With the nesting season over, autumn brings bird box maintenance. One of our bluebird box poles was leaning and inspection revealed the 5" by 5" supporting pole broken at the base. The break was into the ground so fortunately it did not interfere with the rearing of two clutches of bluebirds this summer. The pole was removed, the base dug out and a new pole was prepared. While transferring the box from one pole to the other, it was opened for cleaning. Normally our boxes have about 2 inches of sawdust and wood shavings in the bottom. A surprise awaited us on removing the face of the box. There sitting on top of the wood shavings was a nest with eggs in it. Well, whose eggs were these and what happened to the remains of the bluebird nest?
The top photo shows the contents as removed from the bluebird box. The nest is on top of the wood shavings with some of them incorporated into its structure. The main nesting material is thick and made of coarse and fine grasses with a good addition of feathers. The feathers look like chicken feathers probably from our neighbour's flock. The nest contained 4 creamy coloured eggs with fine streaks and spots of reddish and dark brown. My first comment on observing the contents was, 'Those are not bluebird eggs". But whose were they?
The nest looked like the owner had just flown off for a morning forage. However, we knew that was not so because the box is in close proximity to the house and is observed often. The only activity there has been the flock of 4 to 8 Eastern Bluebirds which have been hanging around our yard this past week. Although we have seen them peek into some of the boxes, they seemed interested only in perching on garden stakes and our various roofs in order to fly out to catch insects. So what could these eggs be from?
Earlier in the summer we had a very active group of Tree Swallows using some of our nest boxes. I think we had 4 successful broods raised. The swallows have long since gone. After they fledged they seemed to move out over the river to feed most of the time. We have not seen a Tree Swallow in the yard for over a month now. The only other activity we have seen has been a White-breasted Nuthatch pair which nested in another of our boxes last year. This year we have not seen them around the boxes but they come regularly to the feeders.
To solve the mystery we first measured the eggs with callipers: 18.4 mm by 13.2 mm (0.72 in by 0.52 in). So we have rather small cream-coloured eggs with brownish spots and streaks. That certainly rules out bluebirds which have pale bluish eggs. So, what about Tree Swallows? It turns out they have pure white eggs that are 19 mm X 14 mm (0.75 x 0.55 inches). So we could rule out Tree Swallows. They did not fit because we saw the bluebirds using the box after they had left the area.
What about Black-capped Chickadees? The literature says their eggs are white with reddish brown dots and are 0.55 X 0.45 inches. Our mystery eggs were bigger than that and we had never seen the chickadees enter the boxes, although we could have missed it.
So we are left with White-breasted Nuthatch as a possibility. The literature says White-breasted Nuthatch eggs are 0.8 X 0.6 inches and white with brown and lilac speckles. Our eggs fit into those dimensions but are just a bit smaller (0.72 X 0.52 inches). And, the colour was right. We concluded the mystery eggs came from a White-breasted Nuthatch.
Let's look more closely at the nesting behaviour of this species. The White-breasted Nuthatch is monogamous. The pair remain together year round and stay together unless one partner dies or disappears. Their territory is 0.1 to 0.15 Km squared (25-37 acres) in wooded areas and larger in semi-wooded habitat. They usually nest in natural tree cavities or abandoned woodpecker holes. The nest is built 3 to 12 metres off the ground. It is lined with fur, fine grass and shredded tree bark. The eggs are incubated by the female for 13 to 14 days and the young fledge in 18 to 26 days. Both adults feed the chicks both in the nest and for 2 weeks after fledging. After this period the young usually leave the adults' territory to find territory of their own.
If the young are unsuccessful at finding their own territory, they become 'floaters'. This is a term for an unpaired bird without a territory. It is these floaters which are thought to make up irregular dispersal of bird species.
White-breasted Nuthatches roost at night and in inclement weather in holes in trees. We have observed them entering nesting boxes in cold weather. They will also take shelter behind loose tree bark in order to avoid bad weather. These birds usually roost alone but in cold weather they will join together presumably to preserve heat. As many as 29 have been found roosting together!
So why did a White-breasted Nuthatch nest in this box after the bluebirds had raised 2 broods? A White-breasted Nuthatch had nesting in a neighbouring box last year. The collected nest contained no evidence of bluebird use but the nuthatch probably threw all of that out before building her nest. It could be that the site of her previous nest had been predated or that it somehow was destroyed. So why did she not remain on the eggs and finish her task? She may have been taken by the Sharp-shinned Hawk that flew periodically through our yard. Or, I suspect it was too late in the season to start a brood (which may also have been her second brood) and she had not become broody because of low hormones caused by the late season.
All my theories are speculation but it is interesting to know that the White-breasted Nuthatch had somehow taken over the bluebird nest and nearly was able to raise her brood. It will be interesting to see who uses the box next year.
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