BRANTA — Marta Cholewa
Timing of breeding and nestling diet of the Great Tit (Parus major) in relation to food supply in primeval forest (Białowieża National Park)
Institution: Wrocław University, Poland
Supervisors: Prof Tomasz Wesołowski
Details: PhD, 2015
ul. Szwoleżerów 18, nr lok. 302, 71-062 Szczecin, Poland
Subject Keywords: Species Keywords: Great Tit, Parus major
In 2008–2011 I studied the breeding phenology and nestling diet of the Great Tit Parus major in the strictly-protected primeval oak-lime-hornbeam stands within Białowieża National Park (hereafter BNP), Poland. My aims were: 1) to investigate which factors affected the onset of breeding, clutch and brood size, brood productivity (percentage of eggs that produced fledglings) and nesting success of Great Tits, 2) to determine whether folivorous caterpillars were the main component of nestling diet, and 3) if the proportion of caterpillars in the chicks’ diet was related to the availability of folivorous caterpillars in the habitat. If the caterpillars were found to be important prey, I aimed to investigate 4) whether the tits adjusted the timing of their breeding to the seasonal peak of this food resource, and also 5) whether the degree of synchronisation between the tits’ breeding season and the caterpillar peak affected the brood productivity and nesting success of the birds.
To date, the published data on the Great Tit’s breeding season and nestling diet has concerned populations living in habitats modified by humans, where the predation pressure was limited and the birds could benefit from anthropogenic food resources, and breeding in nest boxes, usually in high densities. Additionally, the available data on nestling diet were gathered inconsistently and so provide an incomplete picture. As a result, factors that may affect the onset of breeding and composition of nestling diet of Great Tits in natural conditions are mostly unknown.
Long-term studies in the primeval Białowieża forest show that bird species using tree cavities often modify their behaviour when nesting in artificial nest sites elsewhere. Thus, to recognize and better understand the adaptive behavior of Great Tits, including the decision of when to begin breeding, it is important to carry out studies in unmodified habitats in which these adaptations have evolved. Such conditions have been preserved in the strictly protected part of BNP.
I predicted that, in the primeval oak-lime-hornbeam stands of BNP, several factors of varying significance would influence the Great Tits’ decisions on the onset of breeding, and that opposing effects would create a dilemma for the birds.
I expected that defoliating caterpillars would be the main food resource for Great Tit nestlings, but, due to high diversity of invertebrate prey, both the proportion of caterpillars in the chicks’ diet and the availability of folivores in the habitat would not be of critical importance for Great Tit reproduction. I also predicted that the Great Tits’ brood productivity and/or nesting success would be limited by the influence of other factors, such as predation or inclement weather, rather than by variation in caterpillar availability.
In order to collect unbiased data on breeding phenology, clutch and brood size and breeding results of Great Tits nesting in tree holes, I attempted to find the nests of all pairs on selected study plots. Nests were monitored on several visits during the breeding season, and I defined the composition of nestling diet based on direct observations of birds carrying food to their young. Owing to simultaneous observations of the influence of weather changes on three trophic levels (leaf development → emergence and development of folivorous caterpillars → phenology and breeding results of Great Tits), I monitored the degree and importance of synchronisation between these phenomena, and also estimated changes over a longer time perspective (14 years) using data gathered in 1998–2007.
The most significant proximate factor that shaped the Great Tits’ breeding season was temperature during the pre-laying period — the warmer the spring then the sooner the birds began breeding. Additionally, the availability of food for egg-producing females was probably an important factor determining the onset of breeding between seasons — females could not begin forming eggs before sufficient food resources were available (probably at different times in different years). Only some of the birds could ‘predict’ the approach of the period of greatest caterpillar availability, when the chances of raising offspring were highest, based on leaf development and the timing of caterpillar appearance. Moreover, females in poor condition could also use caterpillars as an additional food source that allowed them to attain breeding condition. As such, it seems that caterpillar availability could be the determinant of the onset of the breeding season for the Great Tit, at least in part.
The clutch and brood size and brood productivity of Great Tits breeding in BNP were among the highest observed for this species in Europe, including nest box studies. Predators caused very high nest losses and the frequency of nest failure was similar both within and between years, suggesting no significant pressure for Great Tits to begin breeding earlier or later in the spring. Yet, such a high level of predation, still, could exert a selective pressure to breed as early as possible, as Great Tits commencing breeding early would have a greater opportunity to lay replacement clutches after nest loss, and so a greater chance to rear offspring. In the event of a successful first breeding attempt birds would have also more time to rear a second brood. It is possible that an increased chance to rear offspring was the greatest benefit arising from breeding as early as possible.
Changes in the photoperiod, arrival dates (most Great Tits in BNP are migratory) and duration of nest building did not shape the timing of Great Tit breeding seasons, but they were required preliminaries to the onset of breeding. Additionally, competition for tree holes and food with bird species breeding later did not determine the onset of breeding by Great Tits. Similarly, the influence of female age was not significant because the difference in the timing of breeding between young and older females (which commenced egg-laying an average of two days earlier than youngsters) was much smaller than the variation observed between years. Moreover, such differentiation could be explained by other mechanisms (e.g. optimization of life breeding effort).
Caterpillars constituted the main component of the nestling diet of Great Tits, irrespective of the variation in supply of this food type between and within breeding seasons. The nestling period overlapped widely with the caterpillar season but long-term observations showed that the time of the maximum nestling food demand usually fell after the caterpillar peak. The brood productivity and nesting success of Great Tits which broods synchronized with caterpillar peak availability (where the maximum nestling food demand (at 10 days old) fell within three days of the caterpillar peak) were similar to that observed in asynchronous broods. Hence, variation in food supply did not exert a significant pressure to synchronize broods with the peak availability of caterpillars.
It is most probable that the timing of breeding by Great Tits in BNP resulted from a compromise between the pressure to breed as early as possible and the costs of breeding too early. It seems that Great Tits have reached the optimum in this respect and any advance or delay of the breeding season would lead to a decrease in individual fitness.