Dutch term (edited): (migratie-)intensiteit

I think this is what it is called in English.

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Note added at 1 hr (2015-03-27 00:00:10 GMT)
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See:

"The presence or absence of bird migrants and their speed and direction are therefore known as a function of both height and time, but migration intensity only as a time series.

[…]

However, the nominal bird RCS is obtained empirically, from the slope of the bird density versus reflectivity relationship. This method produces volume-density estimates and thus reveals how the intensity of bird migration, as well as its speed and direction, vary with both height and time." ("Radar Entomology: Observing Insect Flight and Migration", By V. Alistair Drake: http://goo.gl/ie3DZI )

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Note added at 1 hr (2015-03-27 00:03:01 GMT)
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"An overview of bird migration, plus an unusual way to identify migration intensity using the moon." (http://lucystevens.co.uk/2013/05/28/bird-migration-dawn-chor... )

+

"Total visible passage of migrating seabirds was estimated using the following equation: mean migration intensity (individuals/hour) in noted period X daily migration time (10 hours) X days in the migration period." (http://www.britishbirds.co.uk/wp-content/uploads/article_fil... )

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Note added at 1 hr (2015-03-27 00:06:59 GMT)
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Dutch synonym: "trekintensiteit"

"Zowel uit theoretische modellen die de vlieghoogte van vogels voorspellen (Liechti & Bruderer 1998), als uit modellen die de trekintensiteit voorspellen (Erni et al. 2002, van Belle et al. 2007) blijkt dat vogels hun trekvlucht onderbreken wanneer de meewindsnelheid hun eigen snelheid overschrijdt.

[…]

De vogeldichtheid liep op tot een intensiteit van 7 op de door de KLu gebruikte logaritmische vogeltrekschaal van 1 tot 8." (http://www.researchgate.net/publication/230888260_Breken_tre... )

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Note added at 1 hr (2015-03-27 00:09:15 GMT)
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"I. Forecasting migration intensity can improve flight safety and reduce the operational costs of collisions between aircraft and migrating birds." (http://www.jstor.org/discover/10.2307/4539305?sid=2110625696... )

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Note added at 17 hrs (2015-03-27 15:42:31 GMT)
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OK, having seen the additional context, I would like to change my answer to "density" or "intensity".

As far as I know, "intensity" just refers to how many there are. Depends on how they are measuring: bird/minute, bird/day, bird/km, etc. You haven't given us enough context to fully determine that.

See e.g.: "De vogeldichtheid liep op tot een intensiteit van 7" (= the bird density reached an intensity (level) of 7)

"Intensity" (or bird intensity) is used, but "density" is much more common.

Your text says:

"meer onderzoek dient verricht te worden naar de *intensiteit* van broedvogels en trekvogels."

In my Van Dale Groot woordenboek van de Nederlandse taal, "broedvogel" is redirexcted to "broedgast". The dictionary defines "broedgast" as "trekvogel die een gebied alleen bezoekt om er te broeden"

A few refs supporting "(bird) intensity"). There are many more for "(bird) density":

"Dedicated BCU in place.
Bird intensity levels included
in morning brief.
Flying activity mitigations in
place for increased bird
activity (detail in BDN AM)." (MOD Boscombe Down Aerodrome Manual: http://www.raf.mod.uk/rafwaddington/rafcms/mediafiles/3632DD... )
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"Wednesday - Kansas ANG Wylie 89 shortly before touchdown. It was first diverted to Ramstein AB due to high bird intensity and arrived shortly after lunch here at GK. We were on the other side, awaiting take-off of Esso 75 when we got the word that Wylie 89 was inbound. rushing to the other side, we quite didnt make it to our usual vantage point, but at least I caught the bird." (http://fightercontrol.co.uk/forum/viewtopic.php?f=16&p=44798 )

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"TNO, a Dutch R&D Institute, has developed the successful ROBIN (Radar Observation of Bird Intensity) for the Royal Netherlands Airforce. ROBIN is a near real-time monitoring system for flight movements of birds. ROBIN identifies flocks of birds within the signals of large radar systems. This information is used to give Air Force pilots warning during landing and take-off. Years of observation of bird migration with ROBIN have also provided a better insight into bird migration behaviour, which has had an influence on averting collisions with birds, and therefore on flight safety. Since the implementation of the ROBIN system at the Royal Netherlands Airforce the number of collisions between birds and aircraft in the vicinity of military airbases has decreased by more than 50%." (https://en.wikipedia.org/wiki/Bird_strike )
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"The primary objective of a radar study is to describe the general movement through the vicinity of a windfarm (either as an assessment of intensity of bird movement, or as a baseline for assessing the barrier effect post construction)." (http://www.thecrownestate.co.uk/media/5891/2004-05 Best prac... )
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"Avoidance behaviour and flight intensities of birds in an offshore wind farm in the Netherlands" (http://www.bou.org.uk/bouprocnet/marine-renewables-and-birds... )
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"The Netherlands (funded by the Dutch Government under the CO2 reduction
policy)


– Birds: flight patterns, occurrence, intensity, season, day/night in relation to estimated collision risk;

– Birds: disturbance of habitat/forage area;

– Birds: barrier effects;

– Valuation of landscape"
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Note added at 17 hrs (2015-03-27 15:43:38 GMT)
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There are tons of hits for "bird density"

Have a look: https://www.google.co.uk/search?espv=2&biw=1536&bih=858&q="b... (Google search for: "bird density" site:uk)