  Citation: Krams, R.; Krama, T.; Elferts, D.; Daukšte, J.; Raibarte, P.; Br ¯ umelis, G.; Dauškane, I.; Strode, L.; Krams, I.A. High Blood Parasite Infection Rate and Low Fitness Suggest That Forest Water Bodies Comprise Ecological Traps for Pied Flycatchers. Birds 2022, 3, 221–233. https://doi.org/10.3390/birds 3020014 Academic Editor: Jukka Jokimäki Received: 31 December 2021 Accepted: 6 April 2022 Published: 9 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article High Blood Parasite Infection Rate and Low Fitness Suggest That Forest Water Bodies Comprise Ecological Traps for Pied Flycatchers Ronalds Krams 1,2 , Tatjana Krama 1,2 , Didzis Elferts 3 , Jan¯ ına Daukšte 4 , Patr¯ ıcija Raibarte 4 , Guntis Br ¯ umelis 3 , Iluta Dauškane 3 , Linda Strode 3 and Indrikis A. Krams 1,4,5,6, * 1 Department of Biotechnology, Daugavpils University, 5401 Daugavpils, Latvia; ronalds.krams@gmail.com (R.K.); tatjana.krama@du.lv (T.K.) 2 Chair of Plant Health, Estonian University of Life Sciences, 51006 Tartu, Estonia 3 Department of Botany and Ecology, Faculty of Biology, University of Latvia, 1004 R¯ ıga, Latvia; didzis.elferts@lu.lv (D.E.); guntis.brumelis@lu.lv (G.B.); iluta.dauskane@lu.lv (I.D.); linda.strode@lu.lv (L.S.) 4 Institute of Food Safety, Animal Health and Environment BIOR, 1076 R¯ ıga, Latvia; janina.daukste@inbox.lv (J.D.); patricijar9@gmail.com (P.R.) 5 Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, 1004 R¯ ıga, Latvia 6 Institute of Ecology and Earth Sciences, University of Tartu, 50409 Tartu, Estonia * Correspondence: indrikis.krams@ut.ee Simple Summary: Animals tend to settle and reproduce in high-quality habitats which provide large amounts of food and shelter against predators. Although they are attracted by abundant food resources, reproductive output and survival of animals may be affected by some other environmental factors. This study found that Pied Flycatchers (Ficedula hypoleuca) breeding near water bodies were caught in an ecological trap. We found that the number of fledglings, their body mass, and tarsus length were positively correlated to the distance to the lakes. The number of blood-sucking insects, adult body mass at the end of the breeding season, and the distance to the nearest water body were negatively correlated, suggesting that breeding near forest water bodies comes at a cost. More studies must be done to understand the role of different kinds of blood parasites and their vectors in fitness reduction of birds. We recommend avoiding putting bird nest boxes near forest water bodies to mitigate the damaging effects of maladaptive habitat selection of cavity-nesting birds. Abstract: Blood parasites are considered to have strong negative effects on host fitness. Negative fitness consequences may be associated with proximity to areas where blood parasite vectors reproduce. This study tested for relationships between haemosporidian infection prevalence, parasitemia, and fitness parameters of breeding Pied Flycatchers (Ficedula hypoleuca) at different distances from forest water bodies. Prevalence and parasitemias (the intensity of infection) of haemosporidians and vector abundance generally decreased with increasing distance from forest lakes, streams, and bogs. Fledgling numbers were lower, and their condition was worse in the vicinity of water bodies, compared with those located one kilometer away from lakes and streams. At the beginning of the breeding season, adult body mass was not related to distance to the nearest water body, whereas at the end of the breeding season body mass was significantly lower closer to water bodies. Forest areas around water bodies may represent ecological traps for Pied Flycatchers. Installing nest boxes in the vicinity of forest water bodies creates unintended ecological traps that may have conservation implications. Keywords: blood parasites; ecological traps; parasite vectors; Pied Flycatcher 1. Introduction The distribution of biological taxa is spatially arranged. The geographical limits to the distribution of a species are determined by abiotic factors such as precipitation and Birds 2022, 3, 221–233. https://doi.org/10.3390/birds3020014 https://www.mdpi.com/journal/birds