There's actually a fair amount of bluebird research that's been published from Michigan. Benedict Pinkowski wrote a number of articles and short notes from a study area in Macomb in the 1970's. He found that the initial nesting period where eggs were laid was from April 6 to May 15. I guess these two aren't horribly behind, especially given that it's been cold enough that not a lot of stuff is singing. A Depression-era article from Illinois (Musselman, 1939), described how generally the bluebirds would time their breeding such that their eggs would be hatched prior to the return of House Wrens since the wrens apparently will pierce the eggs to try to free up a cavity for their own use. Years with cold snaps forcing the bluebirds to re-lay would lead to much less breeding success since more nests would then be able to be destroyed by wrens. Both eBird and Jon's recent 10 year review show essentially identical bargraphs for House Wren, numbers increase rapidly in the last half of April until they're pretty much back in force by the beginning of May. I hadn't realized that House Wrens had such a negative effect on Bluebirds, but a Wisconsin study of 2600 nests (Radunzel et al, 1997), found that House Wrens accounted for about 40% of the total nest failures in bluebirds, the same precent of the total failures attributable to cats and House Sparrows combined! Interestingly their study also found that bluebirds preferred to nest in standard bluebird boxes, but had higher fledging rates in open topped boxes since wrens avoided these.
The cardinals on the other hand are fully territorial. This one briefly popped up to some soft pishing.
as did this Fox Sparrow, though I only managed to improve my view from completely obstructed to mostly obstructed.
Alsop, FJ. Great Crested Flycatcher Observed Copulating with an Immature Eastern Bluebird. Wilson Bulletin: 83 (3) Jul-Sep, 1971.
Gower, C. The cause of blue color as found in the bluebird (Sialia sialis) and the Blue Jay (Cyanocitta cristata). Auk: 53(2) Apr-Jun 1936.
Pinkowski, BC. Annual productivity and its measurement in a multi-brooded passerine, the Eastern Bluebird. Auk 96(3) Jul-Sep, 1979.
Musselman, TE. The effect of cold snaps up ont the nesting of the Eastern Bluebird (Sialia sialis sialis). JFO 10(1) Jan, 1939.
Wuepper, JT. The Birds of Berrien County Michigan 10 year Summary 2000-2009. Berrien Birding Club. 2011.
Radunzel LA, DM Muschitz, VM Bauldry, and P Arcese. A long-term study of the breeding success of Eastern Bluebirds by year and cavity type. JFO 68 (1) winter 1997.