can you be a bit more detailed about your sampling process?

Hi Fernando,

from a good vantage point, in a boat or land, we start at 3 hours before sunset and count all birds arriving and all birds leaving the colony. we separate the count into 15 minutes intervals. we stop when we cannot distinguish species anymore or cannot see the birds, because of no light. We then subtract leaving from arriving birds and have a netto number. This can be interpreted in many ways according to the two follwing parameters: a. how many birds that are in the colony do not breed (prospectors, helpers, etc) b. according to the phase of the nesting cycle, different proportions of parents can be 1. both away of the nest trying ot find food to feed their offspring 2. one of them stays with young and the other gets food. we apply several of these ratios for subsequently estimating number of br. pairs. and there are more statistical treatments if one does this work three times in three different nesting stages.

Thanks!. I have faced a similar problem but using mist netting for petrels, so I got data from birds entering or leving the colony in a nigth, an unknown part are nom bredders (transients, inmatures etc...) but also an unknown part of the breeders are not "at risk" of being sampled, as do not enter or leave the colony every day (the replacement for incubating is not dayly, and in some periods chicks are not fed every nigth), but for herons I have some experience also in red-beds (purple heron) and counting birds coming and leaving was helped by a study of the redbed as it had a big hill next, so nest were also located with the help of telescope (also for finding nest and colour band them) but It loos that it is not your case. Let my think a bit so maybe I can help a bit, but I do not think this substracting of entering and leaving brds can be of help. I mean, the result won't make any sense or at least it is what I think.

Thanks for the explanation

Hi.

We have used a similar method for couting herons in a roost an hour before sunset, in a locality of Tenerife, Canary Islands, but the roost is used mainly by Little Egret and Cattle Egret, which don't breed in the site, as only Night Heron breeds there. Our interest in this study was to know the total number of birds of each species and, if possible, the proportion of ages, including recently fledged juveniles of Night Heron. Also, to know the main period of arrival of birds to the roost and the directions from which they came. The paper is in preparation yet. 

What about the birds moving in the dark hours which can be underestimated? Have you ever thought using a night vision device? Darkest birds (juveniles) can possibly be discriminated from the adults because of their different pattern. The only limitation could be using it in moon light or star light, according to the device sensititivity.

Thanks Giorgio, my question however is if anyone knows a book or paper where this method is described in more detail

Dear Giorgos,

There is a monograph with 5 parts by a Polish ornithologist, who describes this method investigating the breeding distribution of Squacco Heron Ardeola ralloides. I could give you the title of the part II only:

Josefik, M. 1969. Studies on the Squacco Heron Ardeola ralloides (Scop.) Part II. Secular Changes in the numbers and distribution in the Palearctic range. – Acta orn. Warszawa, 11, No 7, 135-262.

We also used this method for counting the herons numbers in Srebarna breeding colony which is known for the second half of 19 century.

Dear Tanyo,

many thanks for the very valuable information! let me first see if I can find this reference work and if I cannot I will ask you to send me copies only of the description of the method. Very many thanks indeed!