Research published in the Journal of Avian Biology has added to the growing body of evidence on the consequences of supplementary feeding of wild birds, an activity popular in parts of Europe, including Britain, and North America.
Over seven years, the study looked at the changes in the breeding cycle of Great Tits in relation to winter bird-feeding in southern Sweden. As body condition is crucial to reproductive output, the researchers were keen to investigate the role that feeding might play.
Supplementary feeding led to Great Tits producing larger clutches in the spring (Robyn Dalton).
Laying date and clutch size were recorded as measures of the onset of reproduction, and of breeding investment. No experimental manipulation took place over the first three years of the study, but the last four years saw the researchers introduce supplementary feeding to part of the study area. Other parts of the area acted as a control, with no artificial feeding taking place.
It was clear from the results that breeding performance was positively affected by the provision of supplementary food during the winter, with birds from the feeding area laying bigger clutches than those from the control area. There was no difference, however, in the onset of breeding, with laying dates remaining similar.
There was variation in laying date and clutch size over the course of the study, but the influence of artificial feeding on clutch size was consistent, demonstrating a 'carry-over' effect of winter feeding on breeding performance in the spring. The average clutch size in the fed area was 9.23, compared to an average of 8.8 in the area with no winter feeding.
Birds alter their investment in breeding attempts based on their assessment of factors including food availability and their own condition. It seems the fed Great Tits in the study area were better prepared for the breeding season after enjoying a reliable supplementary food source over the most gruelling season.
Passerines such as Great Tit base their investment in breeding attempts on food availabilty and their own body condition (Irene Harrison).
The researchers said: "Winter survival and individual condition at the start of reproduction depend on the ability to obtain the necessary resources to cover this increased energetic challenge, a cost that can be alleviated when human-provided food is available."
The study area, where nestboxes have been provided since 1988, had its feeding and non-feeding zones separated by nearly 1 km. As part of the research, birds were ringed, allowing individuals to be identified and site fidelity to be assessed: 97% of birds recorded at least twice did not move between the zones. This helped to firm up the conclusions of the study, that winter feeding had a real effect on the investment birds were able to put into their breeding attempts.
Although the results of the study indicate that winter feeding has a positive effect, the researchers warned that there could be negative consequences, if birds misinterpret local conditions because of food provided by humans. They said improved survival of birds in poor condition, and the increased risk of predation and pathogens, are worth bearing in mind.
The researchers concluded: "As supplementary feeding is becoming a more popular activity worldwide, it is important to reveal the underlying physiological mechanisms that link life-history stages, their effects on individual fitness, and the causes and consequences of the variation observed among species and populations, to increase beneficial use of food supplementation and prevent potential harmful consequences on wild fauna."
Broggi, J, Watson, H, Nilsson, J, & Nilsson, J-A. 2022. Carry‐over effects on reproduction in food‐supplemented wintering great tits. Journal of Avian Biology. DOI: https://doi.org/10.1111/jav.02969