Colony size and not nest density drives reproductive output in the Common TernSterna hirundo

Abstract

Density is known to be an important factor in population size regulation. Several mechanisms of density limitation have been identified in colonial birds. We studied competition in Common Terns Sterna hirundo to assess whether the factor limiting reproductive output was competition for nest-sites, which is dependent on local nest density, or density-dependent competition for food resources, which is dependent on overall colony size using the same foraging area. We found strong associations of both colony size and nest density with reproductive output in five colonies of Common Terns in three different habitats (one marine, two freshwater). Based on detailed long-term datasets of six separate sub-colonies of the Banter See colony that differed in nest density, we found that reproductive success was not related to nest density but to overall colony size, possibly a result of resource depletion and food competition. We also found carry-over effects of colony size during rearing on post-fledging return rate. These results have important implications for the conservation management plans aimed at recovering declining populations of Common Terns.

Opposing selective pressures may act on the colony size in a waterbird species

“…120 chicks vs. 30-40 chicks) and all grown-up chicks could move freely around each colony, the frequency of interactions between unrelated chicks, as well as between chicks and non-parental adults was undoubtedly higher in the larger colony, likely increasing the level of social stress and deceasing chick survival. Similar conclusions were reached in a recent study on Common Terns in Germany, demonstrating that reproductive parameters are primarily determined by colony size, and not necessarily by nesting densities (Szostek et al 2014).…”

“…Therefore, we may expect that the final fitness balance of terns nesting in the colonies of different size should be much more similar than indicated by the analysis of reproductive success at the moment of fledging. However, since Common Terns can breed in the colonies of up to several thousand pairs in nature (Szostek et al 2014), studies on much broader range of colony sizes would be needed to unequivocally confirm this conclusion.…”

The role of behavior in tern conservation

“…However, if overall metapopulation size has decreased below a minimum level, then Allee effects may be impossible to avoid and population declines will accelerate. At the other extreme, colonies that are too large may suffer density-dependent declines in productivity due to intraspecific competition for food within the foraging range of the colony (Furness and Birkhead, 1984;Tims et al, 2004;Szostek et al, 2014a).…”

“…Knowledge of modes of competition is important when designing artificial habitat. Intraspecific competition for nest sites may be alleviated by providing additional nearby rafts or other nesting habitat, but intraspecific competition for food necessitates creation of habitat outside the foraging range of the main colony (Szostek et al, 2014a).…”

Habitat- and density-dependent demography of a colonial raptor in Mediterranean agro-ecosystems

“…On the other hand, living in large groups imposes significant costs (e.g. Szostek et al, 2014), as lesser kestrels experience density-dependent food depletion in large colonies (Bonal and Aparicio, 2008). First-breeding birds are forced to emigrate from natal sites due to social interactions with adults in colonies at carrying capacity (Serrano and Tella, 2007), and the increase in colony size beyond a certain threshold exerts a negative effect because of its repercussions on nest distance (Serrano et al, 2004).…”