Territory quality affects the relative importance of habitat heterogeneity and interference competition in a long‐lived territorial songbird

Abstract

Density-dependent reproduction is commonly explained by either the habitat heterogeneity (HHH) or individual adjustment (IAH) hypothesis. Under the HHH, high quality territories are assumed to be occupied fi rst. At higher density, occupation of low-quality territories increases due to lower availability of high-quality territories, which reduces mean reproductive success. Alternatively, the IAH assumes that increased competition at higher densities reduces reproductive success in all territories. For birds of prey, HHH plays an important role in territorial species, and IAH in socially breeding species. To test the generality of this hypothesis, we studied the mechanism behind density dependence in raven Corvus corax , a long-lived passerine bird, using long-term population data from a large number of territories.Population density decreased reproduction, which was explained by increased usage of low quality territories at higher density, supporting the HHH. Density reduced reproduction in low quality territories, but not in high and intermediate quality territories. We additionally compared the explanatory power of diff erent models describing brood size, representing IAH, HHH, or a combination of both. Th e best model represented a combination of both hypotheses, in which the eff ect of density depended on territory quality. Our conclusion that both IAH and HHH are supported can be explained by the biology of ravens, where territorial adults not only experience interference competition with other territorial adults, but also with social groups of juveniles and fl oaters. We conclude that the relative importance of IAH and HHH may depend on variation in territory quality and social structure.

Demographic and population responses of an apex predator to climate and its prey: a long‐term study of South Polar Skuas

“…These two hypotheses have been suggested by Grünkorn et al. () to explain negative density‐dependent reproduction in a species sharing many common life history traits with birds of prey, the Raven, Corvux corax . Non‐breeders (i.e., floaters) constitute a significant part of seabird and skua populations (Katzner et al.…”

Food preference of the Black-headed GullChroicocephalus ridibundusdiffers along a rural–urban gradient

“…Using the on-screen digital timer, 'settle time' was defined for each trial as the time elapsed from the start of the observation to the time at which the first food item was taken by a Blackheaded Gull. To account for the potential confounding effect of bird density affecting competition (Grünkorn et al 2014) or behavioural shyness, we described the number of gulls (abundance) prior to each trial semiquantitatively, by classification into the following standardized group sizes: 0, 5, 10, 20, 50, 100. Preference was defined by the food group from which the first food item was consumed, Black-headed Gulls always consumed one food group in its entirety before moving on to the second group.…”

Density‐dependent productivity in a colonial vulture at two spatial scales

“…In this scenario, it is possible to envisage changes in productivity as the result of a combination of both processes (HHH and IAH; Grünkorn et al. ).…”

“…Two hypotheses have been invoked to explain the mechanisms behind density‐dependent breeding output: the habitat heterogeneity hypothesis (HHH) and the individual adjustment hypothesis (IAH, also referred to as interference competition; see Grünkorn et al. ). The HHH propounds that increasing breeding densities lead to the saturation of optimal breeding sites.…”