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A conspiracy news site claims that (emphasis added):

Hundreds of birds have fallen from the sky in The Hague, the Netherlands, during a 5G experiment to see how large the range was and whether the new wireless technology would cause any harm in the local area.

News of the adverse effects suffered by the starlings was slow to break, as initially the birds died in small numbers during the first wave of the experiment. However when a further 150 birds suddenly died at the same time, falling into a public park, people began to take notice and investigate.

What caused the death of 297 birds in a park in The Hague?

If you look around that park you might have seen what is on the corner of the roof across the street from where they died: a new 5G mast, where they had done a test at almost the exact same time as the birds fell from the sky.

And while it does list some sources, they are either dead, some Facebook scribble, or just not proving anything altogether.

I also couldn't find any other reliable source for this event (or one that links it to the 5G test).

No, while 337 starlings (<5% of the population) and 2 common wood pigeons were found dead in a Netherlands park, this was not due to a 5G test. The birds died 4 months after the only conducted 5G test and such "bird death" events are quite common.

Snopes is my first search result for "The Hague dead birds".

They say unexplained bird deaths occurring in a park (Huijgenspark) in The Hague, Netherlands were not caused by a 5G test.

While "It is true a series of mysterious bird deaths has occurred at a park in The Hague," such bird deaths are not uncommon.

Mysterious bird deaths, though great fodder for conspiracy cranks, are not uncommon. Due to their unexplained nature, they are popular with those seeking to stoke fears or make political or religious points. In 2011, for example, Arkansas, Louisiana, and parts of Sweden were the site of thousands of bird deaths in a short time, which the media dubbed “the aflockalypse.” Contemporaneous reporting by the Associated Press made it clear that these mass die-offs are quite common and often unexplained:

Since the 1970s, the U.S. Geological Survey’s National Wildlife Health Center in Wisconsin has tracked mass deaths among birds, fish and other critters, said wildlife disease specialist LeAnn White. At times the sky and the streams just turn deadly. Sometimes it’s disease, sometimes pollution. Other times it’s just a mystery … On average, 163 such events are reported to the federal government each year, according to USGS records.

With 163 mass bird death events a year in the US, it is certainly much more common than expected. In fact, as recently as February 6, there was a mass death event of 20,000 birds (also in the Netherlands)!

Furthermore, the 5G test was performed on June 28, 2018 (source) near Huijgenspark. The earliest news of the dead birds I could find (of the first 30 dead Starlings) were on October 23, 2018 here and here. Thus, around four months elapsed between the 5G test and the first news reports of dead birds. An additional 30 dead starlings were found the next day (source) bringing the total death toll to 60 starlings. Birds (mostly starlings) continued to die over the coming days. On 5 November 2018, The Hague published a news release (later updated on 12 November 2018). Quoted below are excerpts of two non-contiguous paragraphs.

Between Friday, 19 October and Saturday, 3 November 2018, 337 dead starlings and 2 dead common wood pigeons were found.

There is still no evidence that the birds' death was caused by testing the new 5G network for mobile telephones. The Antennebureau (Agentschap Telecom) has confirmed that no 5G tests were conducted around Huygenspark [around the time of the bird deaths].

As the dead birds started dying four months after the test, it is unlikely the test killed the birds. Also, consider the below questions posited by a bird shelter in The Hague on a Facebook post (translated from Dutch with Google).

[There has been much speculation about the 5G test, which has been extensively discussed and is] regarded by almost all parties as non-logical. Why [were] only starlings [found dead]? And why on multiple days? And why "only" 150 total [this post was published when the death toll didn't reach the maximum] when it might be 1000 or more starlings in the [flock]? The animals were not killed by terror and no sick animals were found.

Snopes additionally states that there were no other tests during the period of bird deaths (which concurs with the official statement of The Hague quoted above(emphasis added).

No evidence suggests that any other 5G test ever occurred in The Hague or that a 5G antenna was installed near that park conveniently out of view. We reached out via Twitter to the Dutch company NS, the operator of the train station allegedly involved in the 5G test, and a representative told us that they were “unaware that recent 5G tests were conducted at this location.” A representative of KPN, the largest mobile operator in The Netherlands, told us via Twitter that “I can be very clear about this matter; there are no 5G tests in Den Haag. This is a complete hoax.” Huawei, the cellular provider who took part in the one-day June test, did not respond to our inquiry about a test occurring, but the Dutch equivalent to the FCC asserted that no such test occurred.

This viewpoint is repeated by a Dutch bird shelter here.

As the one test didn't kill the birds, and other tests weren't conducted, 5G cellular network tests didn't kill the birds that fell out of the sky.

Did “hundreds of birds” fall out of the sky in the Netherlands due to a 5G test?

No, while 339 birds did fall out of the sky in a Netherlands park, this was not due to a 5G test. The birds died four months after the 5G test. In addition, birds mysteriously falling dead are "quite common."

The Rotterdam Natural History Museum published a document estimating the number of starlings around Huijgenspark (where the dead birds were found) to be 10,000. ("Ze zagen rond halfacht de zwerm van naar schatting 10.000 spreeuwen in noordoostelijke richting over de nog slapende stad vertrekken.") If this estimate were accurate, less than 5% of starlings around Huijgenspark died from this event. This percentage puts the large number of "hundreds of birds" into perspective.

Snopes also addresses if a 5G cellular network can kill birds (emphasis added).

In Europe, 5G will make use of three frequency ranges: a low-frequency 700MHz “coverage layer,” a 3.4-3.8GHz band which will be the primary bandwidth, and a “super data layer” in the higher frequency 24.25-27.5GHz band. This latter range is more theoretical and is not what has been tested in The Netherlands thus far, as the only known test of 5G in the Hague utilized the 3.4-3.8GHz band. Regardless, all of these frequencies fall within a range considered by the International Commission on Non-Ionizing Radiation Protection (first in 1998 and again in 2009) to be safe:

It is the opinion of ICNIRP that the scientific literature published since the 1998 guidelines has provided no evidence of any adverse effects below the basic restrictions and does not necessitate an immediate revision of its guidance on limiting exposure to high frequency electromagnetic fields … The plausibility of the various non-thermal mechanisms that have been proposed is very low. In addition, the recent in vitro and animal genotoxicity and carcinogenicity studies are rather consistent overall and indicate that such effects are unlikely at low levels of exposure. Therefore, ICNIRP reconfirms the 1998 basic restrictions in the frequency range 100 kHz–300 GHz until further notice.

Snopes also emailed Dr. Eric van Rongen, a member of the Health Council of the Netherlands and the Chairman of the International Commission on Non-Ionizing Radiation Protection.

“Even if there would have been 5G exposure,” he told us, it is “very unlikely that that could cause the [bird] mortality.”

The cause of death is not completely known. It is known that "the starlings must have forcefully flown towards and collided with the ground in a sudden fit of panic, upon arrival or departure from their resting places, and that these impacts were fatal." (The Holland Times) From the same source:

Kees Moeliker, director of the museum, and Erwin Kompanje, a senior researcher, discovered extreme internal bleeding in all examined birds, as a result of ruptured livers. The dead birds had also suffered severe damage to the blood vessels, lungs and heart.

Various viruses ("Usutuvirus, warfarin, bird flu, chlamydia and West Nile (source)") and natural toxins (source) were excluded. Additionally, the dead birds had an above-average body weight so weren't likely to have died of exhaustion (source). The same source additionally states that aside from the internal bleeding due to trauma, the internal organs were healthy.

Snopes actually has more details refuting the sequence of events; because tests of 5G use yet unallocated spectrum, they need special regulatory approval.

One such test did occur in an area generally near Huijgenspark, but it took place on 28 June 2018, and it was not followed by a massive bird die-off. For this test, the Dutch equivalent of the FCC provided a one-day permit for the telecommunication company Huawei to use the 5G frequencies needed for the test:

Huawei has demonstrated a live 5G network in The Hague, using 100 MHz of spectrum in the C-band at 3.5 GHz. As the frequencies are not normally available for mobile services in the Netherlands, the Telecom Agency granted a special one-day permit for the demonstration at the KPN office in the Voorburg area. The band is available for local licensing, but in allotments far smaller than the full 100 MHz width that is standardized for 5G.

No evidence suggests that any other 5G test ever occurred in The Hague or that a 5G antenna was installed near that park conveniently out of view. We reached out via Twitter to the Dutch company NS, the operator of the train station allegedly involved in the 5G test, and a representative told us that they were “unaware that recent 5G tests were conducted at this location.” A representative of KPN, the largest mobile operator in The Netherlands, told us via Twitter that “I can be very clear about this matter; there are no 5G tests in Den Haag. This is a complete hoax.” Huawei, the cellular provider who took part in the one-day June test, did not respond to our inquiry about a test occurring, but the Dutch equivalent to the FCC asserted that no such test occurred.

And the cause for the dead birds was finally identified as physical trauma:

The mystery occurred in the Huijgenspark, close to Hollands Spoor station in The Hague. The surroundings and trees in this part of the city are an important gathering spot for starlings. Initial speculations in Autumn, when the sudden massive deaths occurred, pointed towards possible intentional poisoning of the birds, which, as history shows, is at least worthy of consideration. Other speculations pointed to the possible contamination of the natural waters in which flocks of starlings indulge, and to the testing of a new 5G cell phone tower, believed to have a negative environmental impact. The 5G mast is visible from the site where the flocks of birds died, which didn’t help its case. [...]

Researchers at the National History Museum in Rotterdam performed a necropsy on at least fifteen birds to determine the cause of death and shed light on the mystery. Kees Moeliker, director of the museum, and Erwin Kompanje, a senior researcher, discovered extreme internal bleeding in all examined birds, as a result of ruptured livers. The dead birds had also suffered severe damage to the blood vessels, lungs and heart. But this solved only one part of the mystery. The researchers concluded in Straatgras, the National History Museum magazine, that the starlings must have forcefully flown towards and collided with the ground in a sudden fit of panic, upon arrival or departure from their resting places, and that these impacts were fatal. The WBVR laboratory in Lelystad joined the National History Museum in its conclusion, putting all speculation to rest and eliminating the possibility of human error or experiment, harmful viruses or diseases. As of this moment, the assumed cause of death is that the birds collided with deadly force with each other, with tree branches or with the ground due to being panicked and disoriented.

As with such cases it's probably not really possible to tell what caused the impact/panic. Such events have been observed elsewhere, e.g. in Canada or in the UK.

There is a short report on the swiss national broadcaster, SRF, about the incident, to be found here, in german: https://www.srf.ch/news/wirtschaft/mobilfunk-geruechte-die-wichtigsten-antworten-in-der-5g-debatte

In short it says (translation mine):

Local authorities rebuted 5G-tests on that same day. Dead birds have
been found over the course of several days anyways. Other reports were
from people, that were not actually present at the place.