BIJLSMA RG (2016) Song rate and song type of Wood Warblers Phylloscopus sibilatrix as proxies for mating status, breeding success and detectability.. LIMOSA 89 (1): 2-11.

In the northern Netherlands, song rates of Wood Warblers Phylloscopus sibilatrix were quantified in 2015, mainly by repeatedly recording the number of songs per minute for each territorial male throughout the breeding season, from arrival until departure. A distinction was made between full song and short song, the latter typical of males having acquired a mate. Males started singing upon arrival, and continued to do so until a female showed up. Song rates of unmated males averaged 5.53 songs/min, declining to 3.27 during pair formation, 2.81 during nest-building and less than 1 during egg laying and incubation. Males stopped singing during the nestling stage. As soon as the male had acquired a female, song type changed from full song to short song, the latter always in a lower frequency than full song except during incubation (when 1.11 short songs/min). Unmated males continued singing until late June, when full song came to a dead stop. Polyterritorial males also continued singing until late June, but at a slightly lower frequency than unmated males (4.72 songs/min). Song rate was very stable throughout the day, from sunrise until early evening. A single male that was intensively monitored from arrival (16 May) until after mating (18 June) sang 185,000 times during this period. Body masses of unmated males were similar to that of mated males during the nestling stage (9.71 g and 9.50 g respectively). This may suggest that full singing is a costly enterprise, perhaps as costly as raising chicks.
As song rate and song type in Wood Warblers are closely associated with mating status and breeding stage, variations in the date of female arrivals (3.6-11.2 days after male arrivals) and laying date (7.4 and 24.3 days after female arrival) may result in large variations in annual detection probabilities. Also, the proportion of unmated males in Dutch Wood Warbler populations varied between 0 and 100% per annum (on average 49% on the Veluwe in 1974-2015, and 54% in Drenthe in 2011-15), again resulting in widely differing detection probabilities. Together with the males tendency to form clusters of territories (a mixture of mated and unmated males), polyterritoriality (at least 6-8% in Drenthe, but probably more frequent) and large annual variation in breeding success (naive breeding success varying between 12 and 60% on Veluwe in 1974-2015, and between 25 and 75% in Drenthe in 2011-15), the large variation in timing and mating frequency are likely to cause significant variations in annual detection probablities. Only when detection probabilities are validated each year, and breeding surveys take into account song and call types to interpret mating status and breeding stage, can monitoring of Wood Warblers hope to approach reality and produce biologically meaningful data.

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