Brood Division and the Ontogeny of Caching Behaviour in the North Island robin (Petroica longipes)

<p>The post-fledging period is an important, but understudied, stage of avian development. This is despite the fact that the parental care and behavioural development of young observed during this period contribute significantly toward offspring survival. A key factor that has contributed to the lack of research in this area has been the difficulty with which parents and offspring can be observed during this period. The North Island robin (Petroica longipes) is a small insectivorous passerine native to New Zealand forests. As a result of the historic absence of mammalian predators, North Island robins lack pronounced anti-predator behaviours and are fearless towards humans. This makes them ideal subjects for behavioural studies in the wild because human presence does not alter their daily activities. Using field observations, the present study examined parental care and the development of caching during the post-fledging period in wild North Island robins. Brood division is a form of preferential post-fledging care that is well documented among avian species in the northern hemisphere. In contrast, little is known about the incidence and function of brood division in avian species outside this region. Across two breeding seasons (2014-2015 and 2015-2016), feeding interactions between parents and offspring were observed during nestling and fledgling development to determine the timing of and factors influencing brood division in robins. Brood division occurred around the time young left the nest and was common amongst broods which fledged two or more young. The male parent typically cared for male and larger fledglings and the female parent for female and smaller fledglings. The results of this study match patterns observed in northern hemisphere species suggesting that brood division provides the same adaptive advantages to species regardless of geographical context. Caching, the handling of food to preserve it for future consumption, is an important strategy which allows numerous avian species to deal with natural fluctuations in food supply. In recent decades, caching has become a widely-used paradigm for examining a range of cognitive processes in birds, such as social cognition and spatial memory. However, much is still unknown about how caching develops in young birds, especially in the wild. Over a 12-week period following fledging, the ontogeny of caching and cache retrieval was observed for 34 juvenile robins. Juveniles began caching shortly after achieving foraging independency (approximately 5 weeks after fledging) and their caching rates increased gradually with age. Retrieval of caches began spontaneously as soon as they had begun to cache and retrieval rates remained constant throughout development. Results suggest that caching behaviour in North Island robins is likely to be innate, but that age and experience have an important role in the development of adult caching behaviours. The two studies described in this dissertation examine behaviours that have either been previously difficult to document in the wild or have not been documented in this species. Overall, the results highlight the behavioural similarities between the North Island robin and other avian species exhibiting brood division and caching. Additionally, they also demonstrate the suitability of the North Island robin for future behavioural research given the ease with which these birds can be observed in the wild.</p>

Brood Division and the Ontogeny of Caching Behaviour in the North Island robin (Petroica longipes)

<p>The post-fledging period is an important, but understudied, stage of avian development. This is despite the fact that the parental care and behavioural development of young observed during this period contribute significantly toward offspring survival. A key factor that has contributed to the lack of research in this area has been the difficulty with which parents and offspring can be observed during this period. The North Island robin (Petroica longipes) is a small insectivorous passerine native to New Zealand forests. As a result of the historic absence of mammalian predators, North Island robins lack pronounced anti-predator behaviours and are fearless towards humans. This makes them ideal subjects for behavioural studies in the wild because human presence does not alter their daily activities. Using field observations, the present study examined parental care and the development of caching during the post-fledging period in wild North Island robins. Brood division is a form of preferential post-fledging care that is well documented among avian species in the northern hemisphere. In contrast, little is known about the incidence and function of brood division in avian species outside this region. Across two breeding seasons (2014-2015 and 2015-2016), feeding interactions between parents and offspring were observed during nestling and fledgling development to determine the timing of and factors influencing brood division in robins. Brood division occurred around the time young left the nest and was common amongst broods which fledged two or more young. The male parent typically cared for male and larger fledglings and the female parent for female and smaller fledglings. The results of this study match patterns observed in northern hemisphere species suggesting that brood division provides the same adaptive advantages to species regardless of geographical context. Caching, the handling of food to preserve it for future consumption, is an important strategy which allows numerous avian species to deal with natural fluctuations in food supply. In recent decades, caching has become a widely-used paradigm for examining a range of cognitive processes in birds, such as social cognition and spatial memory. However, much is still unknown about how caching develops in young birds, especially in the wild. Over a 12-week period following fledging, the ontogeny of caching and cache retrieval was observed for 34 juvenile robins. Juveniles began caching shortly after achieving foraging independency (approximately 5 weeks after fledging) and their caching rates increased gradually with age. Retrieval of caches began spontaneously as soon as they had begun to cache and retrieval rates remained constant throughout development. Results suggest that caching behaviour in North Island robins is likely to be innate, but that age and experience have an important role in the development of adult caching behaviours. The two studies described in this dissertation examine behaviours that have either been previously difficult to document in the wild or have not been documented in this species. Overall, the results highlight the behavioural similarities between the North Island robin and other avian species exhibiting brood division and caching. Additionally, they also demonstrate the suitability of the North Island robin for future behavioural research given the ease with which these birds can be observed in the wild.</p>

Parental aggression against dependent young results in task partitioning in a cooperatively breeding bird

In cooperatively breeding species, helpers can alleviate reproductive constraints by assuming the role of primary carers to first-born young, liberating breeders to invest in subsequent broods. However, evidence on how first-born young are transferred to helpers is currently lacking. We propose that breeder–offspring aggression might facilitate inter-brood division and test this idea using data from a wild population of cooperatively breeding pied babblers ( Turdoides bicolor ). After second-brood young hatch, breeders become increasingly aggressive to first-brood fledglings and attack them when they beg for food. After an attack, fledglings reduce begging. Helpers are much less aggressive to begging fledglings and fledglings subsequently tend to target helpers, rather than breeders, when begging for food. In this way, first-born dependent young are transferred to helpers, resulting in a partitioning of tasks among breeders and helpers. Task partitioning in eusocial insects is thought to be determined by the morphological or physiological characteristics of individuals. This complementary study suggests that flexible behavioural strategies may also result in specialized roles in cooperatively breeding vertebrates.

Parental care and brood division in a songbird, the black redstart

Sexual conflict over parental care can be mediated through differences in male and female overall feeding rates, brood division or both. At present, it is not clear whether post-fledging brood division occurs due to sexual conflict over parental investment or is due to bi-parental cooperation, e.g. increase offspring fitness. We provide evidence suggesting that brood division in the black redstart, Phoenicurus ochruros is due to sexual conflict. Males and females had similar feeding contributions during the nestling stage, which is common for most passerine species. After fledging, each parent showed long-term feeding preferences for particular chicks within the brood. In most cases (74%; 17/23) both parents provided care but males tended to feed less fledglings than females did and in about a quarter of cases (26%; 6/23) females fed the whole brood by themselves. The relative amount of male to female post-fledging feedings showed a significant negative relationship with the proportion of fledglings cared for exclusively by the male. These results suggest (1) a close link between the amount of parental care and brood division; (2) sexual conflict can be mediated through brood division; (3) female redstarts appear to loose this conflict more often than male redstarts, with in the extreme cases males showing post-fledging brood desertion. A literature review shows brood division to occur in at least a dozen of songbird species but male black redstarts have the lowest relative post-fledging parental investment, expressed either as feeding rates or number of chicks in care.

Parentally biased favouritism: why should parents specialize in caring for different offspring?

‘Parentally biased favouritism’ occurs when the two parents differentially care for individual offspring or kinds of offspring. Examples in birds include brood division and differential investment by the two parents in relation to the size or sex of the offspring. This paper uses mathematical models to investigate which ideas can, in theory, explain parentally biased favouritism. One previous explanation is that the parents differ in their cost of reproduction and that the parent who consequently invests least concentrates its care on the more valuable offspring. However, a mathematical model predicts the total care given by each parent and received by each offspring, not how much each parent cares for each offspring, and hence does not explain parentally biased favouritism. Parentally biased favouritism towards particular types of offspring can be explained by a difference between the parents in the benefits of caring for a given type of offspring or in the effort incurred in providing care to a given type of offspring, but then it is extreme, with at least one of the parents providing care to only one type of offspring. Parentally biased favouritism towards particular individual offspring (brood division) can be explained by parent–offspring conflict or sexual conflict.