Bluebird and Small Cavity Nester Conservation
Sialis - Bluebirds and other small cavity nesters


Relative sizes of eggs that may be found in Eastern Bluebird nestboxes

Relative sizes of Eastern Bluebird, Tree Swallow and House Wren Eggs. Photo by E Zimmerman

EGG SIZES: from smallest to largest

Common Name

Scientific Name

Avg. length (~inches)

Avg. egg size

Carolina chickadee (CACH) Poecile carolinensis 0.58 14.8 x 11.5
Black-capped Chickadee (BCCH) Poecile atricapillus 0.60 15.2 x 12.2
Chestnut-backed Chickadee (CBCH) Poecile rufescens 0.60 15.3 x 12.0
Boreal Chickadee (BOCH) Poecile hudsonicus 0.60 15.3 x 12.3
Brown-headed Nuthatch (BHNU) Sitta pusilla 0.61 15.5 x 12.3
White-breasted Nuthatch (WHNU) Sitta carolinensis    
Mountain Chickadee (MOCH) Poecile gambeli 0.61 15.6 x 12.3
Tufted Titmouse (TUTI) Baeolophus bicolor 0.63 16.0 x 12.2
House Wren (HOWR) Troglodytes aedon 0.65 16.4 x 12.7
Bewick's Wren (BEWR) Thryomanes bewickii 0.65 16.4 x 12.7
Prothonotary Warbler (PROW) Protonotaria citrea 0.73 18.47 x 14.55
Violet-green Swallow (VGSW) Tachycineta thalassina 0.74 18.7 x 13.1
Tree Swallow (TRES) Tachycineta bicolor 0.74 18.7 x 13.2
House Finch (rare in box) (HOFI) Carpodacus mexicanus 0.74 18.8 x 13.8 (variable length)
Carolina Wren (CARW) Thryothorus ludovicianus 0.75 19.1 x 14.9
Eurasian Tree Sparrow (ETSP) Passer montanus 0.80 19.50 × 14.21
Downy Woodpecker (DOWO) Picoides pubescens   19.5 x 15
Eastern Bluebird (EABL) Sialis sialis 0.81 20.7 x 16.3
Western Bluebird (WEBL) Sialia mexicana 0.83 21 x 16
Mountain Bluebird (MOBL) Sialia currucoides 0.86 21.9 x 16.6
Ash-throated Flycatcher (ATFL) Myiarchus cinerascens 0.88 22.4 x 16.5
Great crested Flycatcher (GCFL) Myarchus crinitus 0.89 22.6 x 17.2
House (English) Sparrow (HOSP) Passer domesticus 0.90 22.8 x 15.4
Purple Martin (PUMA) Progne subis 0.97 24.5 x 17.55
Brown-headed cowbird (BHCO)* Molothrus ater 0.82+ 21.45 x 16.42 (varies by subspecies, from 18.03 x 13.74 to 25.40 x16.76 (BNA)
Bronzed Cowbird (BRCO) Molothrus aeneus (2 subspecies) 0.96 24.5 x 18.35
European (Common) Starling (EUST) Sturnus vulgaris 1.15 29.2 x 21.1

* The Cowbird eggs I have seen are LARGER than House Sparrow eggs. The Peterson guide numbers for Brown-headed are 21.45x16.42 mm for Brown-headed, vs 23.11x18.29 for Bronzed. I have listed the Birds of North America number)


  • Peterson Field Guide of Eastern Birds' Nests, by Hal H. Harrison, 1975
  • Peterson Field Guide of Western Birds' Nests, by Hal H. Harrison, 1979
  • BNA - The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:
Tree swallow.  Photo by Bet Zimmerman.House sparrow.  ChickadeeBewick's Wren.  Photo by S.HarrisBluebird. Photo by Bet Zimmerman.
Tufted titmouse.  Photo by Bet ZimmermanCarolina wren. Photo by Bet ZimmermanHouse wren. Photo by Bet ZimmermanBewick's Wren. Photo by S.Harris.

- click here to see larger photos. 
Unfortunately all the photos are not on the same scale.

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