Marsh Bird Relationships

     Station 43 in South Windsor is considered by many to be one of Connecticut’s premier birding locations. A beautiful eleven-acre marsh, Station 43 is owned by the Hartford Audubon Society and serves as one of the few breeding sites in the state for the elusive Least Bittern.

     On a hot, humid day in late June, I visited the marsh with hopes of catching a glimpse of this small heron. Like many of my past outings, achieving success with bitterns often entails a lot of patience as well as a bit of good luck. 

     As I eagerly anticipated the appearance of a Least Bittern, I couldn’t help but notice all the other wetland species flying about me. Swamp Sparrows, Marsh Wrens and Red-winged Blackbirds were all observed to be defending territory or actively nesting in a confined area of the marsh.  I thought to myself, how do all these birds breed in such close proximity, and still manage to be successful?

     Marshes are low-lying tracts of land that hold water throughout much of the year. They are dominated by thick, rooted vegetation such as cattails, bulrushes and sedges.  

     Birds that inhabit and nest in marshes are some of the least understood species amongst birdwatchers. The difficulty of entering an overgrown wetland in the summer is the probable cause for this. We know even less about the intricate relationships that play out within these lush habitats. My morning observations, though trivial, may shed light on some of those relationships.

     The first species that caught my attention was the Marsh Wren. Its lively song could easily be heard from the middle of the marsh. Since I couldn’t see the bird, I decided to use an ipod recording to help it come into view. Instead, the wren flew low and directly toward me. But before it could land, a Swamp Sparrow came out of nowhere and intercepted the wren, forcing it out of its territory. An agitated male Red-winged Blackbird also flew in to secure its boundary. 

     Marsh Wrens typically nest in isolated pairs, for good reason. This is because wrens are notorious for destroying the eggs and young of neighboring birds such as Red-winged Blackbirds, Swamp Sparrows, Virginia Rails, Sora, Least Bitterns and even other Marsh Wrens. The theory behind these destructive habits includes creating less competition for food with other marsh nesting birds and the expansion of their own breeding territory.

     Playing the Marsh Wren recording brought back memories of a time when I use to conduct call-back surveys for the Connecticut DEP in Glastonbury. I was always amazed at how fast a Virginia Rail could cross a marsh, on foot, when faced with a potential intruder. One rail I remember, parked itself at my feet for over a half-hour calling angrily. Sora is known to be even more aggressive than the Virginia Rail when it comes to defending its territory.

     The Red-winged Blackbird is by far the most conspicuous and energetic species at Station 43. Always in defense of their territorial boundaries, a minute never goes by without a chase or an aggressive response toward another bird. Here is nice description of the male and female blackbird’s territorial habits from the Birds of North America Online;   

     Male establishes and defends territory with clearly delineated boundaries during breeding season. Boundaries may shift within a breeding season (Dickinson and Lein 1987). All activities occur within territories, but male and female also forage, engage in sexual chases, seek extra-pair copulations, and prospect for other breeding opportunities outside territorial boundaries. Defense is based on conspicuousness, song and visual display, and aggressive responses to persistent trespassers (Nero 1956b, Orians and Christman 1968, Peek 1972).

     Existence of female territoriality is controversial. Female is aggressive toward others. Some authors (Nero 1956b, Beletsky 1983a, Hurly and Robertson 1984) assert that females defend “subterritories” within territories of males, but female display perches overlap extensively (Searcy 1986) and the primary aggressive vocalization (teer song) does not deter other females from settling (Yasukawa 1990). Nests are not overdispersed, as would be expected if females were territorial (Picman et al. 1988, Yasukawa et al. 1992a). Dominance status of resident female depends on order of settlement and distance from nest when interacting with other females on male’s territory

     As for my morning search for Least Bitterns, I did finally locate a single bird clinging to an alder bush. Many birders, including myself, raised concerns about the absence of bitterns this spring. Were they no longer breeding at Station 43? What I found out about this year’s late discovery is provided here, courtesy of BoNAO; 

     Because Least Bitterns are highly insectivorous, delayed breeding cycle may be a response to the life cycles of aquatic prey. Emergence of aquatic insects in temperate-zone wetlands peaks in Jun (Orians 1980) when these bitterns are feeding young. Later broods not definitively known, but temporal pattern of nest initiation suggests double-brooding may occur (Kent 1951, Weller 1961). If so, late Jun to mid-Jul in Iowa, with second broods larger than first, owing to the increased availability of food later in the breeding season (Weller 1961).

     In addition to the increased availability of food, I’d like to suggest another benefit to delayed breeding. A delay could help bitterns avoid other nesting birds at a time when they are the most aggressive. A toned down wren or blackbird may be preoccupied with feeding nestlings, rather than territory expansion. This might be enough to reduce the chances of egg loss. 

     Despite all the known conflicts associated with nesting in a marsh, there are a number of important advantages. First, situating nests in and above submerged vegetation helps protect eggs and nestlings from land-dwelling predators like raccoons. Nesting among Red-winged Blackbirds provides additional protection for birds against avian predators such as Blue Jays and Common Grackles.The abundance of aquatic prey that surrounds the nest site is essential to feeding fast growing young.

     If I had to pick one word to describe the relationship between marsh nesting birds, it would have to be tenuous. Even though I singled out the aggressive breeding nature of both the Marsh Wren and Red-winged Blackbird, it is important to realize that each and every wetland species is fully capable of defending its territory, invading nests and destroying eggs to further their own breeding success.