Positives

natural - occupies the parents

generally Cordon Bleus are good sitters

proves husbandry is very good

parent-reared chicks to date fledge earlier than fostered chicks

they almost invariably fledge in very good condition

earlier fledging could be due to less protein in the foster's diet

Negatives

variable live-food requirement between Blue Capped pairs

young birds can lose many clutches while they learn to 'parent'

ejection of chicks if parents overly 'fit' ie too much livefood

chicks abandoned if development isn't 'right' ie too little livefood

can't monitor chicks to move smaller ones to other nests

very hard to access nest to close ring

chicks get less used to 'our food' than when fostered

chicks 'wilder' than when fostered - unused to small cages

overall low ratio of eggs to fledged chicks

to date its been sufficiently mature birds - 18 months plus - which have been successful parent-rearers
if too well fed - hens may start laying at 8 months - these eggs tend to be infertile, laid anywhere and are rarely incubated
at a year old, good, fertile, potential fostering eggs tend to be laid in the nest and incubated but chicks tend to be tossed

to minimise chances of young chicks being tossed by sufficiently mature cocks - do not feed any livefood during incubation

once there are chicks, Tony Jochem in his book 'Breeding Estrildid Finches' advises feeding Pinkies overstimulates even
sufficiently mature cocks so that chicks are tossed
Buffalo worms for the first 10 - 12 days seem to work - 6gms a day worked with a nest of 6 chicks

T.J's 4 - 6 hours of soaking seed for the rearing food feels better than the boiled-seed method, as vitamins are retained

the smaller seeds that many Continentals now use in rearing food and dry seed mix makes sense as suiting small waxbill chicks

due to numbers now being bred is no longer an easy decision

for newcomers to waxbills - fostering is a good way to start
to build up a breeding stud with nice, young birds

then can learn successful parent rearing
knowing there is a viable UK population
in my first season the only parent-rearing success was
from a captive-reared hen

in only 3 months of fostering there were numerous chicks
from different pairs of wild-caught Cordon Bleu parents
(continues to be the case in 2009 using F1's)

good fosters - all fledglings lack any tufts of head down
OK fosters - some fledglings have tufts of head down

poor fosters (or old/ unfit parents) - 1 or more chicks fledge incompletely feathered or can't even fly
try again with eggs from other another Blue-Capped pair
same result?   use such Bengalese to foster rear eggs from good foster Bengalese

common theory is rejection is due to dark skin colour (and down)

am not convinced - Bengies' chicks are much, much noisier - it could be down to sound
it doesn't matter - get some good Bengalese fosters from Gouldian or Parrot finch breeders
once you have a good strain its very easy - good fostering seems to be inherited

only risk a couple of fertile eggs (make up the clutch of say 5 eggs with china eggs) on an untried pair or trio of Society finches
some will happily foster at 8 months
if Bengalese are young - under a year old they may fail the first time -
don't give up on them

if Bengalese are about a year old and have had 2 Blue-Capped nests and failed to rear, then give up on the those Bengies

cock trios every time
can induce cock trios to go clucky in 4 days - 1 china egg added to the nest each morning for 4 consecutive days
so start them going clucky when you know you have a Waxbill nest
no egg-laying cycles
big positive is no distracting extra eggs laid ie is that a Bengalese or a Waxbill egg?

Better feeders

(admittedly this example involves Pin-tailed parrot-finch chicks but hopefully the example is still sound)

the photo below shows two nests which hatched on the same day & had exactly the same food/ regimen/ cage
the trio's chicks are bigger, better developed and better feathered than the pair's chicks
the trio's chicks have already started moving around the nest

what goes in comes out - much, much more droppings in the trio's 3 chick versus the pair's 2 chick nest
not just due to the nest having one more chick - trio's chicks crop's were invariably much fuller

one cock and two hens? - no - the hens compete for the eggs - fewer are consistently brooded so fewer hatch
hen trio - why even go there?

so - cock trios every time

Not tried myself but some people advocate using Zebras to foster

Why?

Bengalese are wonderfuly steady and controllable
A 'trio' of cocks can be made clucky within 3 - 4 days
but
some Bengalese wont feed Blue-Capped Waxbill chicks
the skin colour and/ or the heavy down may contribute

Zebra Finches

Not as controllable and must have a pair breeding 'in sync'
but photo shows dark and light skin colour in same nest
plus quite a lot of down
AND
Zebras don't settle down to roost as early as Bengalese
in winter early roosting limits feeding time and extends 'night'