NIENHUIS J. (2016) Habitat selection of earthworm-eating Black-headed Gulls Chroicocephalus ridibundus: moist soil is good, but recent rain is better . LIMOSA 89 (2): 67-74.

Many Black-headed Gulls winter in cities en villages. In these urban environments they mainly eat bread and other food scraps in parks and earthworms from grasslands. The location where they feed is mainly influenced by weather conditions. In rainy periods earthworms are closer to the surface and more accessible to the gulls. Probably as a result, the number of Black-headed Gulls in parks is lower during rainy periods as birds are foraging on grasslands elsewhere. Frequent counts of Black-headed Gulls in a park in Haren, Groningen, made during five winters were used to assess the effect of timing of precipitation on the selection of feeding sites. Precipitation during the six hours before gulls were counted had a negative effect on the numbers found in the park. Rainfall in the last two hours before a count had a particularly strong effect: over 2 mm of rain in the two hours before a count induced a drop in gull numbers of 75%. The cumulative amount of precipitation over five days prior to the count also had a negative effect on gull numbers: with 50 mm of precipitation or more 58% less gulls were counted in the park. These two effects appeared to reinforce each other. Additional observations conducted at two further sites showed that Black-headed Gulls do react to rain by eating earthworms, mainly during and immediately after rainfall. Earthworms may surface spontaneously in reaction to rain, even if the amount of rain is limited, as shown by numbers of earthworms counted on a footpath in a park at night. The numbers observed were correlated with both the duration (hours) and intensity (mm/hour) of rainfall. If earthworms also show this behaviour during daytime, this could explain the decrease of Black-headed Gull numbers in the park with increasing rainfall.

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