Their decision loomed as researchers exposed their sometimes polygamous relationships and considered how feeding some of their children would affect the whole family.
The best-fed children looked likely to be fed at the expense of the thinnest - but how children whose mothers had two partners would fare was still unclear.
By this week, research in the Dunedin Botanic Garden suggested some dunnocks (hedge sparrows) stayed with one mate for life, while others had multiple partners.
University of Otago PhD student Eduardo Santos said data collected last mating season showed some females mated with, and were supported by, more than one male.
Further work showed some males bred with more than one female, which meant they flew to different sites across the gardens to feed chicks at more than one nest.
A female, whose herb garden nest was her fifth in two years, ''shared'' one of her males with a female that had ''moved in'' at the nearby water garden.
''Those males want to ensure they have a chance to produce as many offspring as they can, and the females want the best chance to bear and then sustain the offspring,'' Mr Santos said.
''So it is not unusual for males to have more than one female, and females to have more than one male.''
Both sexes did all they could to ''attract a mate and procreate'', Mr Santos said.
Females ''popped their tail in the air'' and wiggled it about to attract a mate.
Males pecked the females' reproductive opening to get rid of sperm from previous partners.
''It looks like just hopping about to people, but it is an important part of the reproductive strategy. And in the gardens, it is happening all over the place, just about all of the time.''
The findings were supported by a banding programme that started last year.
The behaviour continues from about 6am every day between September and the end of January.
By this week, the programme had gathered information about more than 210 adults and a similar number of offspring.
It helped researchers identify between 20 and 30 breeding groups, and the more than 100 birds that mated and nested over both nesting seasons in the study.
Mr Santos and assistant Sam van der Horst carried out the programme to see how males and females treated their best-fed offspring.
The researchers would feed half the brood to see whether the adults adjusted their feeding habits to concentrate on the healthiest chicks.
Mr Santos suspected the caregivers - mother, father, shared male, whatever - would eventually ''decide'' to support the chicks that were most likely to survive.
