We tested “the Living child-free hypothesis” (Solovyeva et al., 2019) using our Mute Swan
nesting data, collected on the Kurgalsky Peninsula in 2005-2021. The number of swans varied
from 17–102 pairs/year, depending on temperature in the wintering grounds and the timing of
the ice melt. We observed both territorial pairs and colonial nesting. All nests (n = 858) were
described and mapped. The total number of territorial pairs and their spatial distribution
changed very little, while the number of colonial pairs varied greatly (> 10-fold). Population
declines coincided with years of climatic pessimism. In colonies, the proportion of empty nests
and abandoned clutches increased, and the size of clutches and the success of incubation
decreased, both during the years of the greatest population declines and during the years of
strong population growth. All reproductive parameters of territorial-nesting birds improved
with population growth. Analysis of the breeding sites occupied by birds over many years (both
for territorial and for colonial birds), and for territories and sites in colonies occupied for 1-3
years, found that all long breeders (both in colonies and outside colonies) have improved
reproductive parameters at times of climatic maximum and population growth, and a decrease
in overall reproductive indicators at population peaks due to a strong increase in the number
of short breeders in colonies. Long breeders in their first year of breeding had reproductive
parameters lower than in subsequent years. Thus, the observed “living child-free” effect may
be due to massive breeding attempts of young birds in colonies during the years of population
increases.