Stuart Rae

Fan-tailed Cuckoo egg in Scrubwren clutch

A Fan-tailed Cuckoo egg with two of  Large-billed Scrubwren

I was down in the Shoalhaven area a few days ago and while out for a walk through some riverside forest I found a Large-billed Scrubwren Sericornis magnirostris nest. I saw the adult bird flush as I walked by and was surprised to discover that it had built its nest inside an old nest of a Yellow-throated Scrubwren Sericornis citreogularis - on subsequent research I discovered that this is common behaviour of the species. Then when I inspected the eggs there was a cuckoo egg amongst the scrubwren clutch. The egg were likely from a Fan-tailed cuckoo Cacomantis flabelliformis  - as scrubwrens are common hosts of these birds eggs and young, and there were a few of these around the area, but no other cuckoo species.

The cuckoo egg is noticeably larger than those of the scrubwren

There were two scrubwren eggs, but whether there had been three and the cuckoo had taken one out is not known. The cuckoo egg was quite a good mimic of those of the scrubwren, although slightly larger and more spotted overall, and a little lighter in background colour. The scrubwren eggs also had more defined rings of spots around the broad end, especially one of the eggs. There was a faint ring around the cuckoo egg, but not very distinguishable.

The cuckoo egg has a faint ring of spots on the broad end like those of the scrubwren eggs,
but much more general speckling

The nest was a simple relined structure inside the old Yellow-throated nest which looked old as the leaves in the structure were dried and deteriorating. 

The clutch was in an old Yellow-throated Scrubwren nest

The nest was set about head height, c 190cm, and typical of a Yellow-throated Scrubwren in being suspended from a the end of a thin branch over an open patch of forest floor -in this case a footpath, although often they are hung over stream lines.

The nest was suspended over a human track through riverside forest