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

Published source details 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.
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Summary

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.

During one autumn/winter, tits within the 1.2 km² study area were fed sunflower seeds from 19 automatic feeders. In February, certain groups of willow tits were monitored to estimate winter group size within both the supplementary-fed population and a control (unfed) population in an area of coniferous forest located c.20 km away.

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).

Levels of luteinizing hormone and testosterone increased significantly between February and March in males from the supplementary-fed population; no such changes were detected in females from either of the populations. By March, levels of luteinizing hormone and testosterone in supplementary-fed males were significantly higher than in control males. Although the gonads of both sexes were initially (in February) significantly heavier in supplementary-fed birds, by mid-March they had reached a similar size in both populations.

Thus, although supplementary feeding apparently resulted in larger winter group size (and higher local population density), the authors suggest that this may have increased male–male competition during the establishment of breeding territories.

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