Movement Ecology of Animals

Homepage of Dr. Emily A. McKinnon

Bicknell’s Thrush nests in regenerating clearcuts in the highlands of New Brunswick

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Bicknell’s Thrush (Catharus bicknelli) is one of those birds that real twitchers lust after. It’s hard to identify in the field, especially outside of the breeding season, and it’s range is patchy and limited to northeastern North America and a couple of islands in the Caribbean. Its secretive nature, dense foggy breeding and wintering habitat, and mournful song only add to the allure. Unfortunately, like too many other songbirds, Bicknell’s Thrushes are steadily disappearing. This is particularly so in the highlands of New Brunswick and Nova Scotia, Canada, where I did my Masters research on this species. A recent report by Bird Studies Canada’s High Elevation Landbird Program (HELP) showed a decline of 11% ANNUALLY over a 10-year time period! Yikes! Bicknell’s Thrush is now federally listed as THREATENED in Canada.

Super dense balsam fir forest preferred by Bicknell's Thrush

Super dense balsam fir forest preferred by Bicknell’s Thrush

One potential problem for Bicknell’s Thrush in New Brunswick is a logging practice called Precommercial Thinning (PCT). Bicknell’s Thrush nest in very dense forests that regenerate on mountain-tops in North and North-central New Brunswick. Where I worked, in the Christmas Mountains, the highest peaks are around 600-700m above sea level. Not very big relative to other mountains, but enough, at this high latitude (~46N) to create fairly harsh conditions. Balsam fir and white birch thrive up there, especially after clearcuts. The balsam fir grows back at such a high density that the trees stunt their own growth through crowding. This is no good for logging companies interested in the best bang for their buck. To help improve the growth of the trees in a given patch, loggers go in and ‘thin out’ the forest by PCT, when the trees are about 10-15 years old (about 2-5 cm in diameter). It’s pre-commercial because the trees they cut down aren’t at a size to be of much commercial value – they are just cut and left lying on the ground. What this practice does overall is to remove about 75% of the trees in the forest, with the idea that the remaining 25% will be better off for it.

This practice affects Bicknell’s Thrush because they too love high elevations, and they really really love balsam fir. I collected data on a dozen or so nests of Bicknell’s Thrush during my Masters, and every single one of them was in a balsam fir. The only one that was not entirely in a balsam fir was squished between a fir and a spruce. So when the forests at high elevation are basically perfect Bicknell’s Thrush nesting habitat, loggers come in and cut down 75% of the trees! You can see why this is concerning to people interested in preventing further population declines of this species!

Rolling hills of north-central New Brunswick - few people, lots of logging roads!

Rolling hills of north-central New Brunswick – few people, lots of logging roads!

I wanted to know more about the habitat Bicknell’s Thrush used for nesting, and if they ever nested in forest that had been thinned. Imagine a female Bicknell’s Thrush, returning to North Pole Mountain (yes, the Christmas Mountains have Christmasy names!) in the spring, after spending the winter in the highlands of the Dominican Republic. She gets back to the forest she nested in last year, only to find that most of the trees are gone! Does she go for it anyway? Or fly onward to another mountain? Or just move to the unthinned forest next door in hopes it will stay dense for another nesting season?

I can tell you exactly what one female did.

Finding Bicknell’s Thrush nests is like finding a needle in a haystack, only imagine that the needle is camouflaged and the haystack is several hectares large. Against all odds, we found several Bicknell’s Thrush nests over the course of our project, and in our first year, they were all in unthinned forests. We carefully monitored each nest with minimal disturbance, sometimes using video cameras. We tried to capture all the adults feeding the chicks and band them with a unique colour-combination of plastic rings on their legs. Bicknell’s Thrush are an unusual songbird in that they have multiple ‘dads’ that father and help feed the chicks, so a single nest could have 3 or even 4 parents attending the chicks! One of the few females we captured was nesting in a forest that was scheduled to be thinned. Bicknell’s Thrush, like many songbirds, are highly site-faithful, which means they return to the exact same forest patches for nesting year after year. What would this female (her ID was light-green-mauve-black-silver) do when she came back the next year to a thinned forest?

Can you spot the blue eggs?

Can you spot the blue eggs?

The following spring, we searched and searched in the thinned forests and found no signs of any Bicknell’s Thrushes, let alone nests. In a very small dense patch in a boggy area, right across the logging road from light-green-mauve-black-silver’s original forest patch, we did hear a Bicknell’s Thrush singing. It was such a tiny and miserable forest patch (standing water, dense tangles of dead balsam fir, logging roads on either side of a wedge about 10m wide) that we pretty much gave up finding any nests there and figured the singing male was overly optimistic about his chances. Finally near the end of the nesting season, I decided it would be worth going back to this patch for one final search.

My crew dropped me off and I steeled myself for the scratchy squeeze through the dense boggy patch one more time. I finally popped into a bit of an open area and froze. I had hear a distinct ‘Peer!’ call from very close by. This is a call Bicknell’s Thrush often use, but I noticed on our nest videos that females often gave it when they jumped off the nest, either to let Dad in to feed the kids, or because of some disturbance nearby. Like me. I looked around very carefully. There were no branches with needles on them below my head height – the needled branches were all above me, creating a dense canopy about 5-m tall. Then I saw it – way up high, pressed against the trunk of a balsam fir, a messy clump of nesting material! Could it be? Thankfully balsam fir is pretty easy to climb, so I scaled the tree as quietly as possible and peered in – 3 thrush babies, and pretty old ones too! This nest was close to fledging! But was it a Bicknell’s Thrush nest? It was too high to video, so we instead set up some nets around the nest and tried to catch the parents to confirm that this was indeed a Bicknell’s Thrush nest.

Our first capture that day was one of my favourite field moments ever. It was a colour-banded Bicknell’s Thrush. And low and behold, it was light-green-mauve-black-silver, the same female who nested across the road, about 280m away in the unthinned forest, the year before. I think we had probably only banded 2 or 3 females (they are even more cryptic than males), so it was amazing to see this bird again! She seemed happy and healthy so we released her and left her and her kids alone.

My favorite Bicknell's Thrush, LightGreenMauveBlackSilver

My favorite Bicknell’s Thrush, LightGreenMauveBlackSilver

Colour bands

This bird’s story, and other evidence we collected suggests that Bicknell’s Thrush might be okay in areas with PCT if patches of dense stuff are left for them to use for nesting. It’s an easy way that forest managers can help keep nesting habitat for this species in New Brunswick.

Read our full scientific paper here:

McKinnon, E.A., Askanas, H., and A.W. Diamond. 2014. Nest-Patch Characteristics of Bicknell’s Thrush in Regenerating Clearcuts, and Implications for Precommercial Thinning. Northeastern Naturalist 21(2):259-270.

If you can’t access the full version but want to, tweet me @BirdBiologist or email me emilymckinnon12@gmail.com, and I’ll send you a pdf!

 

 

 

 

 

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