Great Year for Thistle, Bad for Blackbirds

Slenderflower thistle (Carduus tenuiflorus)

The Slenderflower thistle, a close relative of the Italian thistle, is so abundant and vigorous this year that it’s giving the fennel a battle for resources, and may be impacting the Red-winged Blackbird breeding season.  Along the north side path where the normally tall and abundant fennel provided a secure nesting refuge for the blackbirds in past years, the thistle has outraced the fennel to the sun, and the thistle’s roots must be stepping on the fennel’s toes in the soil (first picture below).  

It’s been a bad year for the Red-winged Blackbirds, which usually do their mating, laying, brooding, hatching and fledging business in this area of the park during springtime. Their numbers are a fraction of last year’s.  Part of the problem is that Parks management last winter massacred the standing fennel and other bushes on which the blackbirds relied.  The resulting torn-up soil was perfect for the opportunist thistles. The fennel, which is hardly a pushover in battles for dominance, will eventually prevail, but probably at some cost to its height and density.  And to the birds.  [More below]

Fennel overshadowed by invading Slenderflower thistle, April 27 2017 

Two years ago, at the same time of year, the new fennel in this same area grew vigorously without a single thistle to worry about.  In height and density it looked a lot stronger than this year (See pic below).  [More below]

Fennel in same area growing on May 1 2015.  No thistles. 

Then, in May 2016, Parks management sent its mowing machine on a massacre of the vegetation in the blackbird nursery area (pics below). Eight to twelve feet of standing fennel along the paved path was laid waste in the middle of the nesting season. One nest containing four eggs narrowly escaped destruction but now lay exposed on the edge. How many other nests fell under the roaring cutters nobody knows.  The mower cut another swath twelve feet wide into the heart of the fennel forest, and for good measure knocked down a big bush that was a favorite perch for the males.  [More below]

Mid-May 2016: Park management cuts huge swath of standing fennel in Blackbird breeding area
More mower destruction of vegetation in blackbird breeding area, 2016
Park mower trashed this big bush, a favorite blackbird perch
Strip where mower leveled fennel mid-May 2016, right side of path, is paradise for thistles this year 
Red-winged Blackbird nest almost mowed down, now exposed on edge of fennel forest (May 28, 2016)

In August 2016, after the blackbirds had departed, the Parks Department mower slashed a further six to eight feet of dense bird habitat back from the paved trail, and butchered the abundant vegetation on the hill at the northwest corner of the park.  Result: perfect clearance for thistles to move in and take over.  See pics below.

A second mower massacre in August 2016 destroyed more bird habitat 
The hill in the northwest corner of the park shows abundant bird habitat in July 2016
Same spot: what’s left on August 6, 2016

Note: This post was updated 5/18/2021 to correct the identification of the thistle, erroneously named Italian thistle. It is Slenderflower thistle.

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