The regulation of brood reduction in Booted Eagles Hieraaetus pennatus through habitat heterogeneity

Abstract

Brood reduction, the death of one or more chicks through siblicide or starvation, can occur through density-dependence in fecundity. Brood reduction may arise in territorial breeding systems either as a response to a high level of territorial interference in a situation of high density or as a result of habitat heterogeneity. To test the predictions of the two main hypotheses that attempt to explain how density-dependent fecundity is generated, the Habitat Heterogeneity Hypothesis (HHH) and the Individual Adjustment Hypothesis (IAH), we analysed the relationship between density and fecundity in an expanding population of Booted Eagles in Doñana National Park, Spain, using an 18-year data series. We also studied the occurrence and frequency of brood reduction in the same Booted Eagle population to appreciate further its effects and the factors that influence its occurrence and frequency. Our results support the HHH in the present situation of high density, as fecundity in the better territories (older and more frequently occupied) was higher than in low quality territories and was not affected by population density in high density periods. Nevertheless, the fecundity of high quality territories was affected (although not significantly) by population density in periods of low density, suggesting that the IAH was supported when only high quality territories were occupied. Older territories were used more frequently and chicks in these areas hatched earlier and suffered lower mortality than in new territories. We found a significant negative relationship between mean fecundity and its skewness, a finding that also supports HHH. During years of food shortage, less frequently occupied territories suffered higher rates of brood reduction. Brood reduction in this Booted Eagle population was a consequence of the heterogeneous structure of the habitat, with some territories having a higher probability of brood reduction than others. Parental nutritional condition did not affect brood reduction. The effect of brood reduction on nestling quality and population dynamics is also discussed.

Sibling aggression and brood reduction: a review

“…Nevertheless, some studies found a significant relationship between brood size and nutritional conditions of the young but in the opposite direction; large broods showing better nutritional conditions (FERRER 1994;FERRER & PENTERIANI 2007). This kind of relationship was a consequence of high differences in quality among territories under a density-dependent regulation by habitat heterogeneity Brood reduction hypotheses 11 (CASADO et al 2008). Differences in territory quality are not usually considered in sibling aggression studies.…”

Floater interference reflects territory quality in the Spanish Imperial Eagle Aquila adalberti: a test of a density‐dependent mechanism

“…As the frequency of intrusions and interference increases, individuals show reduced productivity (Lack 1966, Fretwell & Lucas 1970, Dhondt & Schillemans 1983, Fernandez et al 1998, Newton 1998, Sillett et al 2004). Other authors, however, have proposed that density-dependent depression in mean productivity is the result of habitat heterogeneity (Korpimaki 1988, Kadmon 1993, Ferrer & Donazar 1996, Newton 1998, Ferrer & Bisson 2003, Sergio & Newton 2003, Casado et al 2008, Tanferna et al 2013). In a low-density population, individuals entering the breeding population select optimal territories of high quality.…”

“…As density increases, a higher proportion of individuals are forced to occupy poorer quality territories, where their reproductive success is lower. Thus, while productivity in the better territories is main-tained, mean population productivity in the breeding population as a whole decreases (Andrewartha & Birch 1954, Brown 1969, Rosenzweig 1991, Dhondt et al 1992, Sutherland 1996, Rodenhouse et al 1997, Newton 1998, Gill et al 2001, Sutherland & Norris 2002, Casado et al 2008.…”

Tawny owl vocal activity is constrained by predation risk

“…Doñana Protected Area is characterized by a high degree of habitat heterogeneity, and both owl species, like most diurnal raptors and mammalian carnivores, are mainly concentrated in highly productive areas, where prey abundance and breeding success are higher (Veiga and Hiraldo 1990, Viñuela et al 1994, Ferrer and Donázar 1996, Casado et al 2008). Consequently, those areas favoured by tawny owls also seem to be preferred by eagle owls.…”