Online Resource 1: All host nests sampled between 2011 and 2015 in Northeastern and East-central Illinois shrubland habitats.

Host speciesa / Total nests / Parasitized / not parasitized / Parasitismrateb / Probability of parasitismc / SE / Host prefer.d / Hostsizee / Analysesf
Yellow warbler (Setophagapetechia) / 159 / 66 / 93 / 41.5% / 0.286 / 0.037 / Yes / Small / 1,2,3,4,5,6
American goldfinch (Spinustristis) / 263 / 14 / 249 / 5.3% / 0.182 / 0.047 / No / Small / 1,2,3,4,5
Field sparrow (Spizellapusilla) / 388 / 77 / 311 / 19.8% / 0.155 / 0.018 / No / Small / 1-12
Willow flycatcher (Empidonaxtraillii) / 107 / 20/ 87 / 18.7% / 0.190 / 0.040 / No / Small / 1,2,3,4,5,6
Indigo bunting (Passerinacyanea) / 63 / 17 / 46 / 27.0% / 0.379 / 0.073 / Yes / Small / 1,2,3,4,5
Song sparrow (Melospizamelodia) / 34 / 12 / 22 / 35.3% / 0.306 / 0.080 / Yes / Small / 1,2
Cedar waxwing (Bombycillacedrorum) / 142 / 2 / 140 / 1.4% / 0.016 / 0.011 / ---- / Small / 1
Gray catbird (Dumetellacarolinensis) / 383 / 6 / 377 / 1.6% / 0.012 / 0.005 / ---- / Large / 1,5
Eastern towhee (Pipiloerythrophthalmus / 61 / 20 / 41 / 32.8% / 0.325 / 0.064 / Yes / Large / 1,2,3,4,5,6
Northern cardinal (Cardinaliscardinalis) / 326 / 125 / 201 / 38.3% / 0.346 / 0.029 / Yes / Large / 1-12
Red-winged blackbird (Agelaiusphoeniceus) / 200 / 25 / 175 / 12.5% / 0.075 / 0.016 / No / Large / 1,2,3,4,5
Brown thrasher (Toxostomarufum) / 206 / 5 / 201 / 2.4% / 0.016 / 0.007 / ---- / Large / 1,5
American robin (Turdusmigratorious) / 391 / 1 / 390 / 0.3% / 0.002 / 0.002 / ---- / Large / 1
Eastern meadowlark (Sturnella magna) / 1 / 1 / 0 / 100% / ---- / ---- / ---- / ----
Henslow's sparrow (Ammodramushenslowii) / 1 / 0 / 1 / 0% / ---- / ---- / ---- / ----
House finch (Haemorhousmexicanus) / 1 / 1 / 0 / 100% / ---- / ---- / ---- / ----
Red-eyed vireo (Vireo olivaceus) / 1 / 1 / 0 / 100% / ---- / ---- / ---- / ----
White-eyed vireo (Vireo griseus) / 1 / 0 / 1 / 0% / ---- / ---- / ---- / ----
Prairie warbler (Setophaga discolor) / 2 / 1 / 1 / 50.0% / ---- / ---- / ---- / Small / 3,5
Clay-colored sparrow (Spizella pallida) / 2 / 1 / 1 / 50.0% / ---- / ---- / ---- / Small / 3,5
Bell's vireo (Vireo bellii) / 3 / 1 / 2 / 33.3% / ---- / ---- / ---- / ----
Black-billed cuckoo (Coccyzuserythropthalmus) / 4 / 0 / 4 / 0% / ---- / ---- / ---- / ----
Blue Grosbeak (Passerinacaerulea) / 5 / 1 / 4 / 20.0% / ---- / ---- / ---- / Large / 3,5
Rose-breasted grosbeak (Pheucticusludovicianus) / 7 / 2 / 5 / 28.6% / ---- / ---- / ---- / ----
Orchard oriole (Icterus spurius) / 7 / 3 / 4 / 42.9% / ---- / ---- / ---- / Large / 3,5
Yellow-breasted chat (Icteriavirens) / 8 / 2 / 6 / 25.0% / ---- / ---- / ---- / ----
Blue-gray gnatcatcher (Polioptilacaerulea) / 8 / 0 / 8 / 0% / ---- / ---- / ---- / ----
Dickcissel (Spizaamericana) / 10 / 3 / 7 / 30.0% / ---- / ---- / ---- / ----
Chipping sparrow (Spizellapasserina) / 14 / 2 / 12 / 14.3% / ---- / ---- / ---- / ----
Common yellowthroat (Geothlypistrichas) / 14 / 7 / 7 / 50.0% / ---- / ---- / ---- / Small / 3,5
Yellow-billed cuckoo (Coccyzusamericanus) / 16 / 0 / 16 / 0% / ---- / ---- / ---- / ----
Eastern kingbird (Tyrannustyrannus) / 17 / 0 / 17 / 0% / ---- / ---- / ---- / ----
Blue jay (Cyanocittacristata) / 23 / 0 / 23 / 0% / ---- / ---- / ---- / ----
Mourning dove (Zenaidamacroura) / 87 / 0 / 87 / 0% / ---- / ---- / ---- / ----

a Species in bold are considered accepter species based on published cowbird egg rejection rates <20% (Rothstein 1975; Goguen and Mathews 1996; Peer and Sealy 2004).

b Apparent parasitism rate calculate as number of parasitized nests divided by total nests for a given species.

cProbability of parasitism was calculated using a generalized linear model with a binomial distribution and logit link function while controlling for the effect of day of year.

dHosts were classified as preferred (denoted as “Yes”) or not preferred (“No”) if their probability of being parasitized was greater than or less than, respectively, the mean parasitism probability of 0.249 among accepter species (Fig. 2).

eBody sizes of hosts were classified as “Small” if their egg volume was smaller than the mean cowbird egg volume (2926 mm3) and “Large” if their egg volume was larger than the mean cowbird egg volume.

fNumbers refer to the specific analysis(ese) each host species was included in. See Methods (Statistical Analyses) for reference and additional details.

Title: Rates of parasitism, but not allocation of egg resources, vary among and within hosts of a generalist avian brood parasite

Loren Merrill1, Scott J. Chiavacci1, Ryan T. Paitz2, & Thomas J. Benson1⚓

Corresponding author: Loren Merrill

e-mail: ; phone: 217-244-8187

address: Illinois Natural History Survey, Prairie Research Institute, University of Illinois, Urbana-Champaign, 1816 S Oak St. Champaign, IL, 61820, USA