Foraging for high caloric anthropogenic prey is energetically costly

Table 1 Overview of the diet of Herring Gulls breeding on Texel during the chick care of the breeding season

Habitat	Anthropogenic	Marine	Intertidal	Terrestrial	Other-Fresh water	Other-Colony
Frequency of occurrence	17.6%	33.2%	67.6%	9.9%	1.6%	8.0%
Number of samples	699	1319	2683	394	64	316
Common species/types	Plastic packaging (58%) Chicken (20%) Bread (16%) Pork (11%)	Flatfish (44%) Whitefish (37%) Liocarcinus holsatus (25%) Crangon crangon (20%) Small pelagics (12%)	Mytilus edulis (82%) Carcinus maenas (17%) Ensis americanus (11%) Asterias rubens (5%)	Insects (48%) Cattlefeed, grains (18%) Berries and seeds (17%) Earthworm (13%) Birds (13%) Rabbits & rodents (11%)	Rutilus rutilus (86%) Perca fluviatilis (9%)	Larus gull chicks (67%) Larus gull egg (32%)
Availability	Opening hours & waste cleaning operations	Nearby fishing fleets	Low tide	Variable	Unclear	Breeding season
Energetic value	Up to very high ~ 10–25 kJ/g	Moderate to high ~ 4–10 kJ/g	Low to moderate ~ 2–5 kJ/g	Variable ~ 2–9 kJ/g	Moderate ~ 4–6 kJ/g	Moderate ~ 4–8 kJ/g
Digestive constraint	Large bones, platics, metals, glass	Fish bones, scales	Breaking shells with muscular gizzard	Bones, fur, chitin	Fish bones, scales	Bones, down, eggshells

Frequency of occurrence of prey types are shown per foraging habitat based on all samples gathered (n = 3969), together with the most common species or prey types within each habitat groups. Furthermore, an indication is given about the availability, energetic value in kJ g^− 1 wet weight and possible digestive constraints associated with these prey. The analysis was based on regurgitates gathered in the colony during fieldwork between 2006 and 2016, see for more information references [23, 25]

ISSN: 2051-3933