Common chiffchaff / Zilpzalp (Phylloscopus collybita)

In our back yard in Fehraltorf 2020-10-03 12.44.50 Luppmen
Classification: Genus Phylloscopus; Family Phylloscopidae
In the survey of 2008, there was only 1 Chiffchaff in Fehraltorf for every 7 blackbirds, and it was ranked 19th in the village. Yet in summer 2020 I heard them in nearly every recording I make, whether at home by the brook (Luppmen) or in the woods. I presume their population has exploded in the last 10 years.
I see them in middle branches, but not in particularly exposed places.
Song: [Translated from German:] From the middle of August till the end of September the chiffchaff offers regular autumn song. [Link]
Lange Reihen (6-20) von „zilp-zalp“-Rufen, meist abwechselnd in Tonhöhe. (Als bemühe sich der Sänger vergeblich den ersten Ton wieder zu treffen , ohne jedoch weit zu verfehlen.) „Zins zahl, Zins zahl, zahl Zins, Zins Zins zahl“ („Zins“ etwas höher als „zahl“) [Link]
Song a very distinct: "chiff-chaff-chiff-chiff-chaff-chiff", in a regular clock-like rhythm. Each syllable at seemingly random pitch, but no large intervals tonally. Sometimes "get stuck" at one note. [Link]
Calls: Rufe :einsilbiges „hüid“, hinaufgezogen, wandelbar. [Link]
Contact/alarm call a soft, plaintive ascending "hooeet". Similar to W.Warbler, but shorter with a monosyllabic feel. [Link]
Physical details: length=10-11 cm, wingspan=15-21 cm, weight=6-10 g
Habitats: Forest

Song: Seems to consist of 3 notes repeated randomly, occasionally just 2 notes. Though NABU.de says they stop singing at the end of July, I do hear their 3-note song occasionally in October, but just one or two repetitions.
Song attributes: Melody: simple rhythmic, slow, Frequency: high (3-9 KHz) Singing season: 03-01 - 07-31 Dawn chorus start, 35 minutes before dawn.
Call: Repeated rising note, not too loud

♫ Source: XENOCANTO XC656913 - Common Chiffchaff - Phylloscopus collybita - call recorded in Belgium.mp3 (contact call)


Call attributes: Call melody: one note, slow, Frequency: 2-4 KHz, Special sounds: whoop.
First documented observation : 2019-06-04 in Forsthaus.