Baby of brood is ‘best explorer’ in zebra finches

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The youngest members of zebra finch broods “explore more” than older siblings in adult life, say scientists.
Researchers investigated how the birds’ behaviour was affected by the way their parents cared for them as hatchlings.
The team studied broods where females lay and incubate a clutch of eggs over a period of days, resulting in a size hierarchy within the clutch.
They found the youngest birds were more likely to explore their environment as adults in search of food.
The study, published in Animal Behaviour, tested over 100 captive zebra finches’ exploratory behaviour to see whether hatching order, and consequently parental investment, affected their behaviour in adulthood.
Late hatched birds are smaller than their older siblings, and it is the larger hatchlings that “get the lion’s share” when parents bring in food “because they can reach up higher and beg better,” explained research team member Dr Ian Hartley from Lancaster University.
Hatching eggs over a span of time, rather than all at once, is known as “hatching asynchrony” and occurs when eggs are incubated as soon as they are laid.
For zebra finch, this means that birds born up to four days apart can share the same nest and must compete for food.
Researchers wanted to know how the method of rearing affected the behaviour of offspring beyond the nest, once they were living as independent adult birds.
“Hatching asynchrony has much longer-term effects on the individual youngsters than just hatching and surviving and then fledging,” Dr Hartley told BBC Nature.
“It’s not a case of hatching asynchrony affects them until they fledge and then… everything’s equal.”
The study showed that when introduced to a new environment, zebra finches’ behaviour differed depending on whether they were late or early-hatched offspring.
The youngest siblings appeared to be more adventurous when introduced to an unfamiliar test cage than early-hatched offspring or birds from broods that hatched at the same time.
The researchers measured how explorative the zebra finches were by recording how many times they visited bird feeders within the new enclosure.
They found that the youngest offspring in a brood approached the feeders significantly more often than their peers within a 30 minute period.
According to Dr Hartley, the study shows for the first time that hatching order influences birds’ “behavioural repertoires” in adulthood, which can affect their whole “life history as individuals”.

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