Individual study: Effects of over-winter feeding on local population density and reproductive physiology of willow tits Parus montanus near Gothenburg, Göteburg och Bohus, Sweden
Silverin B., Viebke P.A. & Westin J. (1989) An artificial simulation of the vernal increase in day length and its effects on the reproductive system in three species of tits (Parus spp.), and modifying effects of environmental factors: a field experiment. The Condor, 91, 598-608
As part of a broader investigation into the cues stimulating the onset of breeding in tits Parus spp., this study investigated the effects of over-winter supplementary feeding on the local population density and reproductive physiology of willow tits Parus montanus in an area of coniferous forest east of Gothenburg, south-western Sweden.
In mid-February and mid-March, a number of individuals from each population were captured and (following the collection of a blood sample) killed. Levels of reproductive hormones (e.g. luteinizing hormone and testosterone) in the blood plasma were measured, and the testes or ovaries of specimens were weighed.
Thirteen of the 19 feeders were used by mixed groups of tits Parus spp. throughout the winter. Winter group size for willow tits was 3.0 ± 0.4 (n = 9 groups) in the supplementary-fed population and 1.5 ± 0.1 (n = 10) in the control population. The density of willow tit groups within the supplementary feeding area (c.7/km²) was also higher than the average reported for the species (4.8/km²).