Ask SIB: Nesting Anhingas

Seabrook Island Birders (SIB) welcome questions from our community of birding friends! If you have one, just fill out the form on our website or send us an email!

I’ve seen a male and female Anhinga, I assume a mating pair, on Jenkins Point Road in the lagoon rookery on the right.  You know better than I that there are only a few Egret nests in that rookery.  I noticed on several different visits over the last 10 days or so that the Anhinga pair was sometimes sitting on a nest and sometimes they were not and sometimes they were in a big time squabble with a pair of Great Egrets over the nest!  I have photos of the squabble on June 1^st.  I swear the Egrets and Anhinga pair were fighting over the nest!

I was wondering about a few things and thought perhaps you might have some insight on the following:

1.     Do these two species often steal each other’s nest? 

2.     Do Anhinga’s typically nest in the same rookery as Egrets?

3.     Isn’t it late in the season for chicks to hatch? 

Valerie Doane

So for some quick answers, Nancy Brown responded as follows (be sure to “Read More” to see the answers and more photos!):

1. Do these two species often steal each other’s nest? This wouldn’t surprise me, although I didn’t find anything specific to answer this question.
2. Do Anhinga’s typically nest in the same rookery as Egrets? – Yes, they do – and we’ve seen them always at Magnolia Plantation Audubon park and even on Ocean Winds #4 there are a couple Anhinga’s.  All About Birds states: “The Anhinga typically nests in loose groups of several to hundreds of pairs, and sometimes with other colonial waterbirds. The nest is usually in a tree near to water or overhanging it.”
3. Isn’t it late in the season for chicks to hatch? I found that in SC they breed from March – May, and incubation is 25-29 days, then 14-21 days in the nest.  So, no, I don’t think this is too late.

But in order to get more detailed and accurate answers, we went to Bob Mercer, SIB’s “resident naturalist”, who said “I agree with everything you are saying. I did a little digging on Birds of the World and learned a few things.”

Here is a quote from Birds of the World on Anhinga aggression.

Physical Interactions

One of the most territorial of the Pelecaniformes toward conspecifics, but usually unaggressive toward other species. Agonistic behavior between males is common in colonies, especially on nest; will approach intruders by hopping along branches of tree with spread wings and open bill. Rarely, contestants stab each other on head and neck, sometimes grappling aggressively (W. J. M. Vestjens in Cramp and Simmons 1977, Siegel-Causey in press). Aggressive encounters between females are uncommon and less intense than those between males. In a mixed-species colony in Mexico, the Anhinga demonstrated the highest proportion of conspecific aggression of any species. Tends to be socially dominant to most heterospecifics (usually Ciconiiformes) in mixed-species breeding colonies (Burger et al. 1977a).

Nonpredatory Interspecific Interactions

Occurs often in mixed-species flocks at roosts and in the air, especially with other waterbirds. May nest in and near colonies of other waterbirds, but generally interacts little with other species when doing so, and may allow close nesting by heterospecifics. Most ciconiiform birds are socially subordinate to Anhingas; Anhingas may appropriate freshly built nests of Little Blue Heron (Egretta caerulea), Great Egrets (Ardea alba), White Ibises (Eudocimus albus), or Snowy Egrets (Egretta thula; Allen 1961b, Terres 1980b). During breeding, 25% of agonistic encounters were with other species, and Anhingas won 92% of bouts (Burger et al. 1977a). In a roost in coastal Mexico, however, 90% of Anhinga agonistic encounters involved conspecifics, and Anhingas generally were less often involved in aggression than were other species in the same colony (0.12 encounter/bird-h for Anhingas, 0.44 for other species present in roost; Burger et al. 1978).

I then checked Great Egret to see if they are aggressive. This is a quote on them. I colored the key text.

Reacts to presence of other individuals with increasing degrees of threat or hostility, including relatively mild Upright (employed principally by nonterritorial birds at colony or foraging sites), Fluffed Neck (termed Erect Stance by Wiese [Wiese 1976b]; principally at mates during nest relief, but also directed elsewhere, both intra- and interspecifically), Forward (a variable but graded signal directed either toward neighbors of any species or conspecific females as mate attraction proceeds; see also Tomlinson 1976), and Supplanting Attacks (most frequently directed to conspecifics, and often include physical contact).

Later on it said:

Degree of aggressive interaction between Great Egrets and other species apparently varies as a function of vegetational physiognomy. In a N. Carolina heronry with relatively high diversity of plant species and life forms, Great Egrets were more aggressive to their own than to other ardeid species (McCrimmon 1978). In a New Jersey colony in which the vegetation was structurally less complex, Burger (Burger 1978d) reported greater interspecific aggression among ardeids.

So, both species are not usually, but can be aggressive towards other species during mating season. Anhinga are also know to occasionally usurp another birds nest, though it seem relatively rare. I would thing that pond on Jenkins Point provide sparse habitat and nature of the vegetation provided limited nesting opportunity for both species.

Great Egret young are usually off the next by the end of June  and Anhinga by end of July. (there are always exceptions). Anhinga nesting is spread out from Feb to July while Great Egrets is April to through June. It is possible that the Anhinga pair is starting a late nest and were trying to take over an egret nest, but if they are, the Egret nest should not have any young or eggs in it. I suspect it is not a case of nest stealing, but a case of neighbors being just a little too close.

Thank you Valerie Doane for submitting your questions and photos! And thank you for these detailed answers Bob Mercer!