Sky High or Grounded: Nest Site Selection of Herons and Egrets in a Mixed-Species Colony

Abstract

Resource partitioning is crucial for the coexistence of colonial herons, as it allows multiple species to share the same habitat while minimising competition. This study took advantage of a natural experiment in 2006 and 2007 when Black-crowned Night Herons were prevented from breeding at Lake Fetzara in the first year due to the presence of a feral cat. This event provided valuable insight into the spatial and temporal dynamics of nest site selection among coexisting heron species, which consisted of Cattle Egrets (Ardea ibis), Little Egrets (Egretta garzetta) and Squacco Herons (Ardeola ralloides). After the cat was removed, egg-laying began in the core areas of the colony and gradually spread to the periphery. Species that initiated nesting early selected mid-elevation sites near the tree trunk, which likely offered protection from both ground and aerial predators, while also providing some shielding from solar radiation and strong winds. These early selected sites featured larger branches, which conferred greater nest stability. Vertical stratification was evident among the heron species; however, contrary to long-standing assumptions, it was not directly related to body size. Both vertical and horizontal stratification were observed, with nests progressively moving higher and further from the tree trunk as the breeding season advanced. The following year, Black-crowned Night Herons displaced other species to lower heights and positions further from the trunk, highlighting the significant influence of interspecific interactions on nest site selection. This study underscores the complex interplay between nest site selection, biotic interactions and abiotic factors in heron colonies, emphasising the importance of resource partitioning in maintaining species coexistence in densely populated breeding sites.

Keywords: Ardeidae; North Africa; breeding phenology; colonial Waterbirds; competition; resource partitioning.

FIGURE 1

Map of Lake Fetzara depicting the heron colony and associated vegetation. The tamarisk trees ( Tamarix africana ), labelled F to Y, are arranged from east to west. Two clusters of common reed ( Phragmites australis ) are located at the western end.

FIGURE 2

Boxplots of hatching times (using 1 = May 1st) for Cattle Egrets (CE), Little Egrets (LE), Black‐crowned Night Herons (NH; not breeding in the first year) and Squacco Herons (SH) in 2006 and 2007 at Lake Fetzara.

FIGURE 3

Relationship between hatching date and nest location within the colony. The line represents ten groups of two adjacent trees, numbered from east to west: F & G = 1, H & I = 2, and so on, with the western stand of Phragmites numbered 11. Codes: Cattle Egrets (CE), Little Egrets (LE), Black‐crowned Night Herons (NH) and Squacco Herons (SH).

FIGURE 4

(a) Scatterplot showing the horizontal and vertical placement of nests in four trees (Q0, K, L, P) in 2007. (b) Mean position (nest height vs. distance from nest to tree trunk) for all nests in 2006 and 2007. Gold circles represent Cattle Egrets (CE), green diamonds = Black‐crowned Night Herons (NH), orange squares = Squacco Herons (SH) and blue triangles = Little Egrets (LE).

FIGURE 5

Variation in nest height throughout the breeding period for (a) Cattle Egrets and (b) Little Egrets (using May 1st as Day 1).

FIGURE 6

Variation in nest height during the breeding period for (a) Black‐crowned Night Herons and (b) Squacco Herons (using May 1st as Day 1).

FIGURE 7

(a) Variation in distance from nest to tree trunk relative to hatching time for Cattle Egrets (using May 1st as Day 1). (b) Variation in branch diameter relative to hatching time for Cattle Egrets. (c) Variation in branch diameter relative to hatching time for Black‐crowned Night Herons.

FIGURE 8

Proportion of nests in relation to location within the colony for (a) Cattle Egrets and (b) Squacco Herons. The line represents ten groups of two adjacent trees, numbered from east to west: F & G = 1, H & I = 2, and so on, with the western stand of Phragmites numbered 11. Codes: Cattle Egrets (CE), Little Egrets (LE), Black‐crowned Night Herons (NH) and Squacco Herons (SH).