NIENHUIS J (2022) Do Northern Lapwings Vanellus vanellus adjust the start of egg laying to the activity of earthworms?. LIMOSA 95 (1): 28-40.

Most species adjust the timing of breeding to optimal conditions. This is often a combination of the best conditions for females to produce eggs and the best moment to start laying eggs in order to have chicks when their food is abundant. These conditions may differ between years. In warmer springs Northern Lapwings breed earlier, and due to climate warming they consequently advanced the start of egg laying.
Before laying eggs, females have to accumulate sufficient nutrients to produce eggs and energy reserves for incubation. Like most waders, Northern Lapwings collect these energy reserves on the breeding grounds. The predominant food source of Northern Lapwings are earthworms. The more earthworms are present, the fewer time it takes to produce eggs. Earthworm presence is not the same as availability. Lapwings use their vision to hunt and earthworms mainly live belowground.
Detritivore earthworms like the common Lumbricus terrestris surface to feed, mostly at night. At low temperatures during winter this species is inactive. Early in spring they become active and feed aboveground. After having collected sufficient food they surface less frequently in summer. As a consequence there is a period in spring when earthworms are more available for eyesight hunters like plovers. Does this affect the timing of the initiation of egg laying in Northern Lapwings?

Data on the onset of egg laying of Northern Lapwings were collected by volunteers protecting meadow bird nests from farming activities. Only observations from the first peak in egg laying were used as the second peak mainly consists of replacement clutches after failed first breeding attempts.
Surfacing earthworms were counted at night in a firebreak of a 30 year old residential area, where they live under sidewalk tiles. Daily counts were conducted between 9 October 2015 and 28 October 2016 and during four springs (2017-20). Numbers were related to environmental variables using a regression analysis. Based on literature, the Northern Lapwing breeding season was defined as 18 March-1 July. To determine the period with increased numbers of surfacing earthworms, observed numbers were compared to predicted earthworm numbers based on the outcome of the abovementioned initial regression analysis, solely based on the relationships for the non-breeding season.

Earthworm numbers were higher in conditions preventing dehydration, such as for example moisty conditions. During the Northern Lapwing breeding season more foraging earthworms were counted than expected from the predictions based on the non-breeding season. The start of the period of elevated numbers of surfacing earthworms was 2-31 days before the start of egg laying by the Northern Lapwings. Both the onset of higher numbers of surfacing earthworms and the onset of incubation were affected by temperature, but this relationship was much steeper for earthworms. Consequently, in an early (warm) spring, there was a larger gap between the onset of surfacing earthworms and the onset in egg laying. In very late (cold) springs this difference almost disappears, probably as the Northern Lapwings are time stressed to initiate breeding. We conclude, that temperature driven variation in the timing of the onset of (mass) surfacing of earthworms could form an additive ultimate factor affecting the onset of egg laying in Northern Lapwing.

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