Empirical tests of habitat selection theory reveal that conspecific density and patch quality, but not habitat amount, drive long-distance immigration in a wild bird

Out of Africa: Juvenile Dispersal of Black-Shouldered Kites in the Emerging European Population

Simple Summary

Black-shouldered Kite (Elanus caeruleus) is a relative recent colonizer in Europe. The study of juvenile dispersal may help to better understand the patterns of range expansion and colonization. In this study, we provide some information about patterns of juvenile dispersal according to sex, habitat quality, timing of reproduction, and nesting hatching order to test two competing hypotheses about natal dispersal in this poorly studied raptor. We found some evidence supporting the Resources Competition Hypothesis since nestlings hatched from high quality territories and hatched first within the brood stayed closer from natal areas than nestlings hatched from low quality territories or later hatched nestlings.

Abstract

Knowledge of animal dispersal patterns is of great importance for the conservation and maintenance of natural populations. We here analyze juvenile dispersal of the poorly studied Black-shouldered Kite (Elanus caeruleus) monitored in southwestern Spain in an ongoing long-term study initiated in 2003. The European population of Black-shouldered kites is thought to be a recent one funded by colonizing African birds, as no kites have been found in the European fossil record, and the breeding population has progressively expanded to the North in the late 20th and 21st centuries. We obtained information on movements behavior during dispersal from 47 juveniles Kites after marking 384 nestlings with wing tags and three nestlings with radio transmitter. We have tested two competing hypotheses (i.e., the Resources Competition Hypothesis and the Wandering Hypothesis (WH)) that may explain the leptokurtic distribution of the natal dispersal distance in Elanus. After independence, juvenile females dispersed farther from the natal areas than males, as is common in birds. On average, males and females dispersed from their natal areas over 9 (i.e., 26.15 km) and 15 (i.e., 43.79 km) breeding territories, respectively. A male and two females dispersed further than 100 km from their natal nest. Our results indicated some evidence supporting the competition-for-resources hypotheses since nestlings hatched from high quality territories stayed closer from natal areas than nestlings hatched from low quality territories and also nestlings hatched first within the brood also tend to recruit closer to their natal area than later hatched nestlings which tend to disperse further away from their natal area. The information provided by these crucial demographic parameters will be used for the elaboration of future conservation plans for the management of this colonizing species in Europe.

Can attraction to and competition for high-quality habitats shape breeding propensity?

In many animal species, sexually mature individuals may skip breeding opportunities despite a likely negative impact on fitness. In spatio‐temporally heterogeneous environments, habitat selection theory predicts that individuals select habitats where fitness prospects are maximized. Individuals are attracted to high‐quality habitat patches where they compete for high‐quality breeding sites. Since failures in contests to secure a site may prevent individuals from breeding, we hypothesized that attraction to and competition for high‐quality habitats could shape breeding propensity.

Under this hypothesis, we predicted the two following associations between breeding propensity and two key population features. (1) When mean habitat quality in the population increases in multiple patches such that availability of high‐quality sites increases across the population, the resulting decrease in competition should positively affect breeding propensity. (2) When the number of individuals increases in the population, the resulting increase in competitors should negatively affect breeding propensity (negative density dependence).

Using long‐term data from kittiwakes Rissa tridactyla, we checked the prerequisite of prediction (1), that availability of high‐quality sites is positively associated with current mean habitat quality in the population (represented by breeding success). We then applied integrated population modelling to quantify annual fluctuations in population mean breeding success, breeding propensity and number of individuals by breeding status (pre‐breeders, breeders, skippers and immigrants), and tested our predictions.

Our results showed that breeding propensity acts as an important driver of population growth. As expected, breeding propensity was positively associated with preceding mean habitat quality in the population, and negatively with the number of competitors. These relationships varied depending on breeding status, which likely reflects status dependence in competitive ability.

These findings highlight the importance of competition for high‐quality breeding sites in shaping breeding propensity. Thereby, we draw attention towards alternative and complementary explanations to more standard considerations regarding the energetic cost of reproduction, and point to possible side effects of habitat selection behaviours on individual life histories and population dynamics.