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

Abstract

In theory, larger colony size should be favoured by lower per-capita predation rates, whereas smaller colony size should be favoured by reduced parasitism, social stress and competition for food. We conducted an experimental cross-fostering of young between colonies of different size to test whether differences in fitness had an environmental or genetic basis. We induced formation of one large (ca. 100 breeding pairs) and three small (30-40 pairs) Common Tern Sterna hirundo colonies by providing different size patches of nesting area (floating rafts). The larger colony had about 30 % higher reproductive success than the smaller colonies. However, offspring raised in the large colony were in poorer condition, which was indicated by the higher heterophil/lymphocyte ratios, lower hemoglobin concentrations in blood and slower growth rates, suggesting higher parasite loads or higher levels of social stress. By performing a cross-fostering experiment we confirmed that differences in chick condition were not inherited, but could be directly attributed to different rearing conditions experienced in the colonies of varying size. These results suggest that colony size in the Common Tern may be regulated by the opposing selective pressures (predation vs. parasitism/social stress).

Central–periphery gradient of individual quality within a colony of Black‐headed Gulls

“…As changes in the relative proportion of heterophils and lymphocytes are mediated through stress hormones, the H/L ratio has been also recognized as a robust indicator of physiological stress (Davis et al 2008). This hypothesis is consistent with the recent findings for Common Tern Sterna hirundo nestlings, in which elevated H/L ratios were attributed to more frequent agonistic social interactions (Minias et al 2015b). The intensity of agonistic behaviour may also increase as a function of breeding density (Butler & Trivelpiece 1981), which is often highest in the colony centre.…”

“…On the other hand, predators may choose to forage selectively in the parts of the colony occupied by individuals of lower quality (e.g. in the peripheral zone), as it may reduce the time and energy investment in prey capture (Minias et al 2015b). Although we lacked empirical data on the age of gulls nesting in our colony, it seems probable that older individuals might be more likely to settle in the central part of the colony, as similar patterns have already been reported for other colonial waterbirds, including larids (e.g.…”

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

“…Opposite selection pressures are thus acting on colony size as in common terns Sterna hirundo, where large colonies promote higher survival but reduce the quality of chicks (Minias et al, 2015). Similarly, we detected an opposite colony size effect in juvenile survival and fecundity.…”

Extreme group sizes in a colonial bird favored during a rare climatic event

“…With colony-size preferences known to be heritable (Brown and Brown 2000a, Møller 2002, Serrano and Tella 2007, why selection does not remove individuals occupying the less successful colony sizes remains paradoxical. Various hypotheses have been proposed, including that individuals do not have total control over their group size (Sibly 1983, Zemel andLubin 1995), that incomplete estimates of individual fitness lead us to concluding that some colony sizes are best when they actually are not (Minias et al 2015, Brown 2016, and that selection fluctuates spatiotemporally such that different-sized colonies are favored at different times or places (Jones andRiechert 2008, Brown et al 2016) due to changes in food, predators, parasites, or other environmental conditions.…”