/wavelength, where h*c is a constant (the product of Planck's constant * vacuum speed of light): as the wavelength doubles from 400 nm (violet) to 800 nm (near infrared), photon energy halves, because the wavelength's value is in the denominator. Isomerization is followed by a complicated cascade of changes that results in a photoreceptor detecting the absorbed photon and sending a signal centrally. Some animals like pit vipers and one type of beetle can detect IR but do so with special nerve endings and structures that are sensitive to the thermal effects of absorbing IR radiation, which warms up the local structure a little (also how thermopiles work). This doesn't work through the eyes. The normal/old style tungsten light bulb emits most of its radiation inefficiently in the IR, so you can feel its warming effect on your hand but this is visually useless/wasted. Fluorescent tubes and white LEDs do not waste energy in the IR -- they don't emit any radiation there. Steve (Hfx) ________________________________________ From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on behalf of Gerald [naturens@zdoit.airpost.net] Sent: Tuesday, September 9, 2014 9:11 AM To: naturens@chebucto.ns.ca Subject: Re: [NatureNS] Brood parasites Steve, In the ultraviolet or infrared spectrum, which some birds can see, the eggs of the parasite may be similar. -- Gerald On 9/9/14 0:38, Stephen Shaw 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 [naturens-owner@chebucto.ns.ca] on behalf of Randy Lauff [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>> wrote: > I hope brood parasites is the correct term for birds that lay their eggs > in the nest of a different species. > > Are there such parasitic bird species who can also build their own > nests? How do they avoid becoming so successful that they wipe out the > hosts and thereby themselves? > > -- > Gerald >
next message in archive
next message in thread
previous message in archive
previous message in thread
Index of Subjects