Ousted cuckoo?

An adult male Lapland Bunting Calcarius lapponicus 

While surveying birds in northern Norway I came across a Lapland Bunting nest, which is not too unusual as they are common birds in the tundra there, although there was something different about this one. There were chicks in the nest as well as an unhatched egg and an eggshell, again nothing unusual there, except that the eggshell was of the wrong colouring for a Lapland Bunting egg.

The eggshell was a dull blue colour with brown freckling,
unlike the grey-brown background with scribbly line markings of the bunting egg

I immediately thought of a Cuckoo Cuculus canorus having laid an egg in the nest, so I checked the young to see if any were a cuckoo. None were, so what had happened? Thinking it through, the only explanation I can think of is that the buntings had detected the cuckoo when it hatched and ousted it from the nest.

The cuckoo egg did not resemble those of the buntings. It was more like a composite match between a Meadow Pipit Anthus pratensis and a Red-Spotted Bluethroat Luscinia svecica - two other common passerines in the area, and likely species to be targeted by cuckoos. Pipit eggs are grey-brown with dark brown freckles, and bluethroat eggs are dull blue coloured with tiny faint freckles. Yet this egg seemed to still pass unnoticed by the hosts.

The cuckoo eggshell on the left, the unhatched bunting egg on the right

I checked the nest a few days later and all the chicks, four, were alive and similar in plumage - all were buntings. So it seems that a cuckoo had failed in its attempt to dupe the buntings into rearing its chick. They perhaps detected the cuckoo chick when it hatched as different from their own and ejected it. This must happen more often than we assume, for there is an arms race between cuckoos and their hosts. In this case the cuckoo seemed to have laid its egg too late for the chick to gain the advantage of hatching first and ousting the buntings' eggs or newly hatched chicks.

Then, why had the cuckoo laid its egg in a bunting nest, were there too few pipits or bluethroat nests with eggs for it to parasitise in this late cold Spring?

The growing bunting chicks

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